Taxonomic and functional homogenization of butterfly communities along an urban gradient

Urbanization is a major cause of global insect declines, yet some species can persist, and even thrive, in cities. Research on butterflies frequently report reduced diversity in urban habitats compared to rural ones, but less is known about whether urbanization favours butterflies with specific functional traits. Further, few studies have evaluated whether urbanization leads to the biotic homogenization of butterfly communities, despite being frequently reported for other taxa. Here, we investigate how butterfly community composition changes along an urban gradient by surveying 44 sites around Montreal, Quebec, Canada. We test the hypothesis that urban butterfly communities are homogenized at the taxonomic and functional levels. We found clear differences in the structure of urban versus rural butterfly communities with urban areas favouring a few, highly abundant, non-native species. These shifts were defined by losses of native species richness and abundance combined with increases in non-native species abundance. For most of the butterfly community, species with longer flight periods were more common in urban areas. Finally, urban communities were homogenized at the taxonomic and functional levels as demonstrated by reductions in beta diversity and variation in several key functional traits (wingspan, larval diet breadth, oviposition style) in urban habitats compared to rural ones. Overall, urban habitats in this region support less diverse butterfly communities that are homogenized. Since urban areas are growing worldwide, a better understanding of how biotic homogenization arises and what its consequences are will be key to guiding future conservation efforts. Butterfly communities were surveyed across an urban gradient in Montreal, Quebec, Canada. Urban butterfly communities were homogenized at both the taxonomic and functional levels. At the taxonomic level, urban communities were dominated by a few, highly abundant, non-native species. At the functional level, urban communities included more species with longer flight periods. image