Deconstructing the latitudinal diversity gradient of North American mammals by nominal order

Abstract

North American mammals follow a well-established latitudinal diversity gradient in species richness. However, the degree to which species in different mammal clades follow the same latitudinal gradient—and to which each clade contributes to the pattern observed for all mammals remains unknown. Here, we separate the overall mammalian latitudinal diversity gradient by mammal orders and investigate the impact of climate and topography on the distribution of each major mammal clade. We joined an equal-area grid (100 × 100 km cells) of continental North America embedded with environmental variables (n = 10) with mammalian species ranges (n = 753). We used spatial regression models to quantify the relationship between species richness and latitude for all mammals, all mammals excluding select clades, and for each individual subordinate clade (n = 9). We used multiple linear regression and simultaneous autoregressive regression models to determine which environmental variables best explained patterns of species richness for each mammal order. Whereas North American mammals altogether exhibit a strong latitudinal diversity gradient in species richness, most orders deviate from the species richness pattern observed for all mammals and their gradients are weak or entirely absent. Bats (Chiroptera) exhibit the strongest latitudinal gradient—their removal from the pattern for all mammals substantially weakens the total mammalian gradient, more so than when rodents are removed. Environmental variables explain patterns of species richness well for some clades, but poorly for others. The gradient we observe for North American mammals today is likely a combined product of multiple diversification events, dispersals, and climatic and tectonic histories.