The homogenizing influence of agriculture on forest bird communities at landscape scales

Context Agricultural expansion is a principal driver of biodiversity loss, but the impacts on community assembly in agro-ecosystems are less clear, especially across regional scales at which agricultural policies are applied. Objectives Using forest-breeding birds, we (1) tested whether increased agricultural coverage resulted in species communities that were random or more similar than expected, (2) compared the relative influence of agriculture versus distance in structuring communities, and (3) tested whether different responses to agriculture among functional guilds leads to a change in functional diversity across gradients of agriculture. Methods Species abundances were sampled along 229 transects, each 8 km, using citizen science data assembled across a broad region of eastern Canada. Agricultural and natural land cover were each summed within three different-sized buffers (landscapes) around each transect. A null modeling approach was used to measure community similarity. Results Communities were most similar between landscapes that had high agricultural coverage and became more dissimilar as their respective landscapes differed more strongly in the amount of agriculture. Community dissimilarity increased with geographic distance up to about 200 km. Dissimilarity with increasing agriculture was largely due to the disappearance of Neotropical migrants, insectivores and foliage-gleaners from the community as agriculture increased. Functional diversity declined with increasing agriculture but less strongly than species richness and only when agriculture exceeded 40% of the landscape. Conclusions Our results support the hypothesis that increasing agricultural coverage produces a filtering towards communities of agriculture-tolerant forest birds with a loss of functional diversity and high site-to-site community similarity.