Relative roles of ecological and energetic constraints, diversification rates and region history on global species richness gradients

Regions worldwide differ markedly in species richness. Here, for birds and mammals worldwide, we directly compare four sets of hypotheses regarding geographical richness gradients: (1) evolutionary, emphasising heterogeneity in diversification rates, (2) historical, related to differences in region ages and sizes, (3) energetic, associated with variation in productive or ambient energy and (4) ecological, reflecting differences in ecological niche diversity. Among highly independent regions, or 'evolutionary arenas', we find that richness is weakly influenced by richness-standardised ecological niche diversity, questioning the significance of ecological constraints for producing large-scale diversity gradients. In contrast, we find strong evidence for the importance of region area and its changes over time, together with a role for temperature. These predictors affect richness predominately directly without concomitant positive effects on diversification rates. This suggests that regional richness is governed by historical and evolutionary processes, which promote region-specific accumulation of diversity through time or following asymmetrical dispersal.