British plants as aliens in New Zealand cities: residence time moderates their impact on the beta diversity of urban floras

18 December 2017

Anthropogenic activities have weakened biogeographical barriers to dispersal, thereby promoting the introduction, establishment and spread of alien species outside their native ranges. Several studies have identified a number of biological and ecological drivers that contribute to the establishment of plant species in the invaded range. One long-term factor that is generally accepted as a relevant determinant of invasion success is residence time, or time since first introduction into the new region. Residence time is often an important correlate of range extent in the invaded region, such that alien species with longer residence times in the novel environment tend to be more widely distributed. Plant species that were introduced in different regions at different times provide a unique opportunity to examine the effect of residence time on invasion success. In this paper, we examined how residence time affects the beta diversity of alien plants in selected urban floras of New Zealand and of English and Irish cities. We used an intercontinental plant exchange as a model system, comparing groups of species introduced to New Zealand and to the British Isles at different times (i.e., species native to the British Isles, British archaeophytes and British neophytes) and asked if differences in their beta diversity can be related to differences in their residence times. Our results suggest that observed patterns of beta diversity among the urban floras of New Zealand and of English and Irish cities can be attributed to a combination of residence time and of pre-adaptation to urban habitats that evolved, or were filtered in association with human activities, before the species were introduced into the invaded range.