Explaining the enormous variation in species diversity observed across the surface of the Earth is one of the major challenges in ecology. Variation in species richness arises from the underlying complex patterns of geographic range overlap between species. While strong statistical associations between the number of sympatric species and broad-scale environmental gradients suggests a dominant role for ecological niche availability in constraining species coexistence, it has been difficult to discount purely historical explanations. Here we show how phylogenetic models, coupled with functional trait and environmental data, can be used to provide novel insights into the historical and ecological processes generating and maintaining geographic range overlap (i.e. sympatry) and the resulting patterns of species richness.
Results/Conclusions
We show that global variation in the build-up of sympatry among recently diverged lineages of birds cannot be adequately predicted by dispersal-assembly models, based on differences in the rate or time available for range expansion. Instead, we provide evidence that the attainment of sympatry is largely dependent on ecological niche availability, increasing with ecosystem productivity and divergence in functional traits (e.g. beak size) and apparently declining as communities become saturated with species. Our results suggest that ecological controls on sympatry are instrumental in shaping the global distribution of biodiversity and that limits to niche packing will ultimately slow the future build-up of diversity in species rich tropical communities.