Bird community change only partially matches landscape habitat change across five decades in North America

Martha Paola Barajas Barbosa, Kimberly L. Thompson, Nicolas Bianco, Nadja Klein, Jonathan Chase

A common expectation is that as habitats in a landscape change, so do communities within them. However, this niche-based expectation assumes species persist in their favored habitats regardless of how much habitat remains; geometric constraints can counter this when heterogeneous landscapes supply too little of each habitat type for specialists to persist, allowing generalists to dominate. Using North American Breeding Bird Survey data and land-cover reconstructions spanning nearly 50 years, we show environmental heterogeneity decreased slightly, while bird composition became slightly more heterogeneous. Species presence supported niche-based explanations, whereas weighting dominant species revealed patterns consistent with geometric constraints; in both cases, bird community changes were small relative to land-cover shifts. Gains were concentrated among species of intermediate habitat breadth that exploit habitat edges and mosaics, while the broadest generalists declined. Assessing biodiversity responses to land-cover change therefore requires tracking both which species persist and which come to dominate.