Abiotic and biotic constraints across reptile and amphibian ranges

A long‐standing macroecological hypothesis posits that species range limits are primarily determined by abiotic factors (e.g. climate) at poleward boundaries and biotic factors (e.g. competition) at equatorward boundaries. Using correlative environmental niche models we test this hypothesis for 214 amphibian and reptile species endemic to the United States (U.S.). As predicted, we find a closer association between climate and northern (poleward) range limits than at southern (equatorward) boundaries. However when we separately analyze amphibians and reptiles, only reptiles show the predicted pattern; amphibians show the opposite pattern. We also find more unoccupied, but climatically habitable, area beyond species’ southern range limits for reptiles but not amphibians. This suggests that factors other than climate limit distributions at southern boundaries for reptiles and at northern boundaries for amphibians. These contrasting results suggest that even in the same biogeographic regions, this macroecological hypothesis does not hold. Further studies should investigate, preferably via experimental approaches, the proximate and ultimate mechanisms responsible for range limits.