Environment-richness relationships for mammals, birds, reptiles, and amphibians at global and regional scales

Climate has been routinely indicated as a major determinant of broad-scale species richness patterns for a variety of taxa, but studies vary widely in attributing richness variation to the broad-scale distribution of energy, water, ecosystem productivity, habitat heterogeneity, or some combination thereof. Here, I report global and regional environment-richness relationships for the four classes of terrestrial vertebrates (mammals, birds, reptiles, amphibians) using identical sample units and the same set of climate (temperature, precipitation, annual actual evapotranspiration), productivity (normalized difference vegetation index), and topographic (elevation range) variables. My results strongly support concomitant availability of energy and water as the principal constraint on global richness for all vertebrate groups except reptiles, which are largely constrained by temperature. However, environment-richness models for all taxonomic groups varied widely when applied to single (continental-scale) biogeographic realms. In particular, I found strong support for the 'water-energy dynamics hypothesis' that models richness as a function of ambient energy (temperature) in high latitudes and water availability (precipitation) at low latitudes, partially independent of productivity. Ectotherm groups were more constrained by temperature than endotherms, and amphibians were more constrained by water availability than other groups. Although habitat heterogeneity, measured as elevation range, was a consistent contributor to global and regional richness models for all groups, its contribution was always minor compared to other variables. I conclude that temperature and water availability are key variables for modeling broad-scale vertebrate richness, but there remains significant room for taxon-specific modeling approaches and for the inclusion of non-climate factors related to evolutionary history and faunal assembly in different regions.