Within-taxon niche structure: niche conservatism, divergence and predicted effects of climate change

Several studies have observed that taxa below the level of species can vary in the degree to which they differ from one another in the environmental space they occupy. These patterns of within-species niche variation raise the question of whether these differences should be considered when developing models for predicting the potential effects of climate change on species distributions. We address this question with two divergent datasets, one on sister species and subspecies from the European herpetofauna, the other on subspecies of breeding birds in North America. Atlas and observation data come from the Atlas of Amphibians and Reptiles in Europe and the North American Breeding Bird Survey, respectively. We develop boosted regression tree models of climate-distribution relationships and project the predicted geographic range of each taxon using interpolated weather station data and modeled climate for the year 2080. We find differences between models that distinguish the contributions of subtaxa and those that do not, in terms of prediction of both current and future distributions. In comparison to models that ignore sub-taxon structure, models that incorporate this structure generally predict larger areas of suitable conditions, consistently perform better, if only marginally, as measured by cross-validated AUC, and can reveal divergent potential effects of climate change on subtaxa. Differences in niche occupancy and predicted distribution appear between closely related taxa regardless of their phylogenetic distinctness. For these reasons, information on subtaxon membership and phylogeographic structure should be included in modeling exercises when available, in order to identify both the contribution of these units to the niche occupancy of species and the potentially distinct responses of subtaxa to climate change.