Abstract
A major goal of ecology is to determine the causes of the latitudinal gradient in global distribution of species richness. Current evidence points to either energy availability or habitat heterogeneity as the most likely environmental drivers in terrestrial systems, but their relative importance is controversial in the absence of analyses of global (rather than continental or regional) extent.
Here we use data on the global distribution of extant continental and continental island bird species to test the explanatory power of energy availability and habitat heterogeneity while simultaneously addressing issues of spatial resolution, spatial autocorrelation, geometric constraints upon species? range dynamics, and the impact of human populations and historical glacial ice-cover. At the finest resolution (1o), topographical variability is identified as the most important global predictor of avian species richness.
Temperature and productivity are second in importance in multi-predictor and single-predictor models, respectively. While adjusting model outcomes for null expectations based on geometric constraints on species richness improves overall model fit, this has a negligible impact on tests of environmental predictors.
Conclusions concerning the relative importance of environmental predictors of species richness cannot be extrapolated from one biogeographic realm to others or the globe. Rather a global perspective confirms the primary importance of mountain ranges in high energy areas