Limits to elevational distributions in two species of emberizine finches: disentangling the role of interspecific competition, autoecology, and geographic variation in the environment

When species' elevational ranges are wider where putative competitors are absent, researchers have concluded that interspecific competition influences elevational distributions. This overlooks the distinction between factors that limit distributions directly and factors that only influence organisms indirectly through covarying regulators or resources. Because elevation affects organisms indirectly, testing whether competition influences elevational ranges relies on the heretofore untested assumption that the relationship between elevation and factors influencing organisms directly is similar across geography. Focusing on Buarremon brush-finches (Aves: Emberizidae), a group in which distributions represent one of the best examples of the potential role of competition limiting elevational ranges, we show that when distributions are compared along axes of climatic variation, some patterns of elevational range variation do appear to be consistent with predictions of the hypothesis that release from competition underlies expanded elevational ranges in allopatry. However, other patterns of expanded elevational ranges in the absence of putative competitors are better explained by hypothesis related to species' autoecology and geographic variation in the environment. This latter finding cautions against using elevation uncritically as a dimension of ecological niches, and suggests that classical examples of interspecific competition may need re-evaluation.