An evaluation of parapatric distributions among ecologically similar rattlesnakes (Viperidae: Crotalus) in North American warm deserts

Abstract

Abstract Parapatric distributions between ecologically similar species have the potential to reveal the effects of limiting similarity in large-scale patterns of species coexistence. We present a qualitative model that predicts three different types of parapatric boundaries between competing species pairs: gap parapatry, abrupt parapatry and narrow sympatry. Our model integrates both niche-based and neutral theory perspectives of coexistence in the context of climatic tolerances, species equivalence, contingencies of dispersal history, priority effects and a heterogeneous patchwork of optimal and suboptimal habitat. Using species distribution models, tests of niche overlap and phylogeographical data, we evaluated assumptions and conditions of this model for four species of mostly allopatric, rock-dwelling rattlesnakes (Viperidae: Crotalus) inhabiting arid mountain ranges of the Mohave–Sonoran desert system of western North America. Our model was adequate for predicting the different types of range boundaries observed across this guild of ecologically similar species. Where species distributions were separated by a steep climatic gradient without evidence of recent dispersal, we observed gap parapatry; where historical dispersal was asymmetric, we observed abrupt parapatry; and finally, where historical dispersal was symmetric, we observed narrow sympatry.