Ecological limits as the driver of bird species richness patterns along the east Himalayan elevational gradient

Variation in species richness across environmental gradients results from a combination of historical non-equilibrium processes (time, speciation, extinction) and present-day differences in environmental carrying capacities (i.e., ecological limits, affected by species interactions and the abundance and diversity of resources). In a study of bird richness along the sub-tropical east Himalayan elevational gradient, we test the prediction that species richness patterns are consistent with ecological limits using data on morphology, phylogeny, elevational distribution, and arthropod resources. Species richness peaks at mid-elevations. Occupied morphological volume is roughly constant from low to mid-elevations, implying more species are packed into the same space at mid-elevations compared with low elevations. However, variance in beak length, and differences in beak length between close relatives decline with elevation, a consequence of the addition of many small insectivores at mid-elevations. These patterns are predicted from resource distributions: arthropod size diversity declines from low to mid elevations, largely because many more small insects are present at mid-elevations. Weak correlations of species mean morphological traits with elevation also match predictions based on resources and habitats. Elevational transects in the tropical Andes, New Guinea and Tanzania similarly show declines in mean arthropod size and mean beak length, and in these cases likely contribute to declining numbers of insectivorous bird species richness along these gradients. The results imply conditions for ecological limits are met, although historical non-equilibrium processes are likely to also contribute to the pattern of species richness.