Environmental filtering of avian communities along a rural‐to‐urban gradient in Greater Washington, D.C., USA

The composition of avian communities in human‐dominated habitats is thought to be determined by the interaction between species‐specific traits and environmental characteristics. Traits such as dietary habits and habitat specialization influence the vulnerability of species to land use change. As species are excluded from anthropogenic environments, local species pools are differentially sorted from the regional species pool. This sorting process, environmental filtering, is characterized by a decline in the functional diversity of local biotic communities and may result in a loss of regional biodiversity as landscapes are urbanized. Environmental filtering due to urbanization is hypothesized due to an ecosystem stress gradient, which describes a decrease in species richness or abundance with increasing urban intensity. Conflicting patterns of species richness and species abundances have limited our ability to determine whether urban environments filter avian communities. To evaluate the hypothesis that environmental filtering is occurring, we analyzed avian point count data collected along a rural‐to‐urban gradient in metropolitan Washington, D.C. We examined predictions that species richness, functional diversity, and the total and relative abundances of some life history guilds exhibit the pattern expected under the ecosystem stress‐gradient hypothesis. Species richness and functional diversity declined monotonically with increasing impervious surface. Life history guilds, representing species’ diet, foraging, nest, and migration habits, exhibited differential rates of decline across the rural‐to‐urban gradient, resulting in marked shifts in the composition of communities. Our results support the hypothesis that urbanization filters bird communities as a function of avian traits and provide further evidence of trait‐level responses to urban environments.