Interactive effects of land use and other factors on regional bird distributions

Aim We assessed the independent and interactive effects of land use and other factors on regional distributions of lake shore bird assemblages. Location Analyses were restricted to lakes of the northeastern United States (New England, New York, New Jersey). Methods Lake shore bird and habitat data were collected from a regional sample of 186 lakes between 1991 and 1995. Local and regional gradients in environmental attributes of the lakes and their watersheds, and in bird species composition, were characterized using detrended correspondence analysis. Bird assemblages were also characterized with respect to species diversity total bird abundance, and absolute and proportional abundance for neotropical migrants, omnivores and tree foragers. The assemblage measures were compared with the environmental gradients and with the environmental variables from which those gradients were derived using correlations, linear regression and bivariate plots. Results Anthropogenic variables (e.g. human density) and nonanthropogenic variables (e.g. climate, forest composition, lake productivity) were highly confounded regionally, reflecting the influence of climate and geomorphology on land use and, in turn, the influence of land use on the environment. Moving south along the regional gradient to watersheds with more intensive land use, changes in bird assemblage structure were consistent with anthropogenic effects on the avifauna attributable to forest fragmentation: total bird abundance increased, fragmentation-tolerant species were encountered more frequently, the proportional abundance of omnivores increased and the proportional abundances of tree foragers and neotropical migrants decreased. Bird diversity, as assayed using rarefaction, peaked at an intermediate position along the regional gradient, as did the absolute abundances of tree foragers and migrants. Main conclusions We propose that some community attributes peaked at an intermediate position along the regional gradient as a result of the interactive effects of land use and other factors on this region's avifauna, with broad-scale anthropogenic factors constraining assemblages on the urbanized lakes, and climate or other nonanthropogenic factors constraining assemblages on the pristine lakes. Anthropogenic and nonanthropogenic factors of similar scale and magnitude thus acted in concert to shape regional bird distributions.