Productivity, temperature, and the global distribution of terrestrial vertebrates

Despite increasingly well documented patterns of species distributions and their central importance to ecology, biogeography and conservation, there is still disagreement on the identity and generality of their determinants. While some environmental variables have received strong support in certain region-taxon combinations, their validity across the globe and across different taxa to date remains untested. Here I evaluate the significance of core environmental predictors of the distribution of all 28,000 species of terrestrial vertebrates. Geographic richness patterns differ strongly between the four classes and show further disparities when set in relation to total vertebrate richness. Total vertebrate richness is very strongly explained by one single predictor, net primary productivity. Due to its direct effect on energetic needs temperature is expected to be a dominant determinant of the richness of ectotherms (reptiles, amphibians), but to be of only secondary importance for endotherms (mammals, birds). This prediction is borne out by the data, but only for the combined richness of both constituent classes. For endotherms alone again net primary productivity is the core predictor. These relationships are confirmed by multi-predictor modelling and analyses that fully control for spatial autocorrelation. These results illustrate for the first time critical biogeographic patterns. They provide important insights about the interaction of energy availability, energy needs, and taxon-specific drivers of diversification in determining broad-scale patterns of diversity.