Latitudinal and environmental patterns of species richness in lizards and snakes across continental North America

Aim: Little is known about the contributions of individual clades to the broader latitudinal diversity gradients of lizards and snakes in North America. Additionally, the environmental variables underpinning these patterns have not been extensively studied, nor have the relationships between squamate species richness and biome variety, a proxy for habitat heterogeneity. Here, we deconstruct the latitudinal diversity gradient of squamates and investigate lizard and snake species richness in relation to environmental gradients and biomes across continental North America. Location: North America, excluding islands. Taxon: Squamata (lizards and snakes). Methods: We collated a suite of environmental variables (n = 10) and squamate species ranges (n = 1,152) in an equal-area grid system (100 × 100 km cells) spanning continental North America. We deconstructed the latitudinal diversity gradient of squamates and examined species richness patterns for all lizards, all snakes and several individual squamate clades. We used multiple linear regressions to determine which environmental factors are most important for each squamate clade. Finally, we documented species richness within biomes and tested for correlations between species richness and biome variety. Results: Squamates exhibit a strong latitudinal diversity gradient across continental North America. Snakes are more widespread and have a stronger gradient than lizards. Our multiple linear regression results indicate strong associations between species richness and environmental variables for all individual squamate clades analysed; temperature variables are strongly positively correlated with species richness. Biome variety is more strongly correlated with lizard species richness than snake species richness. Squamates are most species-rich in tropical/subtropical forests, deserts and mangroves. Main conclusions: Most snake clades in North America exhibit strong latitudinal diversity gradients, but only two lizard clades, Dactyloidae (anoles) and Gekkota (geckos), display strong gradients and thus drive the overall lizard pattern. Squamate clades differ in their responses to environmental gradients, as reflected by their species richness patterns, with warmer climates generally harboring more species. Lizards are more strongly associated with habitat heterogeneity than snakes are, consistent with results from previous studies. Squamates in more rapidly shifting biomes (e.g. deserts, mangroves) may face a greater risk of extinction from impending climate change.