Environmental prevalence and the distribution of species richness across climatic niche space

Abstract Aim The reciprocal relationship between geographical and ecological niche space known as Hutchinson's duality provides a powerful framework to analyse the relationship between biogeographical distributions and environmental variables. However, little attention has been given to how the structure of niche space is associated with the distribution of species across niche space itself. We examined whether environmental prevalence (i.e. the relative availability of different combinations of climatic conditions) can predict the climate‐richness relationship and consequently influence broad‐scale patterns of species richness. Location Worldwide Methods We tested our hypothesis using distribution data on >10,000 species of amphibians and mammals. We measured the environmental prevalence of five climatic variables associated with broad‐scale patterns of species richness. To this end, we divided each variable into equally spaced intervals representing particular climatic regimes, and calculated the geographical area where each regime is present. We then used species‐area regression models and hierarchical partitioning to test the relationship between environmental prevalence and the number of species in each climatic regime, and measure the relative importance of environmental prevalence and climate in explaining species richness across climatic niche space. Results Our results indicate a strong relationship between environmental prevalence and the number of species found in each climatic regime, especially in temperature‐related variables. Moreover, environmental prevalence was generally a stronger predictor of species richness than climate. Results were similar for amphibians and mammals, being observed at both global‐ and realm‐scales. Main conclusions Our findings suggest that the relationship between climate and species richness can arise from the variation in environmental prevalence. This conclusion challenges the traditional idea that climate itself is a primary driver of the climate‐richness relationship. We argue that environmental prevalence should be considered in future tests of the relationship between environmental variables and species richness.