Functional and phylogenetic convergence of winter and breeding bird communities in the northeastern US

Abstract

Around the world, ecological communities are becoming more similar to one another in a process known as biotic homogenization – an increase in similarity among communities over time. While biotic homogenization has been widely studied among spatial communities, very little attention has been paid to beta diversity between seasonal communities, especially in terms of functional or phylogenetic diversity. In temperate ecosystems, seasonality plays a major role in structuring ecological communities, but anthropogenic pressures are altering community composition. We analyze 40 years of data to study changes in beta diversity between winter and breeding bird communities in the northeastern US. We find evidence of taxonomic, phylogenetic, and functional homogenization between winter and breeding bird communities driven by decreasing turnover. Changes in phylogenetic diversity largely mirrored changes in taxonomic diversity, but functional diversity did not, with functional richness increasing in both seasons despite species richness increasing only in winter. Functional homogenization was driven by 1) decreasing occurrence of winter boreal finches and breeding season aerial insectivores, which reduced the functional space unique to either season, and 2) increasing occurrence of raptors, mergansers, wild turkey, and other functionally distinct species, which expanded the total functional space of both seasons and the shared functional space between seasons. Together, these shifts demonstrate a decline in the distinctiveness of functional space between seasons. Our study is one of the first to describe functional and phylogenetic homogenization between seasons and highlights the importance of considering seasonal homogenization and of using multiple facets of diversity to describe and understand biotic homogenization.