We used ecological niche modelling to study the relative roles of climate and interspecific interactions in defining the parapatric contact between closely related species (
Crotalus mitchelliiand C. stephensi) and to predict refugia during the last glacial maximum. The modelled suitable habitat for C. stephensicorrectly predicts the existing parapatric border between it and C. mitchellii, suggesting that C. stephensi's range at the border is limited by climatic factors. In contrast, the suitable habitat for C. mitchelliidoes not correctly predict the existing parapatric boundary; rather the suitable habitat of this species extends into the range of C. stephensi, suggesting the latter species, not climatic factors, limit the range of C. mitchellii. Modelled refugia of C. stephensiare much smaller than modern suitable habitat and are partially situated at the current parapatric border, whereas the refugia of C. mitchelliiare similar to its current suitable habitat, though also shifted to the south. Ecological niche modelling appears to be a useful tool for studying the interplay between climate and competition in determining boundaries between parapatric species. It also appears to be useful for predicting past suitable habitats of species, because predicted refugia are congruent with independent estimates from molecular phylogeography.