Biotic homogenization of urban floras by alien species. The role of species turnover and richness differences

Question: The spread of alien species has been changing the diversity of plant communities all over the world, perhaps most notably in urban habitats. It has been shown that alien species with different residence times have different impacts on the b-diversity of urban plant communities: archaeophytes tend to contribute to homogenization, while neophytes tend to increase differentiation among sites. However, it has not been determined whether these processes result from changes in species turnover or from differences in species richness. Here, we use an additive partitioning framework to disentangle the contribution of species turnover and richness difference to b-diversity patterns in invaded urban plant communities. Location: Thirty-two cities in ten countries of Central Europe and Benelux. Methods:We analysed the effects of alien species on b-diversity of urban plant communities separately for archaeophytes and neophytes to assess whether the observed patterns differ between these two groups of species with different residence times in the invaded region. We used additive as well as non-additive measures of species turnover and richness difference. For this purpose, we proposed a new index that complements the recently proposed Podani-Schmera index of richness difference. Results: We confirmed the results of earlier studies that neophytes tend to differentiate the urban plant communities, while archaeophytes tend to homogenize, although in some specific habitats they can also contribute to differentiation. The observed changes in b-diversity were related to the turnover component of b-diversity in most cases, especially for neophytes. In contrast, the richness difference component was not significantly different between neophytes and native species. The trends for archaeophytes were less consistent, but inmost habitats their turnover and richness difference were not significantly different fromnative species. Conclusions: Changes in b-diversity of urban plant communities induced by the establishment of alien species reflect mainly species turnover, whereas the richness difference component has small effects restricted to certain habitats only.