Urbanization has detrimental impacts on populations and ecosystems (Czech and Krausman 1997).
Under current rates of urban expansion, developed land in the continental United States is projected to increase by 63% from 2001 to 2051 (Lawler et al. Globally, the proportion of people living in urban environments increased from <30 to over 70% between the years of 19, with the proportion of urban land area projected to increase by 185% by the end of this century (Seto et al. The impact of urbanization on biological communities has become a subject of key conservation concern in the 21st century. 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. 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. Species richness and functional diversity declined monotonically with increasing impervious surface. 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. 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.
Conflicting patterns of species richness and species abundances have limited our ability to determine whether urban environments filter avian communities. 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. 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. As species are excluded from anthropogenic environments, local species pools are differentially sorted from the regional species pool. Traits such as dietary habits and habitat specialization influence the vulnerability of species to land use change.
The composition of avian communities in human-dominated habitats is thought to be determined by the interaction between species-specific traits and environmental characteristics.
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