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Conifers as invasive aliens: a global survey and predictive framework
27 March 2007
Stellenbosch University

Richardson, DM;Rejmánek, M

We summarize information on naturalized and invasive conifers (class Pinopsida) worldwide (data from 40 countries, some with remote states/territories), and contrast these findings with patterns for other ymnosperms (classes Cycadopsida, Gnetopsida and Ginkgoopsida) and for woody angiosperms. Eighty conifer taxa (79 species and one hybrid; 13% of species) are known to be naturalized, and 36 species (6%) are ‘invasive’. This categorization is based on objective and conservative criteria relating to consistency of reproduction, distance of spread from founders, and degree of reliance on propagules from the founder population for persistence in areas well outside the natural range of species. Twenty-eight of the known invasive conifers belong to one family (Pinaceae) and 21 of these are in one genus (Pinus). The Cupressaceae including Taxodiaceae) has six known invasive species (4%) in four genera, but the other four conifer families have none. There are also no known invasive species in classes Cycadopsida, Gnetopsida or Ginkgoopsida. No angiosperm family comprising predominantly trees and shrubs has proportionally as many invasive species as the Pinaceae. Besides the marked taxonomic bias in favour of Pinaceae, and Pinus in particular, invasiveness in conifers is associated with a syndrome of life-history traits: small seed mass (< 50 mg), short juvenile period (< 10 year), and short intervals between large seed crops. Cryptomeria japonica, Larix decidua, Picea sitchensis, Pinus contorta, Pinus strobus, and Pseudotsuga menziesii exemplify this syndrome. Many rare and endangered conifer species exhibit opposite characters. These results are consistent with earlier predictions made using a discriminant function derived from attributes of invasive and noninvasive Pinus species. Informative exceptions are species with small seeds (< 4 mg, e.g. Chamaecyparis spp., Pinus banksiana, Tsuga spp. — mostly limited to wet/mineral substrates) or otherwise ‘non-invasive’ characters (e.g. large seeds, fleshy fruits, e.g. Araucaria araucana, Pinus pinea, Taxus baccata that are dependent on vertebrates for seed dispersal). Most conifers do not require coevolved mutualists for pollination and seed dispersal. Also, many species can persist in small populations but have the genetic and reproductive capacity to colonize and increase population size rapidly. The underlying mechanisms mediating conifer invasions are thus easier to discern than is the case for most angiosperms. Further information is needed to determine the extent to which propagule pressure widespread dissemination, abundant plantings, long history of cultivation) can compensate for low ‘inherent invasiveness’.
Mapping the potential ranges of major plant invaders in South Africa, Lesotho and Swaziland using climatic suitability
27 March 2007
Stellenbosch University

Rouget, M.;Richardson, D.M.;Nel, J.L.;Le Maitre, D.C.;Egoh, B.;Mgidi, T.

Most national or regional initiatives aimed at managing biological invasions lack objective protocols for prioritizing invasive species and areas based on likely future dimensions of spread. South Africa has one of the most ambitious national programmes for managing plant invasions in the world. There is, however, no protocol for assessing the likely future spread patterns needed to inform medium- to longterm planning. This paper presents an assessment of the climatic correlates of distribution of 71 important invasive alien plants, and an analysis of the implications of these findings for future invasions in different vegetation types in South Africa, Lesotho and Swaziland over the next few decades. We used a variant of climatic envelope models (CEMs) based on the Mahalanobis distance to derive climatic suitability surfaces for each species. CEMs were developed using the first three principal components derived from an analysis of seven climatic variables. Most species are currently confined to 10% or less of the region, but could potentially invade up to 40%. Depending on the species, between 2% and 79% of the region is climatically suitable for species to invade, and some areas were suitable for up to 45 plant invaders. Over one third of the modelled species have limited potential to substantially expand their distribution. About 20% of the vegetation types have low invasion potential where fewer than five species can invade, and about 10% have high invasion potential, being potentially suitable for more than 25 of the plant invaders. Our results suggest that management of the invasive plant species that are currently most widespread should focus on reducing densities, for example through biological control programmes, rather than controlling range expansions. We also identify areas of the region that may require additional management focus in the future.
How much evolutionary history in a 10x10m plot?
28 March 2007
Stellenbosch University

Proches, S.;Wilson, J.R.U.;Cowling, R.M.

We use a fully dated phylogenetic tree of the angiospermfamilies to calculate phylogenetic diversity (PD) in four South African vegetation types with distinct evolutionary histories. Since the branch length values are in this case represented by the ages of plant lineages, PD becomes the cumulative evolutionary age (CEA) of assemblages. Unsurprisingly, total CEA increases with family and with species diversity and observed values are the same as expected from random sampling of family lists. However, when random sampling is done from species lists, observed CEAs are generally lower than expected. In vegetation types which have undergone recent diversification—grassland, fynbos and Nama-karoo—co-occurring species are more closely related than expected, but in subtropical thicket the observed CEAs are well described by random sampling. The use of CEA has great potential for assessing the age of biotic assemblages, particularly as the dating of genus and species-level phylogenies become more accurate.
Risk assessment of riparian plant invasions into protected areas
28 March 2007
Stellenbosch University

Foxcroft, L. C.;Rouget, M.;Richardson, D. M.

Protected areas are becoming increasingly isolated. River corridors represent crucial links to the surrounding landscape but are also major conduits for invasion of alien species. We developed a framework to assess the risk that alien plants in watersheds adjacent to a protected area will invade the protected area along rivers. The framework combines species-and landscape- level approaches and has five key components: (1) definition of the geographical area of interest, (2) delineation of the domain into ecologically meaningful zones, (3) identification of the appropriate landscape units, (4) categorization of alien species and mapping of their distribution and abundance, and (5)definition of management options. The framework guides the determination of species distribution and abundance through successive, easily followed steps, providing the means for the assessment of areas of concern. We applied the framework to Kruger National Park (KNP) in South Africa. We recorded 231 invasive alien plant species (of which 79 were major invaders) in the domain. The KNP is facing increasing pressure from alien species in the upper regions of the drainage areas of neighboring watersheds. On the basis of the climatic modeling, we showed that most major riparian invaders have the ability to spread across the KNP should they be transported down the rivers. With this information, KNP managers can identify areas for proactive intervention, monitoring, and resource allocation. Even for a very large protected area such as the KNP, sustainable management of biodiversity will depend heavily on the response of land managers upstream managing alien plants. We suggest that this framework is applicable to plants and other passively dispersed species that invade protected areas situated at the end of a drainage basin.
Reconstructing 50 years of Opuntia stricta invasion in the Kruger National Park, South Africa: environmental determinants and propagule pressure
28 March 2007
Stellenbosch University

Foxcroft, L.C.;Rouget, M.;Richardson, D.M.;Fadyen, S.M.

Many factors influence the spread dynamics and distribution of invasive alien organisms. Despite progress in unravelling the determinants of invasiveness and invasibility, robust, spatially-explicit predictive models for explaining real-world invasion dynamics remain illusive. Reconstructing invasion episodes is a useful way of determining the roles of different factors in mediating spread and proliferation. In many cases, however, human-aided dispersal and other anthropogenic factors blur the roles of natural controlling factors. We describe the reconstruction of an isolated invasion event from a known source: the 50-year invasion history of Opuntia stricta in the Kruger National Park. Our aim was to explore the relative roles of environment and propagule supply in shaping the invasion pattern. Environmental variables (landscape heterogeneity and distance from water sources) were moderately useful for explaining the presence/absence of O. stricta in 1-ha cells across the 660 km2 (53% of cells correctly classified). Adding fire frequency increased the accuracy of the model (68%). However, when we considered the role of propagule pressure (measured as the distance of sites from the known primary invasion focus and putative secondary invasion foci), model accuracy was greatly improved (77%). No environmental variables or propagule pressure correctly explained spatial variation in abundance (expressed as cladode density in 1-ha cells). We discuss implications of the importance of propagule supply for modelling and managing invasions.
Breeding systems of invasive alien plants
28 March 2007
Stellenbosch University

Rambuda, T;Johnson, SD

According to Baker’s Rule, plant species capable of uniparental reproduction are more likely to be successful colonists than are self-incompatible or dioecious species. Controlled pollination experiments carried out on 17 invasive alien plant species in South Africa revealed that 100% were either self-compatible or apomictic, and that 72% of these were capable of autonomous self pollination. The distribution of breeding systems among these invasive aliens is thus strongly skewed towards uniparental reproduction. While all 13 woody species in our survey showed a capacity for uniparental reproduction, this mode of reproduction is very rare among woody plants in general. Thus Baker’s rule, which has generally been considered for shortlived herbaceous plants, may also apply to invasive shrubs and trees. The study species exhibited high levels of fruit set (median = 71.5%). Supplemental hand-pollination experiments carried out on five of the species showed that pollen availability did not limit fruit set. Further work is needed to establish the exact role of uniparental reproduction in various stages of the invasion process. In particular, we need to know whether uniparental reproduction provides reproductive assurance at the population establishment stage (as originally envisaged by Baker) or whether it plays a further role in alleviating pollen limitation in small and established populations
The origins of marine mites: Interpreting geographical and ecological patterns
28 March 2007
Stellenbosch University

Marshall, DJ;Procheş, Ş

Hypotheses are advanced concerning the origins of non-halacarid mites in the marine environment. The hypothesis that oribatid (ameronothroid) and astigmatid (hyadesioid) taxa have had longer marine associations than their prostigmatid and mesostigmatid counterparts, is supported by contrasting geographical, ecological and taxonomic patterns among these mite groups, in particular by the high species to genus ratios, the general lack of generic representation in the terrestrial environment, and well-established marine trophic links of ameronothroid and hyadesioid mites. A second hypothesis that polar and subpolar genera of marine ameronothroid mites (and possibly hyadesioid mites) originated independently of the tropical genera, is suggested by three distinct latitudinal distributions of the major genera of these groups. A third hypothesis that marine origins were caused by glaciation events in the polar regions and competition in the tropics, is supported by species richness patterns across global latitudes, and the apparent habitat separation between rock-dwelling Antarctic and sedimentbased tropical marine mites. A final hypothesis concerns the basis for the return to land by the Antarctic ameronothroid genus Halozetes.
Refining the process of agent selection through understanding plant demography and plant response to herbivory
29 March 2007
Stellenbosch University

Raghu, S.;Wilson, J.R.U.;Dhileepan, K.

Understanding plant demography and plant response to herbivory is critical to the selection of effective weed biological control agents. We adopt the metaphor of ‘filters’ to suggest how agent prioritisation may be improved to narrow our choices down to those likely to be most effective in achieving the desired weed management outcome. Models can serve to capture our level of knowledge (or ignorance) about our study system and we illustrate how one type of modelling approach (matrix models) may be useful in identifying the weak link in a plant life cycle by using a hypothetical and an actual weed example (Parkinsonia aculeata). Once the vulnerable stage has been identified we propose that studying plant response to herbivory (simulated and/or actual) can help identify the guilds of herbivores to which a plant is most likely to succumb. Taking only potentially effective agents through the filter of host specificity may improve the chances of releasing safe and effective agents. The methods we outline may not always lead us definitively to the successful agent(s), but such an empirical, datadriven approach will make the basis for agent selection explicit and serve as testable hypotheses once agents are released.
Population regulation of a classical biological control agent: larval density dependence in Neochetina eichhorniae (Coleoptera: Curculionidae), a biological control agent of water hyacinth Eichhornia crassipes
29 March 2007
Stellenbosch University

Wilson, J.R.U.;Rees, M.;Ajuonu, O.

The release of classical biological control agents has reduced the economic, environmental and social problems caused by water hyacinth, Eichhornia crassipes; however, additional control measures are needed in some locations. Water hyacinth plants were treated with different densities of eggs of the weevil Neochetina eichhorniae Warner, one of the main control agents, under different nutrient regimes in a controlled experiment. Plants were destructively sampled and the development of N. eichhorniae was assessed. The survival of first and second instars declined as larval density increased. Plant nutrient status did not directly affect the mortality rate of larvae, but at higher nutrient concentrations larvae developed faster and were larger at a given developmental stage. It is argued that the density dependence operating in N. eichhorniae occurs through an interaction between young larvae and leaf longevity. Consequently, events which disrupt water hyacinth leaf dynamics, e.g. frost or foliar herbicides, will have a disproportionately large effect on the control agents and may reduce the level of control of the host.
Testing for ecological and genetic allee effect in the invasive shrub Senna Diymobotrya (Fabaceae)
29 March 2007
Stellenbosch University

van Kleunen, M.;Johnson, S.D.

For an introduced plant species to become invasive, it must be able to reproduce even in initially small populations. We tested for Allee effects (reduced reproductive performance of individuals in small populations) in the nonclonal, buzz-pollinated shrub Senna didymobotrya in its invasive range in South Africa. The species is self-compatible, but we found that in its invasive range in South Africa it requires pollinators to set seed. Nearly all stigmas (90%) received pollen, but natural fruit set was very low (3–20%). Pollen receipt and fruit set were not significantly correlated with population size. We thus found no evidence for an ecological Allee effect arising from pollen limitation in small populations. Offspring seedling performance, measured in terms of stem volume and leaf area, was also not significantly correlated with the number of plants in the source population, indicating that genetic Allee effects, such as inbreeding depression, are either absent or of such a small magnitude that they would be unlikely to limit further spread of S. didymobotrya in South Africa.
Temperature- and body mass-related variation in cyclic gas exchange characteristics and metabolic rate of seven species: Broader implications
29 March 2007
Stellenbosch University

Klock, CJ;Chown, SL

The influence of temperature on metabolic rate and characteristics of the gas exchange patterns of flightless, sub-Antarctic Ectemnorhinus-group species from Heard and Marion islands was investigated. All of the species showed cyclic gas exchange with no Flutter period, indicating that these species are not characterized by discontinuous gas exchange cycles. Metabolic rate estimates were substantially lower in this study than in a previous one of a subset of the species, demonstrating that open-system respirometry methods provide more representative estimates of standard metabolic rate than do many closed-system methods. We recommend that the latter, and especially constant-pressure methods, either be abandoned for estimates of standard metabolic rate in insects, or have their outputs subject to careful scrutiny, given the wide availability of the former. V˙ CO2 increase with an increase in temperature (range: 0–15 1C) was modulated by an increase in cycle frequency, but typically not by an increase in burst volume. Previous investigations of temperature-related changes in cyclic gas exchange (both cyclic and discontinuous) in several other insect species were therefore substantiated. Interspecific mass-scaling of metabolic rate (ca. 0.466–0.573, excluding and including phylogenetic non-independence, respectively) produced an exponent lower than 0.75 (but not distinguishable from it or from 0.67). The increase of metabolic rate with mass was modulated by an increase in burst volume and not by a change in cycle frequency, in keeping with investigations of species showing discontinuous gas exchange. These findings are discussed in the context of the emerging macrophysiological metabolic theory of ecology.
Climatic variability and hemispheric differences in insect cold tolerance: support from southern Africa
29 March 2007
Stellenbosch University

Sinclair, BJ;Chown, SL

1. Insects generally survive subzero temperatures by utilizing either freeze tolerance or freeze avoidance. Comparative analyses from a limited data set have indicated that freeze tolerance may be predominant among species at temperate latitudes in the southern hemisphere, while freeze avoidance dominates in the north. 2. We investigated the cold tolerance strategies of cockroaches and beetles from cold regions in southern Africa, including the Cederberg mountains, the Karoo desert and the Drakensberg mountains. 3. We found that 8 of 11 species are freeze tolerant. 4. Overall, 77% of species investigated in the southern hemisphere (n = 27) were freeze tolerant, which is significantly different from the 29% (n = 258) in the north (P < 0·001). 5. There are regular, unpredictable subzero temperature events in the southern African habitats; such variability appears to be a general feature of many cold habitats in the southern hemisphere, which may lead to the observed interhemispheric discrepancies in cold-tolerance strategies.
Potential impacts of future land use and climate change on the Red List status of the Proteaceae in the Cape Floristic Region, South Africa
13 April 2007
Stellenbosch University

Bomhard, B;Richardson, DM;Donaldson, JS;Hughes, GO;Midgley, GF;Raimondo, T;Rebelo, AG;Rouget, M;Thuiller, W

Using spatial predictions of future threats to biodiversity, we assessed for the first time the relative potential impacts of future land use and climate change on the threat status of plant species. We thus estimated how many taxa could be affected by future threats that are usually not included in current IUCN Red List assessments. Here, we computed the Red List status including future threats of 227 Proteaceae taxa endemic to the Cape Floristic Region, South Africa, and compared this with their Red List status excluding future threats. We developed eight different land use and climate change scenarios for the year 2020, providing a range of best- to worst-case scenarios. Four scenarios include only the effects of future land use change, while the other four also include the impacts of projected anthropogenic climate change (HadCM2 IS92a GGa), using niche-based models. Up to a third of the 227 Proteaceae taxa are uplisted (become more threatened) by up to three threat categories if future threats as predicted for 2020 are included, and the proportion of threatened Proteaceae taxa rises on average by 9% (range 2-16%), depending on the scenario. With increasing severity of the scenarios, the proportion of Critically Endangered taxa increases from about 1% to 7% and almost 2% of the 227 Proteaceae taxa become Extinct because of climate change. Overall, climate change has the most severe effects on the Proteaceae, but land use change also severely affects some taxa. Most of the threatened taxa occur in low-lying coastal areas, but the proportion of threatened taxa changes considerably in inland mountain areas if future threats are included. Our approach gives important insights into how, where and when future threats could affect species persistence and can in a sense be seen as a test of the value of planned interventions for conservation.
Plant invasion ecology - dispatches from the front line
13 April 2007
Stellenbosch University

Richardson, DM

This issue of Diversity and Distributions carries papers on a wide range of topics dealing with invasions of introduced plant species. The collection of articles did not arise from a conference or workshop, but grew from a founder population of contributed manuscripts. Some additional papers were solicited to ensure coverage of other established or emerging fields of research in plant invasion ecology. The compilation represents a reasonable cross section of issues that currently occupy plant invasion ecologists. The editorial places the contributions in context and summarizes some key findings. It also suggests some profitable avenues for future research in plant invasion ecology.
An exploratory analysis of geographic genetic variation in southern African nyala (Tragelaphus angasii)
13 April 2007
Stellenbosch University

Grobler, JP;Pretorius, DM;Botha, K;Kotze, A;Hallerman, EM;Jansen van Vuuren, B

We report patterns of genetic variation based on microsatellite, allozyme and mitochondrial control region markers in nyala from geographic locations sampled in South Africa, Mozambique, Malawi and Zimbabwe. Highly significant differences were observed among allele frequencies at three microsatellite loci between populations from KwaZulu-Natal, Limpopo and Malawi, with the Malawi and KwaZulu-Natal groupings showing the highest differentiation ðRST ¼ 0:377Þ: Allozyme frequencies showed minor, non-statistically significant regional differences among the South African populations, with maximum FST values of 0.048–0.067. Mitochondrial DNA analyses indicated a unique haplotype in each location sampled. Since none of these indices of population differentiation showed significant correlation to absolute geographic distance, we conclude that geographic variation in this species is probably a function of a distribution pattern stemming from habitat specificity. It is suggested that translocations among geographically distant regional populations be discouraged at present, pending a more elaborate investigation. Transfer of native individuals among local populations may, however, be required for minimizing the likelihood of inbreeding depression developing in small captive populations.
Concerning invasive species: Reply to Brown and Sax
13 April 2007
Stellenbosch University

Cassey, P.;Blackburn, T.M.;Duncan, R.P.;Chown, S.L.

Biological invasions have commonly occurred, and to a lesser degree continue to do so, without human assistance. It is, however, a combination of the rate and magnitude, as well as the distances and agency involved, that separates human-driven invasion processes from self-perpetuated colonization events. Exotic species are a pervasive and major component of human-induced global change. Decisions to manage invasive species will require judgements to be communicated from scientists to policy makers, because scientists may often be the only ones in the position to make them.
Biological invasions in the Antarctic: extent, impacts and implications
13 April 2007
Stellenbosch University

Frenot, Y.;Chown, S.L.;Whinam, J.;Selkirk, P.M.;Convey, P.;Skotnici, M.;Bergstrom, D.M.

Alien microbes, fungi, plants and animals occur on most of the sub-Antarctic islands and some parts of the Antarctic continent. These have arrived over approximately the last two centuries, coincident with human activity in the region. Introduction routes have varied, but are largely associated with movement of people and cargo in connection with industrial, national scientific program and tourist operations. The large majority of aliens are European in origin. They have both direct and indirect impacts on the functioning of species-poor Antarctic ecosystems, in particular including substantial loss of local biodiversity and changes to ecosystem processes. With rapid climate change occurring in some parts of Antarctica, elevated numbers of introductions and enhanced success of colonization by aliens are likely, with consequent increases in impacts on ecosystems. Mitigation measures that will substantially reduce the risk of introductions to Antarctica and the sub-Antarctic must focus on reducing propagule loads on humans, and their food, cargo, and transport vessels.
Nestedness of Southern Ocean island biotas: ecological perspectives on a biogeographical conundrum
13 April 2007
Stellenbosch University

Greve, M.;Gremmen, N.J.M.;Gaston, K.J.;Chown, S.L.

Aim To use patterns of nestedness in the indigenous and non-indigenous biotas of the Southern Ocean islands to determine the influence of dispersal ability on biogeographical patterns, and the importance of accounting for variation in dispersal ability in their subsequent interpretation, especially in the context of the Insulantarctic and multi-regional hypotheses proposed to explain the biogeography of these islands. Location Southern Ocean islands. Methods Nestedness was determined using a new metric, d1 (a modification of discrepancy), for the indigenous and introduced seabirds, land birds, insects and vascular plants of 26 Southern Ocean islands. To assess the possible confounding effects of spatial autocorrelation on the results, islands were assigned to 11 major island groups and each group was treated as a single island in a following analysis. In addition, nestedness of the six Southern Ocean islands comprising the South Pacific Province (New Zealand islands) was analysed. All analyses were conducted for species and genera, for each of the taxa on its own, and for the complete data sets. Results Statistically significant nestedness was found in all of the taxa examined, with nestedness declining in the order seabirds > land birds > vascular plants > insects for the indigenous species. Vagility had a marked influence on nestedness and the biogeographical patterns shown by the indigenous species. This influence was borne out by additional analyses of marine taxa and small-sized terrestrial species, both of which were more nested than the most nested group examined here, the seabirds. Assemblages of non-indigenous species also showed nestedness, and nestedness was generally more pronounced than in the indigenous species. Surprisingly, vagility had a significant effect on nestedness in these assemblages too. Main conclusions Nestedness analyses provide a quantitative means of comparing biogeographical patterns for groups differing in vagility. These comparisons revealed that vagility has a considerable influence on biogeographical patterns and should be taken into account in analyses. Here, investigations of more vagile taxa support hypotheses for a single origin of the Southern Ocean island biota (the Insulantarctica scenario), whilst those of less mobile taxa support the more commonly held, multi-regional hypothesis. All biogeographical analyses across the Southern Ocean (and elsewhere) will be influenced by the effects of dispersal ability, with composite analyses dominated by sedentary groups likely to favour multi-regional scenarios, and those dominated by mobile groups favouring single origins. Mechanisms underlying nestedness in the region range from nested physiological tolerances in more mobile groups to colonization ability and patterns of speciation in less vagile taxa. Considerable nestedness in the non-indigenous assemblages is largely a consequence of the fact that many of these species are European weedy species.
Human impacts, energy availability and invasion across Southern Ocean Islands
13 April 2007
Stellenbosch University

Chown, SL;Hull, B;Gaston, KJ

Aim Ongoing biological invasions will enhance the impacts of humans on biodiversity. Nonetheless, the effects of exotic species on diversity are idiosyncratic. Increases in diversity might be a consequence of similar responses by species to available energy, or because of positive relationships between human density, energy and propagule pressure. Here we use data from the Southern Ocean island plants and insects to investigate these issues. Location The Southern Ocean Islands ranging from Tristan da Cunha to Heard Island and South Georgia. Methods Generalized linear models are used to explore the relationships between indigenous and exotic species richness for plants and insects on two different islands. Similar models are used to examine interactions between indigenous and exotic species richness, energy availability and propagule pressure at the regional scale. Results Positive relationships were found between indigenous and exotic species richness at local scales, although for plants, the relationship was partially triangular. Across the Southern Ocean Islands, there was strong positive covariation between indigenous and exotic plant species richness and insect species richness, even taking spatial autocorrelation into account. Both exotic and indigenous plant and insect species richness covaried with energy availability, as did human visitor frequency. When two islands with almost identical numbers of human visits were contrasted, it was clear that energy availability, or perhaps differences in climate-matching, were responsible for differences in the extent of invasion. Conclusion In plants and insects, there are positive relationships between indigenous and exotic diversity at local and regional scales across the Southern Ocean islands. These relationships are apparently a consequence of similar responses by both groups and by human occupants to available energy. When visitor frequency is held constant, energy availability is the major correlate of exotic species richness, though the exact mechanistic cause of this relationship requires clarification.
Niche-based modelling as a tool for predicting the risk of alien plant invasions at a global scale
16 April 2007
Stellenbosch University

Thuiller, W;Richardson, DM;Pysek, P;Midgley, GF;Hughes, GO;Rouget, M

Predicting the probability of successful establishment of plant species by matching climatic variables has considerable potential for incorporation in early warning systems for the management of biological invasions. We select South Africa as a model source area of invasions worldwide because it is an important exporter of plant species to other parts of the world because of the huge international demand for indigenous flora from this biodiversity hotspot. We first mapped the five ecoregions that occur both in South Africa and other parts of the world, but the very coarse definition of the ecoregions led to unreliable results in terms of predicting invasible areas. We then determined the bioclimatic features of South Africa's major terrestrial biomes and projected the potential distribution of analogous areas throughout the world. This approach is much more powerful, but depends strongly on how particular biomes are defined in donor countries. Finally, we developed bioclimatic niche models for 96 plant taxa (species and subspecies) endemic to South Africa and invasive elsewhere, and projected these globally after successfully evaluating model projections specifically for three well-known invasive species (Carpobrotus edulis, Senecio glastifolius, Vellereophyton dealbatum) in different target areas. Cumulative probabilities of climatic suitability show that high-risk regions are spatially limited globally but that these closely match hotspots of plant biodiversity. These probabilities are significantly correlated with the number of recorded invasive species from South Africa in natural areas, emphasizing the pivotal role of climate in defining invasion potential. Accounting for potential transfer vectors (trade and tourism) significantly adds to the explanatory power of climate suitability as an index of invasibility. The close match that we found between the climatic component of the ecological habitat suitability and the current pattern of occurrence of South Africa alien species in other parts of the world is encouraging. If species' distribution data in the donor country are available, climatic niche modelling offers a powerful tool for efficient and unbiased first-step screening. Given that eradication of an established invasive species is extremely difficult and expensive, areas identified as potential new sites should be monitored and quarantine measures should be adopted.
The mid-domain effect revisited
16 April 2007
Stellenbosch University

Zapata, FA;Gaston, KJ;Chown, SL

We revisit the proposition that boundary constraints on species' ranges cause species richness gradients ( the mid-domain effect [MDE] hypothesis). In the absence of environmental gradients, species should not retain their observed range sizes as assumed by MDE models. Debate remains regarding the definition of domain limits, valid predictions for testing the models, and their statistical assessment. Empirical support for the MDE is varied but often weak, suggesting that geometric constraints on species' ranges do not provide a general explanation for richness gradients. Criticism of MDE model assumptions does not, however, imply opposition to the use of null models in ecology.
Constraint and competition in assemblages: A cross-continental and modeling approach for ants
16 April 2007
Stellenbosch University

Parr, CL;Sinclair, BJ;Andersen, AN;Gaston, KJ;Chown, SL

The mechanisms leading to structure in local assemblages are controversial. On the one hand, assemblage structure is thought to be the outcome of local interactions determined by the properties of species and their responses to the local environment. Alternatively, this structure has been shown to be an emergent property of assemblages of identical individuals or of random sampling of a regional assemblage. In ants at baits, a combination of environmental stress and interspecific competition is widely held to lead to a unimodal relationship between the abundance of dominant ants and species richness. It is thought that in comparatively adverse environments, both abundance and richness are low. As habitats become more favorable, abundance increases until the abundance of dominant ants is so high that they exclude those that are subordinate and so depress richness. Here we demonstrate empirically that this relationship is remarkably similar across three continents. Using a null model approach, we then show that the ascending part of the relationship is largely constrained to take this form not simply as a consequence of stress but also as a result of the shape of abundance frequency distributions. While the form of the species-abundance frequency distribution can also produce the descending part of the relationship, interspecific competition might lead to it too. Scatter about the relationship, which is generally not discussed in the literature, may well be a consequence of resource availability and environmental patchiness. Our results draw attention to the significance of regional processes in structuring ant assemblages.
Differential responses of thermal tolerance to acclimation in the sub-Antarctic rove beetle Halmaeusa atriceps
16 April 2007
Stellenbosch University

Slabber, S;Chown, SL

Investigations of the responses to acclimation of upper and lower lethal limits and limits to activity in insects have focused primarily on Drosophila. In the present study, Halmaeusa atriceps (Staphylinidae) is examined for thermal tolerance responses to acclimation, and seasonal acclimatization. In summer and winter, lower lethal temperatures of adults and larvae are approximately -7.6 +/- 0.03 and -11.1 +/- 0.06 degrees C, respectively. Supercooling points (SCPs) are more variable, with winter SCPs of -5.4 +/- 0.4 degrees C in larvae and -6.3 +/- 0.8 degrees C in adults. The species appears to be chill susceptible in summer and moderately freeze tolerant in winter, thus showing seasonal acclimatization. Similar changes cannot be induced solely by acclimation to low temperatures in the laboratory. Upper lethal temperatures show a weaker response to acclimation. There are also significant responses to acclimation of critical thermal limits. Critical thermal minima vary between -3.6 +/- 0.2 and -0.6 +/- 0.2 degrees C in larvae, and from -4.1 +/- 0.1 to -0.8 +/- 0.2 degrees C in adults. By contrast, critical thermal maxima vary much less within adults and larvae. These findings are in keeping with the general pattern found in insects, although this species differs in several respects from others found on Marion Island.
Marine alien species of South Africa - status and impacts
16 April 2007
Stellenbosch University

Robinson, TB;Griffiths, CL;McQuaid, CD;Ruis, M

The current status of marine alien species along the South African coast is reviewed and the ecological and economic impacts of these invasions are discussed. In all, 10 confirmed extant alien and 22 cryptogenic species are recorded from the region. All 10 alien species support well-established populations and the majority of these remain restricted in distribution to sheltered bays, estuaries and harbours. Only one species, the Mediterranean mussel Mytilus galloprovincialis, has spread extensively along the coast and caused significant ecological impacts. These include the competitive displacement of indigenous species and a dramatic increase in intertidal mussel biomass. These changes have also increased available habitat for many infaunal species and resulted in enhanced food supply for intertidal predators. Considerable economic benefits have also resulted from this invasion because M. galloprovincialis forms the basis of the South African mussel culture industry.
The Ecological Reserve: Towards a common understanding for river management in South Africa
17 April 2007
Stellenbosch University

van Wyk, E;Breen, CM;Roux, DJ;Rogers, KH;Sherwill, T;van Wilgen, BW

The legal requirement for an Ecological Reserve established in South Africa's water law is commonly regarded by stakeholders as being in direct competition with the needs of humans. This has resulted in much debate and varying interpretations of the meaning and purpose of the Ecological Reserve. However, the requirement for water that is allocated to sustain ecosystem functions is directly aligned with options for human use arising from rivers to deliver a suite of ecosystem goods and services to society. In this paper, we propose a conceptual approach to support a more constructive debate around the role and function of the Reserve in the sustainable use and protection of a suite of benefits to society. The approach proposes that debate be structured around managing for a dynamic ecological state in rivers that would in turn achieve the desired (albeit dynamic) mix of goods and services to a wide range of stakeholders. These stakeholders come from widely differing socioeconomic backgrounds, and their needs may be either for the direct use of water and associated resources located within the macro channels of rivers, or for their use in supporting social and economic activity remote from the river. The paper shows how goods and services concepts can provide an approach that contributes to developing a shared understanding that facilitates decisions on water allocations. The implication is that when water allocations can be evaluated comparatively it creates greater awareness of each other's needs and interdependencies and value is attached to a greater diversity of benefits and costs. This in turn allows for opportunities to achieve more equitable recognition and allocation of the resources associated with rivers. The approach assists in making the conceptual link between goods and services that arise from constructed production systems, and those that arise from natural production systems (i.e. ecosystems). Off-site as well as on-site use of river goods and services (the latter being catered for by the Ecological Reserve) can in this way be brought into debate in a way that promotes wider appreciation of society's diverse uses of river resources. In doing so it promotes interest-based participation as intended by legislation.
The effect of fire season, fire frequency, rainfall and management on fire intensity in savanna vegetation in South Africa
17 April 2007
Stellenbosch University

Govender, N;Trollope, WSW;van Wilgen, BW

1. Fire is important for the maintenance and conservation of African savanna ecosystems. Despite the importance of fire intensity as a key element of the fire regime, it is seldom measured or included in fire records. 2. We estimated fire intensity in the Kruger National Park, South Africa, by documenting fuel loads, fuel moisture contents, rates of fire spread and the heat yields of fuel in 956 experimental plot burns over 21 years. 3. Individual fires were conducted in five different months (February, April, August, October and December) and at five different return intervals (1, 2, 3, 4 and 6 years). Estimated fire intensities ranged from 28 to 17 905 kW m(-1). Fire season had a significant effect on fire intensity. Mean fire intensities were lowest in summer fires (1225 kW m(-1)), increased in autumn fires (1724 kW m(-1)) and highest in winter fires (2314 kW m(-1)); they were associated with a threefold difference between the mean moisture content of grass fuels in winter (28%) and summer (88%). 4. Mean fuel loads increased with post-fire age, from 2964 kg ha(-1) on annually burnt plots to 3972 kg ha(-1) on biennial, triennial and quadrennial burnt plots (which did not differ significantly), but decreased to 2881 kg ha(-1) on sexennial burnt plots. Fuel loads also increased with increasing rainfall over the previous 2 years. 5. Mean fire intensities showed no significant differences between annual burns and burns in the biennial, triennial and quadrennial categories, despite lower fuel loads in annual burns, suggesting that seasonal fuel moisture effects overrode those of fuel load. Mean fire intensity in sexennial burns was less than half that of other burns (638 vs. 1969 kW m(-1)). 6. We used relationships between season of fire, fuel loads and fire intensity in conjunction with the park's fire records to reconstruct broad fire intensity regimes. Changes in management from regular prescribed burning to 'natural' fires over the past four decades have resulted in a decrease in moderate-intensity fires and an increase in high-intensity fires. 7. The highest fire intensities measured in our study (11 000 - > 17 500 kW m(-1)) were significantly higher than those previously reported for African savannas, but were similar to those in South American cerrado vegetation. The mean fire intensity for late dry season (winter) fires in our study was less than half that reported for late dry season fires in savannas in northern Australia. 8. Synthesis and applications. Fire intensity has important effects on savanna vegetation, especially on the dynamics of the tree layer. Fire intensity varies with season (because of differences in fuel moisture) as well as with fuel load. Managers of African savannas can manipulate fire intensity by choosing the season of fire, and further by burning in years with higher or lower fuel loads. The basic relationships described here can also be used to enhance fire records, with a view to building a long-term data set for the ongoing assessment of the effectiveness of fire management.
Evolutionary ecology meets wildlife management: artificial group augmentation in the re-introduction of endangered African wild dogs (Lycaon pictus)
17 April 2007
Stellenbosch University

Graf, JA;Gusset, M;Reid, C;Janse van Rensburg, S;Slotow, R;Somers, MJ

As an alternative to kin selection, group augmentation theory provides a framework for evolutionary mechanisms maintaining cooperative breeding when individual fitness is positively related to group size. It is expected that a cooperator group would accept or adopt unrelated foreigners when it is below a critical threshold size and group members could thus benefit from recruiting additional helpers. In re-introduction attempts, this would allow for a group to be augmented artificially before release, which would enhance its chance to establish itself successfully in the release area. This possibility was tested using endangered African wild dogs Lycaon pictus studied in Hluhluwe-iMfolozi Park, South Africa. Here, we report on the first successful artificial integration of an unrelated adult female with her three male pups into an existing pack. In addition, post-release monitoring data are presented, including how a yearling male displaced the dominant male that adopted him as a pup, adding to the controversy over the evolutionary stability of group augmentation as a route to cooperative breeding. This study thus demonstrates how theory from evolutionary ecology can be applied to practical wildlife management, and vice versa.
Divided we fail: the importance of social integration for the re-introduction of endangered African wild dogs (Lycaon pictus)
17 April 2007
Stellenbosch University

Gusset, M;Skotow, R;Somers, MJ

In South Africa, a plan was launched to manage separate sub-populations of endangered African wild dogs Lycaon pictus in several small, geographically isolated conservation areas as a single meta-population. This intensive management approach involves the re-introduction of wild dogs into suitable conservation areas and periodic translocations among them. Here, we sought to evaluate the relevance of taking the formation of new reproductive units into consideration in promoting such translocation attempts. For this purpose, we analysed the behavioural process of integrating translocated wild dogs into new packs in pre-release holding facilities in Hluhluwe-iMfolozi Park, one of the meta-population conservation areas. In addition, we reviewed findings from other wild dog translocation attempts in regard to the outcome of using variously composed groups and captive-bred animals for the artificial creation of new packs for translocation purposes. We show the importance of social integration before release for wild dog re-introductions and translocations to be successful. We also present a set of proximate factors, including management manipulation of social relationships, which promote pack formation in pre-release holding facilities. This demonstrably enhances the efficiency of costly wild dog re-introductions and translocations, thereby illustrating the implications of sociality for endangered species recovery.
Mongooses on the move: an apparent case of interspecific cooperative vigilance between carnivores
17 April 2007
Stellenbosch University

San, EDL;Somers, MJ

Meerkats (Suricata suricatta) and yellow mongooses (Cynictis penicillata) show antipredator behaviours which, at least in the former species, can take the form of cooperation between pack members. Here, we describe an apparent case of cooperative vigilance between one yellow mongoose and three meerkats while the animals were travelling in open terrain from one termite mound to another. We hypothesize that the observed interspecific interaction, although enabled by similarities in the behaviour of both herpestid species, might have been favoured by the shared use of burrows leading to interspecific tolerance and the high predation pressure put on small groups. To our knowledge, this would constitute the first reported instance of cooperation between two carnivore species related to predator detection.
Current distribution and potential extent of the most invasive alien plant species on La Réunion (Indian Ocean, Mascarene islands)
17 April 2007
Stellenbosch University

Baret, S.;Rouget, M.;Richardson, D.M.;Lavergne, C.;Egoh, B.;Dupont, J.;Strasberg, D.

La R union Island has the largest area of intact vegetation of the islands in the Mascarene archipelago. Biological invasions are the primary threat to biodiversity in the intact habitats of the island (those not already transformed by agriculture and urbanization). Our study aimed to identify areas to prioritize in managing invasive alien plants for biodiversity conservation. We used extensive surveys of 238 distinct untransformed areas on La Reunion to define the current distribution patterns of all invasive species. Using expert knowledge, we compiled maps of the current distribution of the 46 most widespread/important invasive plants at the habitat scale (identified according to vegetation structure). Data from 440 botanical releves for the 20 most threatening invasive alien plant species across the island and climatic envelope models were used to derive climatic suitability surfaces; these were used to map potential distributions for these species. More than 10 species invade 16.7% of the remaining habitat. Five habitat types are invaded by 25 or more species, and eight have fewer than 10 invasive alien plant species. Cluster analysis based on presence/absence of species in the 18 habitat types produced eight groups of species that invade particular habitats. Potential distribution models show that some species have invaded large parts of their potential range (e.g. Fuchsia magellanica, Furcraea foetida, Hiptage benghalensis), whereas others have the potential to increase their range substantially (e.g. Clidemia hirta, Strobilanthes hamiltonianus, Ulex europaeus). Management implications are identified for both groups. Three broad groups of habitats were identified: (i) intact habitats with a low level of invasion (e.g. subalpine shrubland); (ii) moderately invaded habitats with varying levels of intactness (ranging from windward submountain rainforest to the Acacia heterophylla forest); and (iii) habitats with little remaining intact area and high levels of invasion (e.g. lowland rainforest). Different management interventions are appropriate for these three groups.
Biogeographical patterns in the fauna associated with southern African mussel beds
17 April 2007
Stellenbosch University

Hammond, W;Griffiths, CL

Macrofauna communities colonizing intertidal mussel beds were sampled at seven sites between Swakopmund (Namibia) and Salt Rock (KwaZulu-Natal). Mean mussel cover, length, biomass and bed depth were all low in the southeast (former Transkei). Faunal abundance and biomass were minimal along the south coast, increasing up both west and east coasts. Species diversity and richness showed the reverse trends, peaking at south coast sites and dropping off to the west and east. Multivariate analysis grouped the faunal communities into four distinct clusters, corresponding to the recognized Namib, Namaqua, Agulhas and Natal biogeographical provinces. The low mussel biomass and cover in the Eastern Cape can be attributed to intense human exploitation, but the low faunal abundance and biomass along the south coast was unexpected, as was the negative correlation between faunal abundance and diversity. Over recent decades intense exploitation has resulted in marked declines in mussel cover along the east coast, while the M. galloprovincialis invasion has resulted in dramatic increases in mussel cover to the west. The results given here indicate how these changes might have affected the wider intertidal communities in these regions.
Molecular and morphometric assessment of the taxonomic status of Ectemnorhinus weevil species (Coleoptera : Curculionidae, Entiminae) from the sub-Antarctic Prince Edward Islands
19 April 2007
Stellenbosch University

Grobler, G.C.;Janse van Rensburg, L.B.;Bastos, A.D.S.;Chimimba, C.T.;Chown, S.L.

There are long-standing controversies on the taxonomic status of Ectemnorhinus weevil species occurring on the sub-Antarctic Prince Edward Islands. Since the two islands that constitute the Prince Edward Islands archipelago (PEIA), Marion Island (MI) and Prince Edward Island (PEI), differ in terms of alien invasive species such as the introduced house mouse Mus musculus and conservation management strategies, it is important to consider inter-island dynamics when investigating inter-specific relationships. Using a combined molecular phylogenetic and morphometric approach, we attempted to resolve the taxonomic status of the PEIA Ectemnorhinus weevil species. A COI gene phylogeny was inferred following the genetic characterization of 52 Ectemnorhinus weevils from both islands, and morphometric assessment using a set of 15 linear, external measurements was used to differentiate between the two currently recognized species, Ectemnorhinus similis and Ectemnorhinus marioni. Analyses revealed the presence of two genetically and morphometrically distinct species on PEI, whilst evidence for a single species, comprising diverse genetically discrete populations was found on MI. Based on these results, the species unique to PEI has been designated Ectemnorhinus kuscheli n. sp. whilst we confirm the synonymy between E. similis and E. marioni, the two species originally described from MI. E. kuscheli appears to be restricted to PEI, whereas E. similis occurs on both MI and PEI.
Testing the beneficial acclimation hypothesis and its alternatives for locomotor performance
19 April 2007
Stellenbosch University

Deere, JA;Chown, SL

The beneficial acclimation hypothesis ( BAH) is controversial. While physiological work all but assumes that the BAH is true, recent studies have shown that support for the BAH is typically wanting. The latter have been criticized for assessing the benefits of developmental plasticity rather than acclimation. Here we examine the BAH within a strong inference framework for five congeneric species of ameronothroid oribatid mites that occupy marine to terrestrial habitats. We do so by assessing responses of maximum speed, optimum temperature, and performance breadth, measured from - 10 degrees C to 35 degrees C, to four treatment temperatures ( 0 degrees, 5 degrees, 10 degrees, and 15 degrees C). We show that the BAH and its alternatives often make similar empirical predictions. Weak beneficial acclimation is characteristic of one of the more marine species. In the other two upper-shore and marine species, evidence exists for deleterious acclimation and the colder-is-better hypothesis. In the two fully terrestrial species, there is no plasticity. Lack of plasticity is beneficial when cue reliability is low or costs of plasticity are high, and the former seems plausible in terrestrial habitats. However, weak plasticity in the uppershore/marine species and the absence of plasticity in the terrestrial species might also be a consequence of phylogenetic constraint.
Linking molecular physiology to ecological realities
19 April 2007
Stellenbosch University

Chown, SL;Storey, KB

Although molecular physiology and ecology have drifted apart as a consequence of early separation in the questions posed and techniques used, there is a resurgence of interest in forging links between them. Here we explore the reasons for this renewed interest and provide four examples of how this is happening. Specifically, we examine links between molecular physiology and ecological realities in insect responses to thermal stress, vertebrate responses to anoxia, metabolic fuel use and torpor in mammals, and the recently developed "metabolic theory of ecology." Several novel insights are emerging from integrated approaches to these problems that might not have come forward from any single perspective on them. Nonetheless, prospects for linking molecular physiology and ecological realities are likely to remain poor if greater focus is not given to developing these links. Mostly, this is a consequence of the differing approaches and "languages" adopted by these fields. We discuss approaches by which the prospects for synthetic work might be improved.
Protected areas and regional avian species richness in South Africa
19 April 2007
Stellenbosch University

Evans, KL;Rodrigues, ASL;Chown, SL;Gaston, KJ

Protected areas are generally regarded as essential for the long-term maintenance of biodiversity. Evidence for their effectiveness in this regard is, however, somewhat equivocal. Here, we document the relationship between the proportion of protected land and species richness in a region, both with and without taking spatial variation in environmental energy availability into account. Using the South African avifauna as a case study, we find that total and threatened species richness exhibit modest increases with the proportion of protected land. While the protected area network should be expanded, it is essential that conservation efforts also focus on maintaining biodiversity in the wider unprotected landscape that supports high species richness.
Endemicity biases nestedness metrics: a demonstration, explanation and solution
19 April 2007
Stellenbosch University

Greve, M.;Chown, S.L.

Nestedness is frequently investigated to understand complex patterns of species occurrences. Temperature (T) is commonly used for comparisons of nestedness of different-sized datasets. However, the assumptions made for the standardization of this metric have not been fully explored, particularly the effects of endemicity. Here we show that T incorrectly indicates an increase in nestedness with the addition of non-nested endemics to matrices that are not perfectly nested - a consequence of standardizing matrix size by the product of species and sites. This problem is common both to test matrices and to real matrices that are typically subjected to nestedness analyses. The latter are often characterized by substantial numbers of endemics and by variation in the numbers of endemics in different taxa. Standardizing by occupancy resolves this problem, which is demonstrated using a derivative of discrepancy, d1. A small modification to T, such that it standardizes matrices by occupancy, would resolve the current problems with this nestedness metric.
Gas exchange characteristics, metabolic rate and water loss of the Heelwalker, Karoophasma biedouwensis (Mantophasmatodea : Austrophasmatidae)
19 April 2007
Stellenbosch University

Chown, SL;Marais, E;Picker, MD;Terblanche, JS

This study presents the first physiological information for a member of the wingless Mantophasmatodea, or Heelwalkers. This species shows cyclic gas exchange with no evidence of a Flutter period (more typical of discontinuous gas exchange in insects) and no indication that the spiracles are fully occluded during quiescent metabolism. Standard metabolic rate at 20 degrees C was 21.32 +/- 2.73 mu l CO2 h(-1) (mean +/- S.E.), with a Q(10) (10-25 degrees C) of 1.7. Increases in (VCO2)-C-center dot associated with variation in mass and with trial temperature were modulated by an increase in burst period volume and a decline in cycle frequency. Total water loss rate, determined by infrared gas analysis, was 0.876 +/- 0.08 mg H2O h(-1) (range 0.602-1.577, n = 11) whilst cuticular water loss rate, estimated by linear regression of total water loss rate and metabolic rate, was 0.618 +/- 0.09 mg H2O h(-1) (range 0.341-1.363, n = 11). Respiratory water loss rate was therefore no more than 29% of the total rate of water loss. Both total water loss rate and estimated cuticular water loss rate were significantly repeatable, with intraclass correlation coefficients of 0.745 and 0.553, respectively. (c) D 2006 Elsevier Ltd. All rights reserved.
Interactions between environment, species traits, and human uses describe patterns of plant invasions
20 April 2007
Stellenbosch University

Thuiller, W.;Richardson, D.M.;Rouget, M.;Procheş, Ş.;Wilson, J.R.U.

Although invasive alien species (IAS) are a major threat to biodiversity, human health, and economy, our understanding of the factors controlling their distribution and abundance is limited. Here, we determine how environmental factors, land use, life-history traits of the invaders, residence time, origin, and human usage interact to shape the spatial pattern of invasive alien plant species in South Africa. Relationships between the environmental factors and the extrinsic and intrinsic attributes of species were investigated using RLQ analysis, a multivariate method for relating a species-attribute table to an environmental table by way of a species presence/absence table. We then clustered species according to their position on the RLQ axes, and tested these groups for phylogenetic independence. The first three axes of the RLQ explained 99% of the variation and were strongly related to the species attributes. The clustering showed that, after accounting for environmental factors, the spatial pattern of IAS in South Africa was driven by human uses, life forms, and reproductive traits. The seven clusters of species strongly reflected geographical distribution, but also intrinsic species attributes and patterns of human use. Two of the clusters, centered on the genera Acacia and Opuntia, were phylogenetically non-independent. The remaining clusters comprised species of diverse taxonomic affinities, but sharing traits facilitating invasion in particular habitats. This information is useful for assessing the extent to which the potential spread of recent introductions can be predicted by considering the interaction of their biological attributes, region of origin, and human use.
Plant invasions: merging the concepts of species invasiveness and community invasibility
20 April 2007
Stellenbosch University

Richardson, DM;Pysek, P

This paper considers key issues in plant invasion ecology, where findings published since 1990 have significantly improved our understanding of many aspects of invasions. The review focuses on vascular plants invading natural and semi-natural ecosystems, and on fundamental ecological issues relating to species invasiveness and community invasibility. Three big questions addressed by the SCOPE programme in the 1980s (which species invade; which habitats are invaded; and how can we manage invasions?) still underpin most work in invasion ecology. Some organizing and unifying themes in the field are organism-focused and relate to species invasiveness (the tens rule; the concept of residence time; taxonomic patterns and Darwin's naturalization hypothesis; issues of phenotypic plasticity and rapid evolutionary change, including evolution of increased competitive ability hypothesis; the role of long-distance dispersal). Others are ecosystem-centred and deal with determinants of the invasibility of communities, habitats and regions (levels of invasion, invasibility and propagule pressure; the biotic resistance hypothesis and the links between diversity and invasibility; synergisms, mutualisms, and invasional meltdown). Some theories have taken an overarching approach to plant invasions by integrating the concepts of species invasiveness and community invasibility (a theory of seed plant invasiveness; fluctuating resources theory of invasibility). Concepts, hypotheses and theories reviewed here can be linked to the naturalization-invasion continuum concept, which relates invasion processes with a sequence of environmental and biotic barriers that an introduced species must negotiate to become casual, naturalized and invasive. New research tools and improved research links between invasion ecology and succession ecology, community ecology, conservation biology and weed science, respectively, have strengthened the conceptual pillars of invasion ecology.
Pinus: a model group for unlocking the secrets of alien plant invasions
20 April 2007
Stellenbosch University

Richardson, DM

The global-scale natural experiment created by the widespread dissemination of most of the I I I species of pines (genus Pinus, family Pinaceae) has shed light on many aspects of plant invasion ecology. Introductions and the fate of alien pines have been well documented worldwide, facilitating the accurate labelling of species as "naturalized", "invasive", or "non-invasive" using objective criteria. Thirty species are naturalized and 21 are invasive. Three life-history traits that clearly separate invasive from non-invasive taxa define the inherent ability of species to disperse over long distances, win in competition against other plants, and to survive or proliferate under a range of disturbance regimes. The realization of colonization opportunities for potentially invasive pines is determined by extrinsic factors, especially those that mediate seedling establishment. Meta-analysis of many introduction/invasion events revealed the interplay of factors. Detailed studies of pine invasions have elucidated the roles of long-distance seed dispersal and propagule pressure in driving invasions. Enhanced understanding of the ecology of pine invasions has improved our ability to manage these invasions. No other speciose genus of plants has yielded important insights on so many facets of invasion ecology.
Biological invasions as disruptors of plant reproductive mutualisms
20 April 2007
Stellenbosch University

Traveset, A;Richardson, DM

Invasive alien species affect the composition and functioning of invaded ecosystems in many ways, altering ecological interactions that have arisen over evolutionary timescales. Specifically, disruptions to pollination and seed-dispersal mutualistic interactions are often documented, although the profound implications of such impacts are not widely recognized. Such disruptions can occur via the introduction of alien pollinators, seed dispersers, herbivores, predators or plants, and we define here the many potential outcomes of each situation. The frequency and circumstances under which each category of mechanisms operates are also poorly known. Most evidence is from population-level studies, and the implications for global biodiversity are difficult to predict. Further insights are needed on the degree of resilience in interaction networks, but the preliminary picture suggests that invasive species frequently cause profound disruptions to plant reproductive mutualisms.
Management of plant invasions mediated by frugivore interactions
20 April 2007
Stellenbosch University

Buckley, YM;Anderson, S;Catterall, CP;Corlett, RT;Engel, T;Gosper, CR;Nathan, R;Richardson, DM

1. Some of the most damaging invasive plants are dispersed by frugivores and this is an area of emerging importance in weed management. It highlights the need for practical information on how frugivores affect weed population dynamics and spread, how frugivore populations are affected by weeds and what management recommendations are available. 2. Fruit traits influence frugivore choice. Fruit size, the presence of an inedible peel, defensive chemistry, crop size and phenology may all be useful traits for consideration in screening and eradication programmes. By considering the effect of these traits on the probability, quality and quantity of seed dispersal, it may be possible to rank invasive species by their desirability to frugivores. Fruit traits can also be manipulated with biocontrol agents. 3. Functional groups of frugivores can be assembled according to broad species groupings, and further refined according to size, gape size, pre- and post-ingestion processing techniques and movement patterns, to predict dispersal and establishment patterns for plant introductions. 4. Landscape fragmentation can increase frugivore dispersal of invasives, as many invasive plants and dispersers readily use disturbed matrix environments and fragment edges. Dispersal to particular landscape features, such as perches and edges, can be manipulated to function as seed sinks if control measures are concentrated in these areas. 5.Where invasive plants comprise part of the diet of native frugivores, there may be a conservation conflict between control of the invasive and maintaining populations of the native frugivore, especially where other threats such as habitat destruction have reduced populations of native fruit species. 6. Synthesis and applications. Development of functional groups of frugivore-dispersed invasive plants and dispersers will enable us to develop predictions for novel dispersal interactions at both population and community scales. Increasingly sophisticated mechanistic seed dispersal models combined with spatially explicit simulations show much promise for providing weed managers with the information they need to develop strategies for surveying, eradicating and managing plant invasions. Possible conservation conflicts mean that understanding the nature of the invasive plant-frugivore interaction is essential for determining appropriate management.
Who cites who in the invasion zoo: insights from an analysis of the most highly cited papers in invasion ecology
20 April 2007
Stellenbosch University

Pysek, P;Richardson, DM

The citation frequency of papers on invasion ecology published between 1981 and 2003 and that had accumulated at least 30 citations on the Web of Science on 9 August 2006 was analysed. The dataset comprised 329 papers and 27,240 citations. For each paper, the total number of citations was recorded and the annual citation rate (number of citations per year) was calculated. Papers were classified into broad research fields: plant invasions, animal invasions, biological control, and general papers (reviews and syntheses). Eight papers were cited more than 300 times, five of them dealt with general topics, and the mean value of the total number of citations across the whole data set is 82.8 +/- 73.1. The mean annual citation rate is 11.5 +/- 11.3 citations per year; six studies received on average at least 50 citations each year. About a half (50.8%) of papers in the data set deal with plant invasions. General papers are significantly more cited than papers from the other categories. The annual citation rate increased with time over the analysed period (1981-2003), by 1.0 citations per year. To compare the trends in invasion ecology with those in other fields of ecology, comparable data were compiled for population ecology and dynamics, and global change. The annual citation rate for invasion ecology as a whole increased faster than that for population ecology and dynamics, but not exponentially as is the case with studies on global change. The best-cited papers on invasion ecology were distributed among most of the top ecology journals. Those published in Oikos, Journal of Ecology, Ecological Applications and BioScience are cited 3.8-5.8 times more than the average for these journals (based on the impact factor). Papers on biodiversity, community ecology, impact, invasibility, dispersal, population ecology, competition, resources, genetical issues, biological control and species invasiveness received the highest total number of citations. However, measured by the annual citation rate, the hottest current topics in invasion ecology are the effect of global change on invasions, the role of natural enemies, character of the invasion process, evolutionary aspects, invasibility of communities and ecosystem processes. Some topics are disproportionally more cited than studied and vice versa. Studies on plant and animal invasions differ in focus: the topics of invasibility, biodiversity, resources, species invasiveness and population genetics are more emphasized in botanical studies, dispersal, competition, impact and pathways in papers dealing with animal invasions. Studies of grasslands and marine environment are most frequently cited in botanical and zoological studies, respectively. Most of the highly cited papers deal with multiple species; only 14 plant species and four animal species are the primary focus of one or more of the highly-cited papers. Twenty-two authors (4.5% of the total involved in the papers analysed), each with seven or more contributions cited at least 30 times, together contributed 49.4% of the most-cited papers, and attracted 55.6% of the total number of citations.
Novel ecosystems: theoretical and management aspects of the new ecological world order
20 April 2007
Stellenbosch University

Hobbs, RJ;Arico, S;Aronson, S;Baron, JS;Bridgewater, P;Cramer, VA;Epstein, PR;Ewel, JJ;Klink, CA;Lugo, AE;Norton, D;Ojima, D;Richardson, DM

We explore the issues relevant to those types of ecosystems containing new combinations of species that arise through human action, environmental change, and the impacts of the deliberate and inadvertent introduction of species from other regions. Novel ecosystems (also termed 'emerging ecosystems') result when species occur in combinations and relative abundances that have not occurred previously within a given biome. Key characteristics are novelty, in the form of new species combinations and the potential for changes in ecosystem functioning, and human agency, in that these ecosystems are the result of deliberate or inadvertent human action. As more of the Earth becomes transformed by human actions, novel ecosystems increase in importance, but are relatively little studied. Either the degradation or invasion of native or 'wild' ecosystems or the abandonment of intensively managed systems can result in the formation of these novel systems. Important considerations are whether these new systems are persistent and what values they may have. It is likely that it may be very difficult or costly to return such systems to their previous state, and hence consideration needs to be given to developing appropriate management goals and approaches.
A global indicator for biological invasion
20 April 2007
Stellenbosch University

McGeoch, MA;Chown, SL;Kalwij, JM

"Trends in invasive alien species' is one of only two indicators of threat to biodiversity that form part of the Convention on Biological Diversity's (CBD) framework for monitoring progress toward its "2010 target" (ie., the commitment to achieve by 2010 a significant reduction in the current rate of biodiversity loss). To date, however, there is no fully developed indicator for invasive alien species (JAS) that combines trends, derived from a standard set of methods, across species groups, ecosystems, and regions. Here we provide a rationale for the form and characteristics of an indicator of trends in MS that will meet the 2010 framework goal and targets for this indicator We suggest single and composite indicators that include problem-status and inanagement-status measures that are designed to be flexible, readily disaggregated, and as far as possible draw on existing data. The single indicators at national and global scales are number of IAS and numbers of operational management plans for MS. Global trends in IAS are measured as the progress of nations toward the tat-gets of stabilizing JAS numbers and the implementation of MS management plans. The proposed global indicator thus represents a minimum information set that most directly addresses the indicator objective and simultaneously aims to maximize national participation. This global indicator now requires testing to assess its accuracy, sensitivity and tractability. Although it may not be possible to achieve the desired objective for a global indicator of biological invasion by 2010 as comprehensively as desired, it seems possible to obtain trend estimates for a component of the taxa, ecosystems, and regions involved. Importantly, current indicator development initiatives will. also contribute to developing the mechanisms necessary for monitoring global trends in MS beyond 2010.
Body size patterns in Drosophila inhabiting a mesocosm: interactive effects of spatial variation in temperature and abundance
20 April 2007
Stellenbosch University

Warren, M;McGeoch, MA;Nicolson, SW;Chown, SL

Body size is a major component of fitness. However, the relative contributions of different factors to optimal size, and the determinants of spatial and temporal variation in size, have not been fully established empirically. Here, we use a mesocosm of a Drosophilidae assemblage inhabiting decaying nectarines to investigate the influence of spatial variation in temperature on adult body size in Drosophila simulans Sturtevant. Two treatments were established; one in the sun where developing larvae were exposed to high temperatures and the other in the shade where temperature conditions were milder. The simple developmental effects of temperature differences (i.e. larger flies are likely to emerge from cooler environments), or the simple effects of stressful temperatures (i.e. high temperatures yield wing abnormalities and smaller flies), were overridden by interactive effects between temperature and larval density. Emergences were lower in the sun than shade, probably as a result of temperature-induced mortality. However, flies attained the same final sizes in the shade and sun. In addition, abnormally winged flies were clustered in the shaded treatments. In the shade treatments, where emergences were higher than in the sun, stressful conditions as a result of high larval density likely resulted in wing abnormalities and small size. Consequently, there was little spatial variation in size across the mesocosm, but substantial spatial variation in abundance. Under natural conditions both mortality and non-lethal effects of temperature and/or crowding are likely to play a role in the evolution of body size.
Determinants of terrestrial arthropod community composition at Cape Hallett, Antarctica
20 April 2007
Stellenbosch University

Sinclair, BJ;Scott, MB;Klok, CJ;Terblanche, JS;Marshall, DJ;Reyers, B;Chown, SL

The distribution and abundance of free-living arthropods from soil and under stones were surveyed at the Cape Hallett ice-free area (ASPA No. 106), North Victoria Land, Antarctica. A total of 327 samples from 67 plots yielded 11 species of arthropods comprised of three Collembola: Cryptopygus cisantarcticus, Friesea grisea and Isotoma klovstadi and eight mites: Coccorhagidia gressitti, Eupodes wisei, Maudheimia petronia, Nanorchestes sp., Stereotydeus belli, S. punctatus, Tydeus setsukoae and T wadei. Arthropods were absent from areas occupied by the large Adelie penguin colony. There was some distinction among arthropod communities of different habitats, with water and a lichen species (indicative of scree slope habitats) ranking as significant community predictors alongside spatial variables in a Canonical Correspondence Analysis. Recent changes to the management plan for ASPA No. 106 may need to be revisited as the recommended campsite is close to the area of greatest arthropod diversity.
Caterpillars benefit from thermal ecosystem engineering by wandering albatrosses on sub-Antarctic Marion Island
20 April 2007
Stellenbosch University

Sinclair, BJ;Chown, SL

Wandering albatrosses (Diomedea exulans) nest on Southern Ocean islands, building elevated nests upon which they incubate eggs and raise chicks, and which the chicks occupy through winter. The nests support high invertebrate biomass, including larvae of the flightless moth Pringleophaga marioni. Here we argue that high biomass of P marioni in the nests is not associated with nutrient loading as previously suspected, but that higher temperatures in the nests increase growth and feeding rate, and decrease deleterious repeated cold exposure, providing fitness advantages for P marioni. Thus, wandering albatrosses may be serving as thermal engineers, modifying temperature and therefore enabling better resource use by P marioni.
Effects of experimental harvesting on recruitment of an alien mussel Mytilus galloprovincialis
24 April 2007
Stellenbosch University

Robinson, TB;Branch, GM;Griffiths, CL;Govender, A

Successful fisheries management is underpinned by an understanding of the processes that underlie the population dynamics of exploited stocks. This study considered the effects of experimental harvesting on recruitment of Mytilus galloprovincialis along the west coast of South Africa, where harvesting of this alien species is being contemplated. In particular, the role of settlement habitat availability in the form of adult mussels was analysed. To track the effects of a spectrum of harvesting intensities, five treatments were implemented: F = 0 (i.e. a control), F = 0.3, F = 0.6, F = 0.9 and F = 1. At these harvesting intensities 0%, 30%, 60%, 90% or 100% of mussel biomass was removed respectively at the outset of the experiment. A significant negative exponential relationship (p < 0.01) was found between M. galloprovincialis recruit density and harvesting intensity, with intensities greater than F = 0.3 dramatically reducing recruitment. This pattern was recorded throughout the intertidal zone and remained temporally constant over 2 years. Significant positive linear relationships (p < 0.01) between recruit density and adult mussel biomass or density indicate a strong correlation between availability of settlement habitat and recruitment. It is likely that the high recruit density recorded at low harvesting intensities (2000–20 000 per 0.01 m2) exceeds the level required for population maintenance. However, if settlement habitat is eliminated or significantly reduced, as was achieved by F = 0.3 or above, recruitment may become limiting. Thus, to protect stock replenishment, harvesting of M. galloprovincialis in this region should take place at intensities less than F = 0.3.
Perspective: The challenge of ecologically sustainable water management
25 April 2007
Stellenbosch University

Bernhardt, E.;Bunn, S.E.;Hart, D.D.;Malmqvist, B.;Timo-Muotka, T.;Naiman, R.J.;Pringle, C.;Martin Reuss, M.;van Wilgen, B.W.

Sustainable water resource management is constrained by three pervasive myths; that societal and environmental water demands always compete with one another; that technological solutions can solve all water resource management problems; and that environmental solutions to protect and maintain freshwater resources are more expensive and less dependable than technological solutions. We argue that conservation and good stewardship of water resources can go a long way toward meeting societal demands and values. Furthermore, water requirements to sustain ecosystem health and biodiversity in rivers and their associated coastal systems can be well aligned with options for human use and deliver a suite of ecosystem goods and services to society. However, to achieve ecologically sustainable water management, we propose several key issues that must be addressed. The objective of this opinion paper is to stimulate discussion across traditional discipline boundaries with the aim of forging new partnerships and collaborations to meet this pressing challenge of ecologically sustainable water management.
Ecotones: Marginal or central areas of transition?
25 April 2007
Stellenbosch University

Kark, S;van Rensburg, BJ

Areas of environmental transition, where ecological communities coincide, are sometimes termed ecotones. These regions often correspond with sharp environmmental gradients. Ecotones occur at multiple spatial scales, ranging from transittions between biomes to local small-scale transitions. In recent years ecotones have received increasing scientific attention after being neglected for years, as studies historically often focused on distinct communities. However, it is still debbatable whether these transitional regions are speciation and biodiversity hotspots that deserve special conservation interest or are actually areas that hold marginal populations that depend on other parts of the range for the maintenance of their biodiversity and therefore should not deserve primary investment. This paper discusses some of the recent advancements in our understanding of the role of ecotones in ecology, evolution, and conservation.
Tourists' willingness to pay to view otters along the Wild Coast, South Africa: a potential for increased ecotourism
25 April 2007
Stellenbosch University

Dumasile, L.;Somers, M.J.;Walters, M.;Nel, J.A.J.

This article reports on tourists’ willingness to pay to view Cape clawless otters Aonyx capensis along the Eastern Cape Wild Coast, South Africa. We collected data from a survey carried out using a structured, selfadministered questionnaire. We analyzed 67 completed questionnaires, of which 60 respondents (89.5%) indicated that they would be interested in viewing otters and would be prepared to pay an extra fee, over and above the normal entrance fees, for a trained guide to show them otters. The remaining 7 (10.4%) respondents indicated that they would reconsider and pay if this would create jobs for members of the local community. Most (98.4% and 91.8%) of the respondents also indicated that they would still pay even if there was only a 50% or 25% chance of seeing otters. Most of the respondents were willing to pay either less than R50.00 (ca US$8.00) or R50.00- R100.00 to view otters regardless of the chances of seeing them. We conclude from our results that otters do have the potential to increase ecotourism in the area, and also to contribute financially to the poverty stricken local Dwesa community.
Riparian scrub recovery after clearing of invasive alien trees in headwater streams of the Western Cape, South Africa
25 April 2007
Stellenbosch University

Galatowitsch, S;Richardson, DM

Headwater rivers are the predominant kind of aquatic ecosystem in South Africa's Cape Floristic Region. Invasion by alien woody taxa (especially Acacia longifolia and Acacia mearnsii) have altered these rivers in recent decades, replacing indigenous vegetation and altering ecosystem functioning. Aliens have been systematically cleared in watercourses since 1995 as part of a national program ("Working for Water") to increase water production and improve water quality. Cleared sites are mostly left to recover to their pre-invasion state without additional intervention. We compared the vegetation of seven rivers that vary in their extent of invasion and clearing to identify factors limiting recolonization. Dense invasions cleared 3-6 years before the survey differ in vegetation structure and composition from uninvaded or lightly invaded/cleared riparian reaches in the following ways: (1) diminished extent of indigenous trees in the canopy and increased importance of shrubs or reinvading alien trees, (2) reduced species richness, (3) reduced likelihood of occupancy for more than one-third of common, indigenous species, and (4) reduced incidence of indigenous tree regeneration. Overall, indigenous tree regeneration is very low and not disturbance-triggered which will likely result in slow recovery without additional intervention. We recommend focusing active revegetation on common riparian scrub trees (i.e., Metrosideros angustifolia, Brachylaena neriifolia, Brabejum stellatfolium, and Erica caffra). These species tolerate open habitats favored by alien trees, eventually forming closed canopies required by shade-tolerant species. Accelerating establishment of these small trees is likely critical for shifting cleared riparian corridors from a state that favors alien reinvasion. Effective establishment strategies will need to be developed in the context of hydrologic impairment, since alien-invaded rivers in this region typically have reduced flow. (C) 2004 Elsevier Ltd. All rights reserved.
Residence time and potential range : crucial considerations in modelling plant invasions
11 May 2007
Stellenbosch University

Wilson, John R. U.;Richardson, David M.;Rouget, Mathieu;Proches, Serban;Amis, Mao A.;Henderson, Lesley;Thuiller, Wilfried

A prime aim of invasion biology is to predict which species will become invasive, but retrospective analyses have so far failed to develop robust generalizations. This is because many biological, environmental, and anthropogenic factors interact to determine the distribution of invasive species. However, in this paper we also argue that many analyses of invasiveness have been flawed by not considering several fundamental issues: (1) the range size of an invasive species depends on how much time it has had to spread (its residence time); (2) the range size and spread rate are mediated by the total extent of suitable (i.e. potentially invasible) habitat; and (3) the range size and spread rate depend on the frequency and intensity of introductions (propagule pressure), the position of founder populations in relation to the potential range, and the spatial distribution of the potential range. We explored these considerations using a large set of invasive alien plant species in South Africa for which accurate distribution data and other relevant information were available. Species introduced earlier and those with larger potential ranges had larger current range sizes, but we found no significant effect of the spatial distribution of potential ranges on current range sizes, and data on propagule pressure were largely unavailable. However, crucially, we showed that: (1) including residence time and potential range always significantly increases the explanatory power of the models; and (2) residence time and potential range can affect which factors emerge as significant determinants of invasiveness. Therefore, analyses not including potential range and residence time can come to misleading conclusions. When these factors were taken into account, we found that nitrogen-fixing plants and plants invading arid regions have spread faster than other species, but these results were phylogenetically constrained. We also show that, when analysed in the context of residence time and potential range, variation in range size among invasive species is implicitly due to variation in spread rates, and, that by explicitly assuming a particular model of spread, it is possible to estimate changes in the rates of plant invasions through time. We believe that invasion biology can develop generalizations that are useful for management, but only in the context of a suitable null model.
The effects of temperature, body mass and feeding on metabolic rate in the tsetse fly, Glossina morsitans centralis
14 May 2007
Stellenbosch University

Terblanche, JS;Chown, SL

Metabolic rate variation with temperature, body mass, gender and feeding status is documented for Glossina morsitans centralis . Metabolic rate [mean ± SE; VCO2 = 19.78 ± 3.11 uL CO 2 h −1 in males (mean mass = 22.72 ± 1.41 mg) and 27.34 ± 3.86 uL CO 2 h −1 in females (mean mass = 29.28 ± 1.96 mg) at 24 °C in fasted individuals] is strongly influenced by temperature, body mass and feeding status, but not by gender once the effects of body mass have been accounted for. A significant interaction between gender and feeding status is seen, similar to patterns of metabolic rate variation documented in Glossina morsitans morsitans. Synthesis of metabolic rate-temperature relationships in G. m. centralis , G. m. morsitans and Glossina pallidipes indicate that biting frequency as well as mortality risks associated with foraging will probably increase with temperature as a consequence of increasing metabolic demands, although there is little evidence for variation among species at present. Furthermore, metabolic rate – body mass relationships appear to be similarly invariant among these species. These data provide important physiological information for bottom-up modelling of tsetse fly population dynamics.
Effects of habitat fragmentation on bird communities of sand forests in southern Mozambique
16 May 2007
Stellenbosch University

Wilson, J.W.;van Aarde, R.J.;van Rensburg, B.J.

We investigated the influence of forest fragment size and isolation on the bird assemblages in the species- and endemic-rich sand forests of the Maputaland Centre of Endemism, southern Mozambique. Point centre surveys were conducted across 12 sand forest patches that varied in size and isolation. Patch size and isolation had little influence on bird species richness, but the number of individuals decreased significantly with increasing isolation. Furthermore, bird abundances were correlated to a combination of the size and isolation of patches. Many forest specialists, in particular large-bodied frugivores, were highly sensitive to reduced patch size and increased distances between patches. Further fragmentation of the landscape may therefore impair the ability of these forests to support viable populations of forest specialists.
The value of community-based conservation in a heterogeneous landscape: an avian case study from sand forest in Maputaland, South Africa
16 May 2007
Stellenbosch University

van Eeden, DG;van Rensburg, BJ;Bothma, J. du P.;De Wijn, M

A local community adjacent to Tembe Elephant Park, which is the largest protected portion of sand forest in South Africa, recently nominated a portion of their land for a community-based natural resource management project to promote conservation in the region. The present study compared sand forest bird assemblages found in the communal land area with that of the Tembe Elephant Park. The communal land comprised unique avian sand forest assemblages, characterised by more indicator species, and higher species richness and site fidelity values than the Park. This demonstrates the biological importance of the communal land for sand forest conservation, especially from an avian perspective.
Invasive alien plants and South African rivers: a proposed approach to the prioritization of control operations
25 May 2007
Stellenbosch University

van Wilgen, BW;Nel, JL;Rouget, M

1. A number of parallel initiatives in South Africa have been addressing the prioritization and management of invasive alien plant species, the prioritization of rivers for the conservation of biodiversity, and broad-scale planning for water resource management. This paper has combined aspects of these approaches to develop a composite index of prioritization of quaternary catchments for alien plant control purposes. 2. We calculated, for each quaternary catchment, a simple composite index that combined estimates of (i) the number of invasive alien plant species present; (ii) the potential number of invasive alien plant species that would be present if they occupied the full range as determined by climatic envelope models; (iii) the degree of habitat loss in rivers; and (iv) the degree of water stress. Each of the four components contributed between one and four to the combined index, which had a range of values between four and 16. 3. We used a geographic information system to map the distribution of priority catchments for invasive alien plant control. Of the 1911 quaternary catchments in South Africa and Lesotho, just over one-third (650) were in the highest priority category with an index of 13 or more. A relatively small proportion (273, or 14%) of the catchments had the maximum scores of 15 or 16. 4. The approach identified priority areas that have not currently been identified as such, and should provide decision makers with an objective and transparent method with which to prioritize areas for the control of invasive alien plants. We anticipate debate about the way in which components of the index are calculated, and the weight given to the different components, and that this will lead to the transparent evolution of the index. Improvements would also come about through the addition of a more comprehensive list of species, and through the addition of further components.
Distribution and habitat choice of Cape clawless otters Aonyx capensis in South Africa
25 May 2007
Stellenbosch University

Nel, J;Somers, MJ

Cape clawless otters, Aonyx capensis,are widely distributed in South Africa, as elsewhere on the continent. They occur in a wide variety of environments and most aquatic habitats, from freshwater lakes to the marine littoral,and even in episodic rivers in arid areas, provided freshwater sources are adequate and sufficient food is available. This animal is not much affected by turbid water as it locates prey by touch, and usually forages close to shores or banks. Evidence of presence in given localities and habitats, distributed over a large area of the Northern,Western, and Eastern Cape provinces, was deduced from signs (faecal deposits or distinctive tracks) on land. Accepting the inherent pitfalls of this approach we nevertheless feel using it is acceptable for a first approximation of habitat preferences over a large geographical area. Results point to areas with dense reed beds and a rocky substrate on banks being used most intensively, probably on account of a localized high food biomass.
Diet of the South African large-spotted genet Genetta tigrina (Carnivora, Viverridae) in a coastal dune forest
25 May 2007
Stellenbosch University

Roberts, PD;Somers, MJ;White, RM;Nel, J

We studied seasonal variations in the diet of the large-spotted genet Genetta tigrina Schreber, 1776 in the coastal dune forest of the Dwesa Nature Reserve, Eastern Cape Province, South Africa. The food items with the highest relative percentage occurrence were Coleoptera, Orthoptera and Mammalia. However, by volume they ate mostly grass then followed by Coleoptera and Orthoptera. Main prey items originated from the litter layer or low lying bushes, such as arachnids, insects, myriapods, and most mammals. The latter included ten rodent (main staple: Dendromus sp.), two golden mole and two shrew species, from 10–100 g mass. They were represented dependent on species density, but switching between seasons and habitats occupied. Birds appeared under-represented in the diet for a semi-arboreal carnivore, although this correlates with data from other studies. Remains of birds in the diet, however, peaked during winter and spring probably as a result of the main nesting period in spring. There were some variation in diet between habitats (riparian, forest and beach) and seasons. Our results show the South African large-spotted genet to have an opportunistic, generalist diet.
Rates and causes of mortality in Endangered African wild dogs Lycaon
25 May 2007
Stellenbosch University

Woodroffe, R;Davies-Mostert, H;Ginsberg, J;Graf, J;Leigh, K;McCreery, K;Mills, G;Pole, A;Rasmussen, G;Robbins, R;Somers, M;Szykman, M

Effective species conservation depends upon correctly identifying the threats that cause decline or hinder recovery. Because estimates of the relative viability of different populations of Endangered African wild dogs Lycaon pictus are most strongly influenced by adult and pup mortality, we analysed rates and causes of mortality in eight wild dog populations under study in southern and eastern Africa. The probabilities of detecting wild dog deaths were influenced by the monitoring methods used. The least biased estimates of mortality causes were obtained through intensive monitoring of adio-collared individuals; this is impossible for pups, however. Mortality patterns varied substantially between populations. Rates of human-caused mortality were higher for wild dogs radio-collared outside protected areas than for those collared inside, but rates of natural mortality were comparable, suggesting that anthropogenic mortality is additive to natural mortality. The relative importance of factors such as snaring and infectious disease also varied regionally. Hence, although our analyses identified no new threats beyond those highlighted in a 1997 rangewide Action Plan, they suggest that local plans will be valuable to target conservation activities more precisely.
Invasive alien plants infiltrate bird-mediated shrub nucleation processes in arid savanna
19 June 2007
Stellenbosch University

Milton, S.J.;Wilson, J.R.U.;Richardson, D.M.;Seymour, C.L.;Dean, W.R.J.;Iponga, D.M.;Procheş, Ş.

1 The cultivation and dissemination of alien ornamental plants increases their potential to invade. More specifically, species with bird-dispersed seeds can potentially infiltrate natural nucleation processes in savannas. 2 To test (i) whether invasion depends on facilitation by host trees, (ii) whether propagule pressure determines invasion probability, and (iii) whether alien host plants are better facilitators of alien fleshy-fruited species than indigenous species, we mapped the distribution of alien fleshy-fruited species planted inside a military base, and compared this with the distribution of alien and native fleshy-fruited species established in the surrounding natural vegetation. 3 Abundance and diversity of fleshy-fruited plant species was much greater beneath tree canopies than in open grassland and, although some native fleshy-fruited plants were found both beneath host trees and in the open, alien fleshy-fruited plants were found only beneath trees. 4 Abundance of fleshy-fruited alien species in the natural savanna was positively correlated with the number of individuals of those species planted in the grounds of the military base, while the species richness of alien fleshy-fruited taxa decreased with distance from the military base, supporting the notion that propagule pressure is a fundamental driver of invasions. 5 There were more fleshy-fruited species beneath native Acacia tortilis than beneath alien Prosopis sp. trees of the equivalent size. Although there were significant differences in native plant assemblages beneath these hosts, the proportion of alien to native fleshyfruited species did not differ with host. 6 Synthesis. Birds facilitate invasion of a semi-arid African savanna by alien fleshyfruited plants, and this process does not require disturbance. Instead, propagule pressure and a few simple biological observations define the probability that a plant will invade, with alien species planted in gardens being a major source of propagules. Some invading species have the potential to transform this savanna by overtopping native trees, leading to ecosystem-level impacts. Likewise, the invasion of the open savanna by alien host trees (such as Prosopis sp.) may change the diversity, abundance and species composition of the fleshy-fruited understorey. These results illustrate the complex interplay between propagule pressure, facilitation, and a range of other factors in biological invasions.
A case study of landholder attitudes and behaviour towards the conservation of Renosterveld, a critically endangered vegetation type in the Cape Floral Kingdom, South Africa
20 June 2007
Stellenbosch University

Winter, SJ;Prozesky, H;Esler, KJ

Attitudes and behaviour of private landholders towards the conservation of a highly transformed and critically endangered habitat, Overberg Coastal Renosterveld (OCR) (a grassy-shrubland of the Cape Floral Region, SA) are described. Personal, semi-structured interviews were conducted with landholders, representing 40 properties in the Overberg region, on topics such as their management and utilisation of OCR, the depth of their knowledge of its conservation importance, what they perceive its value to be, and extent of their willingness to conserve it. General attitudes towards conservation incentives and provincial conservation authorities were also investigated. Farmers more willing to conserve were younger, did not necessarily have a better education, and owned larger farms (> 500 ha) with a greater amount of remnant renosterveld (>300 ha), than those less willing to conserve. Attitudes towards OCR were largely negative, due to associated problem plants and animals, and because it is not economically advantageous to retain it. However farmers are of the opinion that provision of incentives and increased extension support will provide practical positive inducements for conservation. Landholder education is paramount to prevent further transformation of critically endangered habitats. The success of private-conservation programs depends on: • the attitudes of landowners towards (i) the particular habitat or species to be conserved (which can vary depending on the type of land use practised, and the associated benefits and disadvantages of that habitat type); and (ii) the conservation agency or extension officers responsible for that area; • the willingness of landowners to participate in a conservation program, which is influenced by landowner age, farm size and the amount of natural habitat left to conserve.
Alien plant invasions—incorporating emerging invaders in regional prioritization: A pragmatic approach for Southern Africa
03 July 2007
Stellenbosch University

Mgidi, TN;Le Maitre, DC;Schonegevel, L;Nel, JL;Rouget, M;Richardson, DM

Plant invasions are a serious threat to natural and semi-natural ecosystems worldwide. Most management-orientated research on invasions focuses on invaders that are already widespread and often have major impacts. This paper deals with “emerging” invaders—those alien species with the potential to become important problems without timely intervention. A climate matching procedure was developed to define areas of South Africa, Lesotho and Swaziland that could be invaded by 28 plant species that had previously been classified as emerging invaders. Information on the location of populations of these species in the study area was combined with information on their distributions (as native or alien) in parts of Australia and the United States of America. These two countries had the best available distribution data for this study. They also share many invasive alien plant species with South Africa. Climatic data obtained for weather stations near points of known occurrence in these countries were used to define the climatically suitable areas for each species in the study area. Almost 80% of the remaining natural environment in southern Africa was found to be vulnerable to invasion by at least one of these species, 50% by six or more and 24% by 16 or more species. The most vulnerable areas are the highveld grasslands and the eastern escarpment. The emerging invaders with the greatest potential range included Acacia podalyriifolia and Cortaderia selloana. The globally important invaders Ulex europaeus and Lythrum salicaria had a more limited invasion potential but could still become major invaders. There was no relationship between the extent of the climatically suitable areas for the different species and an expert ranking of their invasion potential, emphasising the uncertainties inherent in making expert assessments based on very little information. The methods used in this analysis establish a protocol for future modelling exercises to assess the invasion potential of other emerging invaders.
Home away from home - objective mapping of high-risk source areas for plant introductions
01 August 2007
Stellenbosch University

Richardson, DM;Thuiller, W

Prevention is the best way to slow the escalation of problems associated with biological invasions. Screening of potential introductions is widely applied for assessing the risk of species becoming invasive. Despite advances in the understanding of the determinants of invasiveness, screening still relies heavily on assessments of the potential of species to 'fit in' to the broad environmental conditions of a target region. Most screening systems ask whether species are native to, or are known to be naturalized or invasive in, regions with 'similar' climatic/environmental conditions to the target region. The level of similarity required to make the species a high-risk introduction is generally not specified. This paper describes a protocol for making such assessments more objective, using South Africa as a test case. Using nonparametric niche-based modelling (generalized additive model; GAM) calibrated on the current distribution of each South African biome, we mapped regions of the world that are climatically similar to South African biomes. Lists were produced of countries with the largest areas climatically similar to South Africa overall, and to each biome separately. Validation of the usefulness of the approach was sought by evaluating whether the main invasive plant species in South African biomes occur naturally, or have adventive ranges, in regions mapped as analogous to South African biomes. A very large part of the world is climatically similar to South Africa, with eight countries having larger areas of land classified as climatically similar to South African biomes than the total area of South Africa. Almost all the most prominent invasive species in South African biomes occur naturally or are invasive outside their natural range in areas with similar climates to those that occur in parts of South Africa. This confirms the value of objective climate matching in screening protocols. We examined climatic conditions for a representative sample of major invasive plants from other parts of the world. The analysis identified several species that are already invasive in regions that have matched climates in South Africa but that are not yet introduced or, if already present, have not yet invaded large areas. For example, the following known invasive species should be considered high-risk species in South African grasslands: Alliaria petiolata, Cytisus scoparius, Gleditsia triacanthos, Heracleum mantegazzianum, Hieracium pilosella, Juniperus communis, Pinus contorta, P. monticola, P. ponderosa, P. sylvestris, Prunus laurcerasus, and P. serotina. Objectively matched climatic regions are also useful as a first-cut assessment when evaluating species with no invasive history.
Shell availability and use by the hermit crab Clibanarius virescens along the Eastern Cape coast, South Africa
21 August 2007
Stellenbosch University

Nakin, MD;Somers, MJ

Patterns of shell availability and use of the intertidal yellow-banded hermit crab Clibanarius virescens were studied by means of field sampling. Data were collected at three sites (Dwesa, Nqabara and Mendwana) along the Eastern Cape coast, South Africa. The most striking result was that at all three sites there was a difference between observed frequencies of shells used compared to that which was expected from their availability as live animals or empty shells on the rocks and the strandline. This suggests selection by the crabs. Crabs commonly used shells of Burnupena cincta and Burnupena pubescens and amongst other types, rarely used the shells of Cymatium dolarium and Stramonita capensis. Fewer types of shells were used than were available. A wider diversity of shell species were occupied at Dwesa (N = 11) and Nqabara (N = 10) than at Mendwana (N = 6). The diversity of shells used by the hermit crabs in this study falls within the globally predicted levels given body size of the study species and the latitude of the studied population.
Population structure, propagule pressure, and conservation biogeography in the sub-Antarctic: lessons from indigenous and invasive springtails
21 August 2007
Stellenbosch University

Myburgh, M;Chown, SL;Daniels, SR;Jansen van Vuuren, B

The patterns in and the processes underlying the distribution of invertebrates among Southern Ocean islands and across vegetation types on these islands are reasonably well understood. However, few studies have examined the extent to which populations are genetically structured. Given that many sub-Antarctic islands experienced major glaciation and volcanic activity, it might be predicted that substantial population substructure and little genetic isolation-by-distance should characterize indigenous species. By contrast, substantially less population structure might be expected for introduced species. Here, we examine these predictions and their consequences for the conservation of diversity in the region. We do so by examining haplotype diversity based on mitochondrial cytochrome c oxidase subunit I sequence data, from two indigenous (Cryptopygus antarcticus travei, Tullbergia bisetosa) and two introduced (Isotomurus cf. palustris, Ceratophysella denticulata) springtail species from Marion Island. We find considerable genetic substructure in the indigenous species that is compatible with the geological and glacialogical history of the island. Moreover, by employing ecological techniques, we show that haplotype diversity is likely much higher than our sequenced samples suggest. No structure is found in the introduced species, with each being represented by a single haplotype only. This indicates that propagule pressure is not significant for these small animals unlike the situation for other, larger invasive species: a few individuals introduced once are likely to have initiated the invasion. These outcomes demonstrate that sampling must be more comprehensive if the population history of indigenous arthropods on these islands is to be comprehended, and that the risks of within- and among-island introductions are substantial. The latter means that, if biogeographical signal is to be retained in the region, great care must be taken to avoid inadvertent movement of indigenous species among and within islands. Thus, quarantine procedures should also focus on among-island movements.
Phylogeography of Eupodes minutus (Acari : Prostigmata) on sub-Antarctic Marion Island reflects the impact of historical events
21 August 2007
Stellenbosch University

Mortimer, E;Jansen van Vuuren, B

Marion Island, situated similar to 2,300 km south-east of Cape Town, South Africa, has experienced multiple volcanic and glaciation events during its history. To better understand the impact of these events on species' genetic structure, we determined the phylogeographic population structure of the mite, Eupodes minutus. We included 57 individuals sampled from 11 localities across the island. Our analyses based on the mitochondrial COI gene suggest a population expansion as would typically be expected when species recover after being confined to refugia. Standard phi (Phi) statistics and a spatial analysis of molecular variance (SAMOVA) identified unique populations on the south-western and south-eastern sides of the island. We argue that multiple volcanic events on the southern side of Marion, in combination with glaciations, effectively isolated these populations from each other.
Conflicting human interests over the re-introduction of endangered wild dogs in South Africa.
22 February 2008
Stellenbosch University

Gusset, Markus;Maddock, Anthony;Gunther, Glen J.;Szykman, Micaela;Slotow, Rob;Walters, Michele;Somers, Michael J.

In South Africa, a plan was launched to manage separate sub-populations of endangered African wild dogs (Lycaon pictus) in several small, geographically isolated, conservation areas as a single meta-population. This intensive management approach involves the re-introduction of wild dogs into suitable conservation areas and periodic translocations among them. To assess the attitudes towards re-introduced wild dogs, we conducted a questionnaire survey of multiple stakeholders—local community members, private landowners and tourists—in and around Hluhluwe-iMfolozi Park (HiP), one of the meta-population conservation areas. Here, we document conflicting human interests over the re-introduced wild dogs. Tourists in HiP, on the one hand, expressed overwhelmingly positive opinions about wild dogs across personal details of the respondents, but especially after having seen free-ranging wild dogs. On the other hand, we found misconceptions and perceptions that were more negative among the rural population around HiP, again largely independent of personal details of the participants, although educated respondents voiced more favourable views of wild dogs. These negative attitudes were in particular due to perceived and real threats of livestock losses. In a follow-up questionnaire survey, we also discovered apparent shortcomings of a previous short-lived conservation education programme among the local communities adjacent to HiP. Consequently, the mitigation of the conflict between wild dogs and rural people requires an understanding of the conditions under which livestock predation occurs, the encouragement of practices that prevent such predation, and increasing local tolerance of co-existence with wild dogs through both economic and non-monetary incentive schemes as well as continued conservation education.
The (bio)diversity of science reflects the interests of society
25 February 2008
Stellenbosch University

Wilson, J.R.U.;Procheş, Ş.;Braschler, B.;Dixon, E.S.;Richardson, D.M.

For ecologists to develop robust generalizations and principles, a broad taxonomic and geographic spread of research is required, but, in practice, most generalizations are based on the research of individual scientists and groups, and their choice of study organism is affected by many different factors. We analyzed researchers’ choice to assess potential biases. In particular, by comparing the relative representation of species in the scientific literature and on the Internet, we explored how the choice of study organism is influenced by societal interests. While there is a strong positive correlation between output in the scientific literature and on the web, deviations from this general pattern suggest that, when compared with societal biases, research agendas are more directly influenced by economic priorities and practical limitations, and less by geographical and sociopolitical barriers. Although the range of biological research reflects the needs of society, there are still large taxonomic and geographic gaps. By focusing on specific groups, we are developing an in-depth knowledge of certain taxa, but if ecologists are to develop generalizations, we may need to widen our research scope.
Efforts going to the dogs? Evaluating attempts to re-introduce endangered wild dogs in South Africa
25 February 2008
Stellenbosch University

Gusset, Markus;Ryan, Sadie J.;Hofmeyr, Markus;Van Dyk, Gus;Davies-Mostert, Harriet T.;Graf, J.A.;Owen, Cailey;Szykman, Micaela;Macdonald, Dave W.;Monfort, Steven L.;Wildt, David E.;Maddock, Anthony H.;Mills, M Gus L.;Slotow, Rob;Somers, Michael J.

1. We evaluated one of the most extensive efforts to date to re-introduce an endangered species: attempts to establish an actively managed meta-population of African wild dogs Lycaon pictus in South Africa. 2. Using an information-theoretic approach, known-fate modelling in program mark was employed to estimate the survival of re-introduced wild dogs and their offspring, and to model covariate effects relative to survival. Multiple a priori hypotheses on correlates of re-introduction success were tested (collated from extensive individual experiences) using different re-introduction attempts as natural quasi experiments. 3. Survival analyses revealed that the determinants of re-introduction success can be reduced to two factors relevant for management, suggesting that wild dog re-introductions should be attempted with socially integrated animals that are released into securely fenced areas, unless measures are implemented to mitigate human-related mortalities outside protected areas. 4. Synthesis and application. This study illustrates that monitoring and evaluation of conservation efforts, complimented with expert knowledge, forms the foundation of informed decision-making to underpin management recommendations with scientific evidence, particularly if the proposed actions are controversial.
The relative importance of environment, human activity and space in explaining species richness of South African bird orders
14 March 2008
Stellenbosch University

Wilson, J. W.;van Rensburg, B. J.;Ferguson, J. W. H.;Keith, M.

Aim To assess the relative importance of environmental (climate, habitat heterogeneity and topography), human (population density, economic prosperity and land transformation) and spatial (autocorrelation) influences, and the interactions between the above-mentioned predictor groups, on species richness patterns of various avifaunal orders. Location South Africa. Methods Generalised linear models were used to determine the amount of variation in species richness, for each order, attributable to each of the different predictor groups. To assess the relationships between species richness and the various predictor groups, a deviance statistic (a measure of goodness of fit for each model) and the percentage deviation explained for the best fitting model were calculated. Results Of the 12 avifaunal orders examined, spatially structured environmental deviance accounted for most of the variation in species richness in 11 orders (averaging 28%) and 50% or more in seven orders. However, orders comprising mostly water birds (Charadriiformes, Anseriformes, Ciconiformes) had a relatively large component of purely spatial deviance compared with spatially structured environmental deviance, and much of this spatial deviance was due to higher order spatial effects, such as patchiness, as opposed to linear gradients in species richness. Although human activity, in general, offered little explanatory power to species richness patterns, it was an important correlate of spatial variation in species of Charadriiformes and Anseriformes. The species richness of these water birds was positively related to the presence of artificial water bodies. Main conclusions Not all bird orders showed similar trends when assessing, simultaneously, the relative importance of environmental, human and spatial influences in affecting bird species richness patterns. Although spatially structured environmental deviance described most of the variation in bird species richness, the explanatory power of purely spatial deviance, mostly due to nonlinear geographical effects such as patchiness, became more apparent in orders representing water birds. This was especially true for Charadriiformes, where the strong anthropogenic relationship has negative implications for the successful conservation of this group.
Invasion of grasslands by silver wattle, Acacia dealbata (Mimosaceae), alters beetle (Coleoptera) assemblage structure
14 March 2008
Stellenbosch University

Coetzee, B.W.T.;van Rensburg, B.J.;Robertson, M.

Alien plants are rapidly invading natural areas in South Africa but their impacts on biodiversity, particularly on arthropods, are poorly understood. We examined the impact of silver wattle, Acacia dealbata, invasion on Coleoptera assemblages in the grasslands of the Drakensberg region, South Africa. Baited pitfall trap samples from uninvaded grassland and grassland sites invaded by A. dealbata were sorted into morphospecies. The composition of Coleoptera assemblages, attracted by cow dung, differed significantly between invaded and uninvaded grassland habitats. Coleoptera richness and especially abundance values were found to be significantly lower in the invaded stands compared to grassland. Mean body size showed significant differences when compared between the two habitat treatments. The invaded sites exhibited a marked decline in the larger and less abundant species. Also, parataxonomic units unique to the grassland were, in general, characterized by larger body sizes compared to those unique to the invaded sites. Clearly, A. dealbata invasion of grasslands has substantial negative impacts on Coleoptera diversity.
Translocating lions into an inbred lion population in the Hluhluwe-iMfolozi Park, South Africa
31 March 2008
Stellenbosch University

Trinkel, M;Fergyson, N;Reid, A;Reid, C;Somers, MJ;Turelli, L;Graf, J;Syykman, M;Cooper, D;Haverman, P;Kastberger, G;Packer, C;Slotow, R

A fundamental problem in conservation biology is the risk of inbreeding in fragmented and declining populations. In the Hluhluwe-iMfolozi Park (HiP), a small, enclosed reserve in South Africa, a large lion Panthera leo population arose from a founder group of five individuals in the 1960s. The HiP lion population went through a persistent decline and showed indications of inbreeding depression. To restore the genetic variation of the inbred HiP lion population, new lions were translocated into the existing population. Translocated females formed stable associations and established enduring pride areas with other translocated lionesses, but did not bond into native female prides. The translocated male coalition was more successful in gaining and maintaining residence in a pride than the translocated lone male that split off on his own from the male coalition. Litter size and cub survival was about twice as high for pairings involving at least one translocated parent than for pairings of two native lions. It is therefore possible to infuse new genes rapidly and successfully into a small, isolated lion population. Such translocations may become an important adaptive management tool as lion populations become increasingly fragmented.
Growth form and population genetic structure of Azorella selago on sub-Antarctic Marion Island
08 April 2008
Stellenbosch University

Mortimer, Elizabeth;McGeoch, Melodie A.;Daniels, Savel R.;Jansen van Vuuren, Bettine

Seven community complexes have been described across sub-Antarctic Marion Island, amongst these fellfield that comprise low plant cover dominated by Azorella selago Hook. f. Azorella is considered a keystone species since it forms nutrient rich environments for microarthropod communities and epiphytic plants. Two distinct growth forms typify Azorella, namely discrete cushions and continuous mats. Whether these continuous mats normally consist of a single large cushion individual, or whether several individual plants merge, interdigitating to form a continuous area, remains unclear. As such, it is important to obtain some measure of Azorella growth dynamics before embarking on phylogeographic studies. Previous genetic studies indicated that several of these microarthropod species are significantly substructured across Marion Island, but it remains unclear whether similar subdivisions characterize Azorella. We used chloroplast sequence data (trnH-psbA) and amplified fragment length polymorphism (AFLP) to investigate these questions. No sequence variation characterized the trnH-psbA region in Azorella across Marion Island. In contrast, the AFLP results indicated that an A. selago mat comprises multiple individuals. We argue that mats can be formed through at least two processes namely fragmentation, where parts of the cushion plant die off creating open areas for the establishment of different individuals and/or to a high density of interdigitating individuals merging to form the mat. Fragment data further indicated significant substructure for Azorella across Marion Island (FST=0.101, P=0.01) and we attribute this to past vicariance.
Coalescence methods reveal the impact of vicariance on the spatial genetic structure of Elephantulus edwardii (Afrotheria, Macroscelidea)
08 April 2008
Stellenbosch University

Smit, Hanneline A.;Robinson, Terence J.;Jansen van Vuuren, Bettine

Within the Macroscelidea 15 species of elephant-shrews are recognized, of which nine occur in the southern African subregion. The Cape rock elephant-shrew ( Elephantulus edwardii ) is the only strictly endemic South African elephant-shrew species. Recent distribution data suggest that E. edwardii is continuously distributed from Namaqualand in the Western Cape Province to Port Elizabeth in the Eastern Cape Province. Molecular sequences from the mitochondrial cytochrome b gene and variable control region indicate significant substructure within the Cape rock elephant-shrew across its distribution. Our data unequivocally showed the presence of a northern Namaqua and central Fynbos clade with four evolutionary lineages identified within the latter. The geographical delimitation of the northern and central clades corresponds closely with patterns reported for other rockdwelling vertebrate species, indicating a shared biogeographical history for these taxa in South Africa. A coalescent method revealed the effects of ancestral polymorphism in shaping the Namaqua and Fynbos populations since their divergence ~1.7 million years ago. Furthermore, our analyses uncovered a distinct Karoo lineage(s) that does not correspond to any of the previously described and/or currently recognized species, and we therefore argue for the possible recognition of a new sister taxon to E. edwardii. The taxonomic affinities of this clade were examined by sequencing corresponding regions from the type specimens of species described in the past, but which presently are synonimized within E. edwardii . Our results reveal the morphological misidentification of one of these types, accentuating the problems of field identification.
The distribution and spread of the invasive alien common myna Acridotheres tristis L. (Aves: Sturnidae), in southern Africa
15 April 2008
Stellenbosch University

Peacock, Derick;van Rensburg, Berndt;Robertson, Mark

The common myna is an Asian starling that has become established in many parts of the world outside of its native range due to accidental or deliberate introductions by humans. The South African population of this species originated from captive birds that escaped in Durban in 1902. A century later, the common myna has become abundant throughout much of South Africa and is considered to pose a serious threat to indigenous biodiversity. Preliminary observations suggest that the common myna’s distribution is closely tied to that of humans, but empirical evidence for this hypothesis is lacking. We have investigated the relationships between common myna distribution, human population size and land-transformation values at a quarter degree resolution in South Africa. Common mynas were found more frequently than expected by chance in areas with greater human population numbers and land transformation values. We also investigated the spatial relationship between the bird’s range and the locations of South Africa’s protected areas at the quarter-degree scale. These results indicate that, although there is some overlap, the common myna distribution is not closely tied to the spatial arrangement of protected areas. We discuss the original introduction, establishment and rate of spread of the common myna in South Africa and the neighbouring countries and contrast the current distribution with that presented in The atlas of Southern African Birds. We also discuss the factors that affect the common myna’s success and the consequences that invasion by this species is likely to have, specifically in protected areas.
Riparian vegetation : degradation, alien plant invasions, and restoration prospects
23 April 2008
Stellenbosch University

Richardson, D. M.;Holmes, P. M.;Esler, K. J.;Galatowitsch, S. M.;Stromberg, J. C.;Kirkman, S. P.;Pysek, P.;Hobbs, R. J.

Rivers are conduits for materials and energy; this, the frequent and intense disturbances that these systems experience, and their narrow, linear nature, create problems for conservation of biodiversity and ecosystem functioning in the face of increasing human influence. In most parts of the world, riparian zones are highly modified. Changes caused by alien plants — or environmental changes that facilitate shifts in dominance creating novel ecosystems — are often important agents of perturbation in these systems. Many restoration projects are underway. Objective frameworks based on an understanding of biogeographical processes at different spatial scales (reach, segment, catchment), the specific relationships between invasive plants and resilience and ecosystem functioning, and realistic endpoints are needed to guide sustainable restoration initiatives. This paper examines the biogeography and the determinants of composition and structure of riparian vegetation in temperate and subtropical regions and conceptualizes the components of resilience in these systems. We consider changes to structure and functioning caused by, or associated with, alien plant invasions, in particular those that lead to breached abiotic- or biotic thresholds. These pose challenges when formulating restoration programmes. Pervasive and escalating human-mediated changes to multiple factors and at a range of scales in riparian environments demand innovative and pragmatic approaches to restoration. The application of a new framework accommodating such complexity is demonstrated with reference to a hypothetical riparian ecosystem under three scenarios: (1) system unaffected by invasive plants; (2) system initially uninvaded, but with flood-generated incursion of alien plants and escalating invasion-driven alteration; and (3) system affected by both invasions and engineering interventions. The scheme has been used to derive a decision-making framework for restoring riparian zones in South Africa and could guide similar initiatives in other parts of the world.
Ingress of Lantana in dry tropical forest fragments: Edge and shade effects
23 April 2008
Stellenbosch University

Raizada, P.;Sharma, G.P.;Raghubanshi, A.S.

Invasion of native communities by exotic species is considered the second largest threat to global biodiversity. Since the tropics are highly populated, they experience greater pressure of invasive species, as the process of plant species invasion gets exacerbated with increased anthropogenic interventions. Seasonal tropical forests are especially susceptible to invasion. In India, dry tropical forest accounts for 28.6% of the total forest cover. These forests are under immense anthropogenic pressure due to rapid industrialization and related land-use changes in the past few decades, leading to forest fragmentation. Lantana (Lantana camara L.) had been introduced in India in the early 19th century as an ornamental plant; but now it is growing densely throughout India. Field observations in the dry tropical forests of India indicated that lantana is spreading fast. However, it is not uniformly distributed across a forest fragment. The objective of the present study was to assess the spread of lantana across forest fragments and to establish a relationship between lantana cover and tree canopy opening due to dry tropical forest fragmentation.
Spider responses to alien plant invasion: the effect of short- and long-term Chromolaena odorata invasion and management
11 August 2008
Stellenbosch University

Mgobozi, Mandisa P.;Somers, Michael J.;Dippenaar-Schoeman, Ansie S.

Invasions by alien plants into unspoiled ecosystems are a cause for concern because the pristine systems are important stores of biodiversity. Indeed, the introduction of non-indigenous species into protected areas is a direct threat to conservation. Consequently, it is fundamental to document the impact that alien invasive plants have on native communities and to determine if, and at whatrate, native communities re-establish following the removal of invasives. Chromolaena odorata is one of the most important invader species in the savanna biome in South Africa. It reduces vegetation heterogeneity in grasslands, savannas and forests. Spiders, as ecological indicators for change, were used to investigate the impact of (i) C. odorata invasions, (ii) C. odorata invasion durations, and (iii) the impact of clearing C. odorata on abundance, assemblage patterns, diversity and estimated species richness of spiders. The progressive invasion of C. odorata with increasing invasion duration brings with it changes in native spider abundance, assemblage patterns, diversity and estimated species richness. Native spider assemblages do re-establish after clearing C. odorata without further management intervention. Small lingering differences are observed between the native and the cleared sites, suggesting that other features of the habitat may have been affected by the invasion and clearing. Synthesis and applications: Alien clearance is an essential and invaluable management tool. There are a substantial number of programmes that aim to control alien invasive plants but very little is known about the way in which biodiversity recovers after alien plant removal. Our data show that the removal of alien invasive plants benefits biodiversity with immediate effects, highlighting that management should be carried out to control this invasive, even if the area has been invaded for a long period of time. These data are important for policy-forming and informing policy-makers that alien invasion and removal are critical for biodiversity conservation management.
Environmental factors, regional body size distributions, and spatial variation in body size of local avian assemblages
05 November 2008
Stellenbosch University

Greve, M.;Gaston, K.J.;van Rensburg, B.J.;Chown, S.L.

Aim To determine how well variation in median body size of avian assemblages is predicted by (1) the environmental models usually employed in analyses of Bergmann’s rule and (2) random sampling from the regional body size frequency distribution. If body size frequency distributions of local assemblages represent a random sample of a regional frequency distribution, then geographic variation in body sizes of assemblages might be a consequence of the determinants of spatial variation in species richness rather than direct influences on body size per se. Location Southern Africa. Methods Median body masses (as a measure of body size) of avian assemblages were calculated for quarter degree grid cells across South Africa and Lesotho. The relationship between median body mass and four environmental variables (minimum and maximum monthly temperatures, precipitation, and seasonality in the normalized difference vegetation index, as a measure of seasonality in productivity) was examined using general linear models first without taking spatial autocorrelation into account, and accounting for it by fitting an exponential spatial covariance structure. Model fit was assessed using the Akaike Information Criterion and Akaike weights. At each species richness value, random assemblages were sampled by either drawing species randomly from the regional body mass frequency distribution, or drawing species from the regional body mass frequency distribution with a probability proportional to their geographic distribution in the area. The ability of randomizations to predict actual body masses was examined using two-tailed Fisher exact tests. Results: Seasonality in productivity was the only environmental variable that remained a significant predictor of body mass variation in spatially explicit models, though the positive relationship was weak. When species richness was included in the models it remained the only significant predictor of size variation. Randomizations predicted median body mass poorly at low species richness, but well at high richness. Main conclusions: Environmental models that have previously been proposed explain little of the variation in body mass across avian assemblages in South Africa. However, much of the variation in median mass of assemblages could be predicted by randomly drawing species from the regional body mass frequency distribution, particularly using randomizations in which all species were drawn from the regional body mass frequency distribution with equal probability and at high species richness values. This outcome emphasizes the need to consider null expectations in investigations of the geographic variation in body size together with the likely environmental mechanisms underlying spatial variation in average size. Moreover, it suggests that in the South African avifauna, spatial variation in the body sizes of assemblages may be determined indirectly by the factors that influence geographic variation in species richness.
Human activity facilitates altitudinal expansion of exotic plants along a road in montane grassland, South Africa
05 November 2008
Stellenbosch University

Kalwij, Jesse;Robertson, Mark;van Rensburg, Berndt

Question: Do anthropogenic activities facilitate the distribution of exotic plants along steep altitudinal gradients? Location: Sani Pass road, Grassland biome, South Africa Methods: On both sides of this road, presence and abundance of exotic plants was recorded in four 25m-long road-verge plots and in parallel 25 x 2 m adjacent land plots, nested within 5 altitudinal levels: 1500, 1800, 2100, 2400, and 2700 m a.s.l. Exotic community structure was analyzed using Canonical Correspondence Analysis while a two-level nested Generalized Linear Model was fitted for richness and cover of exotics. We tested the upper altitudinal limits for all exotics along this road for spatial clustering around four potential propagule sources using a t-test. Results: Community structure, richness and abundance of exotics were negatively correlated with altitude. Greatest invasion by exotics was recorded for adjacent land in the 1500 m level. Of the 45 exotics, 16 were found at higher altitudes than expected and observations were spatially clustered around potential propagule sources. Conclusions: Spatial clustering of upper altitudinal limits around human inhabited areas suggests that exotics originate from these areas, while exceeding expected altitudinal limits suggests that distribution ranges of exotics are presently underestimated. Exotics are generally characterised by a high propagule pressure and/or persistent seedbanks, thus future tarring of the Sani Pass may result in an increase of exotic species richness and abundance. This would initially result from construction-related soil disturbance and subsequently from increased traffic, water run-off, and altered fire frequency. We suggest examples of management actions to prevent this.
The maintenance of a positive spatial correlation between South African bird species richness and human population density
05 November 2008
Stellenbosch University

Hugo, Sanet;van Rensburg, Berndt

Aim To investigate explanations for the maintenance of a positive spatial species richness – human population density correlation at broad scales, despite the negative impact of humans on species. These are (hypotheses 1 – 4): (1) human activities that create a habitat mosaic and (2) a more favourable climate, and (3) adequate conservation measures (e.g., enough natural habitat), maintain the positive species richness – human density correlation; or (4) the full range of human densities decrease the correlation’s slope without changing its form. Location South Africa Methods Avian species richness data from atlas distribution maps and human population density data derived from 2001 census results were converted to a quarter-degree resolution. We investigated the number of land transformation types (anthropogenic habitat heterogeneity), irrigated area (increasing productivity), and other covarying factors (e.g. primary productivity) as predictors of species richness. We compared species richness – human density relationships among regions with different amounts of natural habitat, and investigated whether the full range of human densities decrease species richness in relation to primary productivity. Results Hypotheses 1, 2 and 3 were supported. Human densities and activities that increase habitat heterogeneity and productivity are important beneficial factors to common species, but not to rare species. The species richness – human density relationship persists only at low land transformation levels, and no significant relationship exists at higher levels. For common species, the relationship becomes nonsignificant at lower land transformation levels than for rare species. Main conclusions The persistence of the species richness – human density relationship depends mostly on the amount of remaining natural habitat. In addition, certain human activities benefit especially common species. Common species seem to be more flexible than rare species in response to human activity and habitat loss.
The potential for large carnivores to act as biodiversity surrogates in southern Africa
10 November 2008
Stellenbosch University

Dalerum, Fredrik;Somers, Michael J.;Kunkel, Kyran E.;Cameron, Elissa Z.

Biodiversity in southern Africa is globally extraordinary but threatened by human activities. Although there are considerable biodiversity conservation initiatives within the region, no one has yet assessed the potential use of large carnivores in such actions. Surrogate approaches have often been suggested as one such way of capitalizing on large carnivores. Here we review the suitability of the large carnivore guild (i.e., brown hyaena Hyaena hyaena, spotted hyaena Crocuta crocutta, cheetah Acinonyx jubatus, leopard Panthera pardus, lion Panthea leo and African wild dog Lycaon pictus) to act as surrogate species for biodiversity conservation in southern Africa. We suggest that the guild must be complete for the large carnivores to fully provide their role as ecological keystones. The potential for large carnivores to act as umbrella and indicator species seems limited. However, self-sustaining populations of large carnivores may be useful indicators of unfragmented landscapes. Moreover, diversity within the large carnivore guild may reXect overall biodiversity. Although the global appeal of the large African carnivores makes them important international Xagships, we stress that international conservation funding must be linked to local communities for them to be important also locally. In summary, we suggest that the Xagship value of these large carnivores should be used to promote biodiversity conservation in the region, and that the suggested relationship between large carnivore diversity and overall biodiversity is empirically tested. Finally we suggest that direct conservation activities should focus on enhancing the keystone values of large carnivores through complete guild conservation and restoration.
Dynamics of a small re-introduced population of wild dogs over 25 years: Allee effects and the implications of sociality for endangered species’ recovery
18 November 2008
Stellenbosch University

Somers, Michael J.;Graf, Jan A.;Szykman, Micaela;Slotow, Rob;Gusset, Markus

We analysed 25 years (1980–2004) of demographic data on a small re-introduced population of endangered African wild dogs (Lycaon pictus) in Hluhluwe-iMfolozi Park (HiP), South Africa, to describe population and pack dynamics. As small populations of cooperative breeders may be particularly prone to Allee effects, this extensive data set was used to test the prediction that, if Allee effects occur, aspects of reproductive success, individual survival and population growth should increase with pack and population size. The results suggest that behavioural aspects of wild dogs rather than ecological factors (i.e. competitors, prey and rainfall) primarily have been limiting the HiP wild dog population, particularly a low probability of finding suitable mates upon dispersal at low pack number (i.e. a mate-finding Allee effect). Wild dogs in HiP were not subject to component Allee effects at the pack level, most likely due to low interspecific competition and high prey availability. This suggests that aspects of the environment can mediate the strength of Allee effects. There was also no demographic Allee effect in the HiP wild dog population, as the population growth rate was significantly negatively related to population size, despite no apparent ecological resource limitation. Such negative density dependence at low numbers indicates that behavioural studies of the causal mechanisms potentially generating Allee effects in small populations can provide a key to understanding their dynamics. This study demonstrates how aspects of a species’ social behaviour can influence the vulnerability of small populations to extinction and illustrates the profound implications of sociality for endangered species’ recovery.
Diversity and depletions in continental carnivore guilds: implications for prioritizing global carnivore conservation
28 January 2009
Stellenbosch University

Dalerum, Fredrik;Cameron, Elissa Z.;Kunkel, Kyran;Somers, Michael J.

Large carnivores are important ecosystem components but are extinction prone due to small populations, slow growth rates and large area requirements. Consequently, there has been a surge of carnivore conservation efforts. Such efforts typically target local populations, with limited attention to the effects on the ecosystem function of predator guilds. Also, there is no framework for prioritizing these efforts globally. We compared taxonomic and functional diversity of continental carnivore guilds, compared them with the corresponding guilds during the Late Pleistocene and synthesized our results into suggestions for global prioritizations for carnivore conservation. Recent extinctions have caused taxonomically and functionally depleted carnivore guilds in Europe and North and South America, contrasting with guilds in Africa and Asia, which have retained a larger proportion of their carnivores. However, Asia is at higher risk of suffering further extinctions than other continents. We suggest three priorities of contrasting urgency for global carnivore conservation: (i) to promote recovery of the threatened Asian species, (ii) to prevent species in the depleted guilds in Europe and North and South America from becoming threatened, and (iii) to reconstruct functionally intact sympatric guilds of large carnivores at ecologically effective population sizes.
Alien and native birds in South Africa: patterns, processes and conservation
17 June 2009
Stellenbosch University

Hugo, Sanet;van Rensburg, Berndt

The spatial distribution of alien species richness often correlates positively with native species richness, and reflects the role of human density and activity, and primary productivity and habitat heterogeneity, in facilitating the establishment and spread of alien species. Here, we investigate the relationship between the spatial distribution of alien bird species, human density, and anthropogenic and natural environmental conditions. Next, we examined the relationship between the spatial distribution of alien bird species and native bird species richness. We examined alien species richness as a response variable, using correlative analyses that take spatial autocorrelation into account. Further, each alien bird species was examined as a response variable, using logistic regression procedures based on binary presence-absence data. A combination of human density and natural habitat heterogeneity best explained the spatial distribution of alien species richness. This contrasts with the results for individual alien species and with previous studies on other non-native taxa showing the importance of primary productivity and anthropogenic habitat modification as explanatory variables. In general, native species richness is an important correlate of the spatial distribution of alien species richness and individual alien species, with alien species being more similar to common species than to rare species.
Spatial congruence between ecotones and range restricted species: implications for conservation biogeography at the sub-continental scale
17 June 2009
Stellenbosch University

van Rensburg, Berndt;Levin, Noam;Kark, Salit

Aim To examine whether at a sub-continental scale range-limited species tend to occur close to areas of transition between vegetation boundaries more often than expected by chance. Location South Africa and Lesotho. Methods We examined the relationship between the distance of a grid square to ecological transition areas between vegetation types and both avian and frog range-limited species richness in the quadrate. We used quadrates at a spatial resolution of quarter degree (15' x 15'  676 km2). Spatial congruence between areas representing range-restricted species and those representing ecological transition zones was assessed using a random draw technique. Results Species richness and range size rarity are generally negatively correlated with distance to transition areas between vegetation communities when analysed for the whole region for both groups. Although this relationship becomes weaker after controlling for environmental energy and topographic heterogeneity, the explanatory power of distance to transition areas remained significant, and compared to the different biomes examined, accounts for most of the variation in bird richness (20%), frog richness (18%), range restricted bird species (17%), and range restricted frog species (16%) in the savanna biome. The random draw technique indicated that areas representing range restricted species were situated significantly closer in space to those areas representing transition areas between vegetation communities than expected by chance. Main conclusions We find that at the sub-continental scale, when examined for South Africa, areas of transition between vegetation communities hold concentrations of range-limited species in both birds and frogs. We find that South African endemic / range-limited birds and frogs are located closer to ecological transition zones than endemics and non-endemics combined. This has important implications for ongoing conservation planning in a biogeographical context.
Factors associated with suni distribution in Tembe Elephant Park, South Africa: implications for management and conservation of a small antelope
17 June 2009
Stellenbosch University

Belton, Lydia;Dalerum, Fredrik;van Rensburg, Berndt

While small antelopes are becoming increasingly threatened, knowledge of their biology is often limited. Management recommendations are therefore frequently based on insufficient information. One declining small antelope is the suni in South Africa. A reduction in sand forest as well as habitat destruction by increased nyala populations are the suggested causes of suni declines. However, no recent study has quantified the factors that underlie suni distribution in southern Africa. Here we investigate whether the vegetation types, vegetation structure or occurrences of sympatric browsers affect suni distribution within Tembe Elephant Park, South Africa. We found that vegetation type and vegetation structure interacted in their effects on suni distribution, but there were no effects of sympatric browsers. Suni were found most often in sand forest, but were also present in closed woodland. Vegetation structure was, however, related to the local distribution of suni only in sand forest. This highlights that suni are not exclusively bound to sand forest, and also that vegetation structure within the sand forest may be as important as the vegetation type. We suggest that the current conservation assessment of this subspecies is based on insufficient data, and that a more detailed assessment of their distribution and the factors that determine the same is critically needed.
Home range use of free-ranging cheetah on farm and conservation land in Botswana
22 July 2009
Stellenbosch University

Houser, AnnMarie;Somers, Michael J.;Boast, Lorraine K.

Cheetah ( Acinonyx jubatus) movements should be considered when developing management strategies for long-term survival and coexistence with humans. Although work has been done in Namibia, South Africa and Tanzania little data on the home range and territory size of cheetah in Botswana has been published. This study aimed to estimate male and female home range sizes and daily movement on farmland and a game reserve in Botswana. Cheetahs were monitored from October 2003 to April 2007. The cheetah were fitted with cell/GPS or VHF collars and released back into their home range. Single male home ranges were 494 km2 and 663 km2 and a coalition of two males had a home range of 849 km2 (fixed kernel method). The females’ home ranges were 241 km2 and 306 km2 (fixed kernel method). Females travelled a mean distance of 2.16 ± 0.07 km/day (range; 0–20 km/day) compared to 6.13 ± 0.30 km/day (range; 0–39 km/day) in males. Female maximum daily travel increased from 4.17 km/day when cubs where in the den to 8.16 km/day when cubs had left the den.
Does the self-similar species distribution model lead to unrealistic predictions?
15 August 2009
Stellenbosch University

Hui, Cang;McGeoch, Melodie A.

J. Harte et al. demonstrated that the power law form of the species–area relationship may be derived from a bisected, self-similar landscape and a community-level probability rule. Harte’s self-similarity model has been widely applied in modeling species distributions. However, R. D. Maddux showed that this self-similarity model generates biologically unrealistic predictions. We resolve the Harte–Maddux debate by demonstrating that the problems identified by Maddux result from an assumption that the probability of occurrence of a species at one scale is independent of its probability of occurrence at the next. We refer to this as a ‘‘non-heritage assumption.’’ By altering this assumption to one in which each species in the community has an occupancy status that is partially inherited across scales (a scale-heritage assumption), the predictions of the self-similarity model are neither mathematically inconsistent nor biologically unrealistic. Harte’s self-similarity model remains an important framework for modeling species distributions. Our results illustrate the importance of considering patterns of species co-occurrence, and the way in which species occupancy patterns change with scale, when modeling species distributions.
Capturing the ‘droopy tail’ in the occupancy-abundance relationship
15 August 2009
Stellenbosch University

Hui, Cang;McGeoch, Melodie A.

The intraspecific occupancy–abundance relationship is a widely used descriptor of species distributions, with potential value to conservation and pest management for predicting species abundance from occurrence data. Six traditional and commonly used models for the description of this relationship include the Poisson, exponential, metapopulation, fractal, negative binomial, and improved negative binomial models. However, these models deviate systematically from empirical data. We present a model (the “droopy-tail” model or DTM) that improves estimates of occupancy at coarse scales by taking the percolation effect of scale on the occupancy–abundance relationship into account. The DTM is a fundamental departure from previous models because the slope of the scaling curve limits to negative infinity (rather than to zero) at coarse scales in addition to providing a good fit to empirical data at all spatial scales.
Modeling species distributions by breaking the assumption of self-similarity
15 August 2009
Stellenbosch University

Hui, Cang;McGeoch, Melodie A.

Species distributions are commonly measured as the number of sites, or geographic grid cells occupied. These data may then be used to model species distributions and to examine patterns in both intraspecific and interspecific distributions. Harte et al. (1999) used a model based on a bisection rule and assuming self-similarity in species distributions to do so. However, this approach has also been criticized for several reasons. Here we show that the self-similarity in species distributions breaks down according to a power relationship with spatial scales, and we therefore adopt a power-scaling assumption for modeling species occupancy distributions. The outcomes of models based on these two assumptions (self-similar and power-scaling) have not previously been compared. Based on Harte’s bisection method and an occupancy probability transition model under these two assumptions (self-similar and power-scaling), we compared the scaling pattern of occupancy (also known as the area-of-occupancy) and the spatial distribution of species. The two assumptions of species distribution lead to a relatively similar interspecific occupancy frequency distribution pattern, although the spatial distribution of individual species and the scaling pattern of occupancy differ significantly. The bimodality in occupancy frequency distributions that is common in species communities, is confirmed to a result for certain mathematical and statistical properties of the probability distribution of occupancy. The results thus demonstrate that the use of the bisection method in combination with a power-scaling assumption is more appropriate for modeling species distributions than the use of a self-similarity assumption, particularly at fine scales.
The effect of landscape heterogeneity on host–parasite dynamics
15 August 2009
Stellenbosch University

Su, Min;Li, Wenlong;Li, Zizhen;Zhang, Fengpan;Hui, Cang

Environmental heterogeneity has been shown to have a profound effect on population dynamics and biological invasions, yet the effect of its spatial structure on the dynamics of disease invasion in a spatial host–parasite system has received little attention. Here we explore the effect of environment heterogeneity using the pair approximation and the stochastic spatially explicit simulation in which the lost patches are clustered in a fragmented landscape. The intensity of fragmentation is defined by the amount and spatial autocorrelation of the lost habitat. More fragmented landscape (high amount of habitat loss, low clustering of lost patches) was shown to be detrimental to the parasitic disease invasion and transmission, which implies that the potential of using artificial disturbances as a disease-control agency in biological conservation and management. Two components of the spatial heterogeneity (the amount and spatial autocorrelation of the lost habitat) formed a trade-off in determining the host–parasite dynamics. An extremely high degree of habitat loss was, counter-intuitively, harmful to the host. These results enrich our understanding of eco-epidemiological, host–parasite systems, and suggest the possibility of using the spatial arrangement of habitat patches as a conservation tool for guarding focal species against parasitic infection and transmission.
How does the spatial structure of habitat loss affect the eco-epidemic dynamics?
15 August 2009
Stellenbosch University

Su, Min;Hui, Cang;Zhang, Yanyu;Li, Zizhen

Habitat loss is considered as one of the primary causes of species extinction, especially for a species that also suffers from an epidemic disease. Little attention has been paid to the combined effect of habitat loss and epidemic transmission on the species spatiotemporal dynamics. Here, a spatial model of the parasite–host/prey–predator eco-epidemiological system with habitat loss was studied. Habitat patches in the model, instead of undergoing a random loss, were spatially clustered by different degrees. Not only the quantity of habitat loss but also its clustering degree was shown to affect the equilibrium of the system. The infection rate and the probability of successful predation were keys to determine the spatial patterns of species. The epidemic disease is more likely to break out if only a small amount of suitable patches were lost. Counter-intuitively, infected preys are more sensitive to habitat loss than predators if the lost patches are highly clustered. This result is new to eco-epidemiology and implies a possibility of using spatial arrangement of suitable (or unsuitable) patches to control the spread of epidemics in the ecological system.
The effect of migration on the spatial structure of intraguild predation in metapopulations
15 August 2009
Stellenbosch University

Su, Min;Zhang, Yanyu;Hui, Cang;Li, Zizhen

We consider the effects of migration on intraguild predation (IGP) via lattice models and pair approximation. The following results can be found: there exists asymmetry between two consumers’ migration effects on the spatial structure of IGP; migration of each consumer reduces its own clumping degree; the spatial structure is influenced by the ecological neighborhood.
On species-area and species accumulation curves: A comment on Chong and Stohlgren’s index
15 August 2009
Stellenbosch University

Hui, Cang

Chong and Stohlgren [Chong, G.W., Stohlgren, T.J., 2007. Species–area curves indicate the importance of habitats’ contributions to regional biodiversity. Ecol. Indic. 7, 387–395] presented a combined ranking index to measure the habitats’ contribution to regional biodiversity, in which the species–area curves perform poorly in the prediction of the species richness in multiple plots. After re-examination of the dataset (17 vegetation types of the Rocky Mountain National Park, CO, USA), I present the reason for this poor performance: (1) species–area curve is not identical with species accumulation curve; (2) the latter is steeper than the former due to: (a) rare species normally appearing at coarser spatial scales and (b) beta diversity being positively correlated to the distance between samples. This result implies that Chong and Stohlgren’s ranking index is scale dependent. The total number of species in different habitats could serve as an intuitive indicator that is highly correlated with the Chong–Stohlgren index.
From introduction to equilibrium: reconstructing the invasive pathways of the Argentine ant in a Mediterranean region
15 August 2009
Stellenbosch University

Roura-Pascual, Nuria;Bas, Josep M.;Thuiller, Wilfried;Hui, Cang;Krug, Rainer;Brotons, Lluis

Determining the geographical range of invasive species is an important component of formulating effective management strategies. In the absence of detailed distributional data, species distribution models can provide estimates of an invasion range and increase our understanding of the ecological processes acting at various spatial scales. We used two complementary approaches to evaluate the influence of historical and environmental factors in shaping the distribution of the Argentine ant (Linepithema humile), a widespread, highly invasive species native to South America. Occurrence data were combined with environmental data at incremental spatial scales (extent and resolution) to predict the suitable range of the ant invasion using ecological niche models. In addition, we also used a spread model that simulated the jump dispersal of the species to identify the most plausible scenarios of arrival of L. humile in the NE Iberian Peninsula at local scales. Based on the results of both modelling practices, we suggest that L. humile might have reached its maximum geographic range at regional scales in the NE Iberian Peninsula. However, the species does not appear in equilibrium with the environment at small spatial scales, and further expansions are expected along coastal and inland localities of the Costa Brava. Long-distance jumps are ultimately responsible for the spread of the Argentine ant in the area. Overall, our study shows the utility of combining niche based models with spread models to understand the dynamics of species’ invasions.
Spatiotemporal dynamics of the epidemic transmission in a predator-prey system
15 August 2009
Stellenbosch University

Su, Min;Hui, Cang;Zhang, Yanyu;Li, Zizhen

Epidemic transmission is one of the critical density-dependent mechanisms that affect species viability and dynamics. In a predator-prey system, epidemic transmission can strongly affect the success probability of hunting, especially for social animals. Predators, therefore, will suffer from the positive density-dependence, i.e., Allee effect, due to epidemic transmission in the population. The rate of species contacting the epidemic, especially for those endangered or invasive, has largely increased due to the habitat destruction caused by anthropogenic disturbance. Using ordinary differential equations and cellular automata, we here explored the epidemic transmission in a predator-prey system. Results show that a moderate Allee effect will destabilize the dynamics, but it is not true for the extreme Allee effect (weak or strong). The predator-prey dynamics amazingly stabilize by the extreme Allee effect. Predators suffer the most from the epidemic disease at moderate transmission probability. Counter-intuitively, habitat destruction will benefit the control of the epidemic disease. The demographic stochasticity dramatically influences the spatial distribution of the system. The spatial distribution changes from oil-bubble-like (due to local interaction) to aggregated spatially scattered points (due to local interaction and demographic stochasticity). It indicates the possibility of using human disturbance in habitat as a potential epidemic-control method in conservation.
Dogs on the catwalk: Modelling re-introduction and translocation of endangered wild dogs in South Africa
03 September 2009
Stellenbosch University

Gusset, Markus;Jakoby, Oliver;Muller, Michael S.;Somers, Michael J.;Slotow, Rob;Grimm, Volker

In South Africa, a plan was launched to manage separate sub-populations of endangered African wild dogs (Lycaon pictus) in several small, geographically isolated conservation areas as a single meta-population. This intensive management approach involves the re-introduction of wild dogs into suitable conservation areas and periodic translocations among them. Despite the initial failures and high costs associated with wild dog re-introductions and translocations, there is no predictive framework available to quantify which management protocol is the most efficient. We therefore developed an individualbased model of wild dog population and pack dynamics, which accounts for the wild dogs’ social complexity. The model appeared to capture the essential characteristics of a real wild dog population from Hluhluwe-iMfolozi Park, South Africa and to be relatively robust to parameter uncertainty, suggesting that the model is valid enough for addressing management problems. The model enabled us to quantify a critical initial number of packs (two) and individuals per pack (six) necessary for a re-introduced wild dog population to establish itself in the release area. We also found a practically feasible intervention regime at which a re-introduced wild dog population had the best chance of persistence: intermittently adding packs (at least every 6 years) and harvesting disperser groups (as often as every 4 years) for translocation to other release sites, without threatening the small source population. This study demonstrates that individual-based models can be a powerful decision-support tool in re-introduction planning and provides insight into how populations made up of social groups have dynamics, and ultimately persistence, determined by individual behaviour.

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