Niche filling limits the accumulation of Himalayan songbirds
In the progress of an adaptive radiation, species rapidly form early associated with ecological diversification, and production slows down as environments fill up (ecological opportunity becomes limited). The extent to which environments are actually full with species remains controversial. Here, we estimate phylogenetic relationships among all Himalayan songbirds to study the accumulation of ecological differences important to species coexistence.
Based on a phylogeny for the 358 species distributed along the eastern elevational gradient, we show that body size and shape differences evolved early in the radiation, with the elevational band occupied by a species evolving later. These results are consistent with competition for niche space limiting species accumulation. Even the elevation dimension appears to be approaching ecological saturation, as close relatives outside the assemblage are on average old (>5My) separated. Further, elevational distributions are well explained by resources, notably the abundance of arthropods, and not by differences in diversification rates in different elevational zones. We consider a model of adaptive radiation, in which new species are produced uniformly through time but randomly in niche space, and show how this is broadly consistent with the Himalayan pattern, with some notable exceptions.