Biotic homogenization of three insect groups due to urbanization.

Cities are growing rapidly, thereby expected to cause a large-scale global biotic homogenization. Evidence for the homogenization hypothesis is mostly derived from plants and birds, whereas arthropods have so far been neglected. Here, I tested the homogenization hypothesis with three insect indicator groups, namely true bugs, leafhoppers, and beetles. In particular, I was interested whether insect species community composition differs between urban and rural areas, whether they are more similar between cities than between rural areas, and whether the found pattern is explained by true species turnover, species diversity gradients and geographic distance, by non-native or specialist species, respectively. I analyzed insect species communities sampled on birch trees in a total of six Swiss cities and six rural areas nearby. In all indicator groups, urban and rural community composition was significantly dissimilar due to native species turnover. Further, for bug and leafhopper communities, I found evidence for large-scale homogenization due to urbanization, which was driven by reduced species turnover of specialist species in cities. Species turnover of beetle communities was similar between cities and rural areas. Interestingly, when specialist species of beetles were excluded from the analyses, cities were more dissimilar than rural areas, suggesting biotic differentiation of beetle communities in cities. Non-native species did not affect species turnover of the insect groups. However, given non-native arthropod species are increasing rapidly, their homogenizing effect might be detected more often in future. Overall, the results show that urbanization has a negative large-scale impact on the diversity specialist species of the investigated insect groups. Specific measures in cities targeted at increasing the persistence of specialist species typical for the respective biogeographic region could help to stop the loss of biodiversity.