UC Irvine

Over large spatial scales, debate remains concerning the relative importance of ecological and evolutionary processes as drivers of species richness gradients. Ecological hypotheses propose that contemporary environmental variables are the primary drivers of species diversity, whereas evolutionary hypotheses, such as the metabolic theory and the time-for-speciation hypothesis, propose diversification rates or time. Previous work has compared ecological and evolutionary variables to quantify relationships with richness of three North American freshwater fish families and concluded that their richness gradients are driven by contemporary climate. We compiled the first time-calibrated phylogenetic tree consisting of 836 native North American freshwater fish species to evaluate both ecological and evolutionary hypotheses as contributors to species richness. Our results showed that climate, measured as actual evapotranspiration, is the strongest predictor of richness. In contrast, family ages and diversification rates are poor predictors of species richness, providing no support for the time-for-speciation hypothesis or metabolic theory. However, the contribution of species mean root distance, a phylogenetic metric, is greater than previously documented. We also find that climatic and phylogenetic variables together explain most of the variation in the broad-scale richness pattern. We further demonstrate that mean annual temperature indirectly shapes richness through species mean root distance and actual evapotranspiration, which we interpret as the result of local extirpations of fish species in response to Pleistocene glacial cycles. We argue that including historical and evolutionary processes alongside climate provides a stronger explanation for richness gradients than either considered separately.