Time best explains global variation in species richness of amphibians, birds and mammals

Aim The general pattern of higher species richness in tropical areas has been long recognized but the underlying cause is still debated. Two major hypotheses have emerged in recent years. The Rate Hypothesis attributes this pattern to a high rate of diversification, whereas the Time Hypothesis attributes it to greater lineage age. Here, we revisited these two hypotheses with global data sets of amphibians, birds and mammals. Location Global. Methods To test these hypotheses we evaluated the relationship between crown age and species richness, and between diversification rate and species richness within biogeographical regions. We also compared diversification rates of tropical and temperate clades, and assessed the usefulness of two phylometrics, evolutionary distinctiveness (ED) and evolutionary rate (ER), as proxies of age and diversification rate. Finally, we used those phylometrics in a grid cell approach to explore the spatial distribution of clade age and diversification rate. Results We found species richness of these tetrapods is best described by time (age of lineages) and that diversification rates are not significantly different between tropical and temperate areas. In addition to time, we found that historical biogeography, in some cases, has an influence on species richness patterns. In turn, this suggests that the latitudinal diversity gradient is a result of the gradient in climatic stability, with younger assemblages (hence, fewer species) occupying higher latitudes. Main conclusion Our results indicate that time, and not rate of diversification, best describes species richness patterns of amphibians, birds and mammals, and that this pattern is a 'climate effect' ultimately deriving from the latitudinal gradient in climatic stability.