Interactions between climate, competition, and habitat in limiting Himalayan bird distributions
Understanding the determinants of species distributions is critical for elucidating broader patterns of diversity, determining community organization patterns and dynamics, and predicting species responses to global change. Yet the relative importance of abiotic and biotic factors in shaping species distributions is still poorly known for most species, as they are often difficult to separate empirically. We utilized differences in climate and habitat, as well as the well-documented bird diversity gradient across the Himalayas, to test the relative importance of these factors in limiting bird distributions along elevational gradients. We conducted community-wide bird and vegetation surveys along five elevational gradients from two locations in the western Himalayas differing in climate and species richness and featuring shifted habitat zonation. In addition, locally deployed weather stations simultaneously recorded temperature at six locations along each gradient. We incorporated habitat, temperature, and competitor abundance as covariates into a modelling framework to predict species abundance for over 70 bird species while accounting for imperfect detection.
Many bird species showed displaced elevational distribution patterns both between sites and between slopes within sites. Shifts in distributions were related to both differences in local temperature as well as displaced habitat zonation, but temperature dominated as a significant predictor of bird abundance, occurring in over 80% of the top-ranked models. The presence or absence of congeneric species on some slopes, but not others, led to range contractions, expansions, or shifts, suggesting competition also plays a role in constraining the distributions of some species. While we expect species ranges may shift with climate change, our results suggest that such shifts may be hindered by “boundaries” enforced by habitat and competition.