Biotic homogenization at the community scale: disentangling the roles of urbanization and plant invasion

Aim Urbanization as a major global trend profoundly changes biodiversity patterns, and homogenization of urban biota due to expanding exotic species and declining native species is of increasing concern. Previous studies on this topic have mostly taken place at large scales that include high habitat heterogeneity. Here, we aimed at disentangling the effects of urbanization and plant invasion on species composition through the analysis of similarity patterns of urban plant assemblages at the community scale where species interact. Location Berlin, Germany. Methods We analysed how different levels of urbanization, specific components of the urban matrix and the dominance of a native (Betula pendula) versus an exotic tree species (Robinia pseudoacacia) affect alpha and beta diversity of urban woodland understorey vegetation in sixty-eight 100-m2 plots. Results Exotic dominance reduced alpha diversity, but not beta diversity of the total species pool. Comparing beta diversity among different species groups revealed significant but divergent effects of exotic dominance, habitat connectivity and levels of urbanization in native and non-native species assemblages. In particular, urbanity proved to homogenize the native species pool, whereas the beta diversity of the non-native species pool showed a more pronounced response to exotic dominance. Main conclusions Our data provide evidence that both the urban context and the dominance of exotic species can modify homogenization processes at the community level. These novel insights into the mechanisms of biotic homogenization of urban floras may contribute to mitigating the effects of urbanization on biodiversity.