A growing interest in and understanding of green roof arthropod diversity and associated ecological functions provides the chance to compare species found on green roofs in cities around the world. Globalization may confer a competitive advantage to urban exploiters resulting in homogenization or convergence of communities in cities. An international dataset characterizing green roof beetle inhabitants allows us to test whether the biotic homogenization hypothesis might apply to green roofs, like it has been tested in other urban patches such as lawns, or whether green roofs support regionally distinct communities. In this paper, we compare beetle communities from five cities known for their effective green roof implementation in Europe and North America: Portland, USA; Halifax, Canada; Basel, Switzerland; Berlin, Germany; and Neubrandenberg, Germany. We identified different size sub-sets of the communities in our dataset, as a proxy for different life history strategy, and compared these. Consistent with the biotic homogenization hypothesis, we predicted that the smaller and potentially r-selected portions of these green roof communities would be more similar to each other than their larger, potentially k-selected, counterparts. We also predicted that there would be a high proportion of non-native species present in all locations, and that species most common to all cities would be generalists.
We found that both city and roof type were import organizers of communities in both size sub-sets, whereas roof age was not. This supports findings from previous work in Portland which demonstrated an increase in beetle abundance and species richness on green roofs with habitat elements. We also found that the effect of city was greater for the larger sub-sets compared to the smaller ones. At the same time, a majority of the top ten most abundant species found in the North American cities were non-native compared to a majority of native European ones. Together these findings suggest either resilience against homogenization or simply that not enough time has occurred for it to develop. More research is needed to track drivers of homogenization such as functional composition and trophic interactions. For example, thirty-five percent of species were found only once in the global dataset, suggesting a strong barrier to establishment that could be environmental or predatory. As green roofs become more common in urban areas, more research is needed to identify both widespread and rare species of ecological and economic importance.