A comparative analysis of metacommunities reveals contrasting drivers of alpha and beta diversity in urban butterflies

Abstract Urbanization reduces biodiversity and accelerates biotic homogenization, including among pollinators. However, questions remain concerning urban pollinator ecology and conservation, particularly: to what extent does pollinator alpha and beta diversity decline across urban landscapes, and what drives these patterns? We investigated the impacts of local and landscape characteristics on community diversity in four increasingly urban butterfly metacommunities, collecting data on butterfly diversity, butterfly host plant richness, flowering plant richness, and impervious surface area in the landscape. Suburban and urban metacommunities supported similarly low levels of butterfly diversity compared to a natural peri-urban metacommunity. Host plant richness increased butterfly diversity, but floral richness was correlated with reduced diversity, suggesting that many species-rich gardens are low in diversity of plants that support butterflies. In contrast, dissimilarity was driven by both local and landscape scale factors. Dissimilarities in site area and landscape impervious surface area drove overall dissimilarity, while these factors in addition to host and floral communities drove butterfly turnover (loss with replacement) and nestedness (loss without replacement). Our findings demonstrate that alpha and beta diversity are driven by different factors, and both must be considered to understand urban butterfly ecology and conservation. While butterfly species are filtered out of landscapes with high amounts of impervious surface area, even small sites can support diverse butterfly communities if they contain rich assemblages of butterfly host plants. Our comparative analysis of metacommunities provides a framework for future studies in human-altered environments.