Species with broader environmental tolerances are expected to be more widely distributed than specialist species, leading to the prediction of a positive correlation between niche breadth and geographic range size. This relationship is often evaluated based on data derived from the broad-scale geographic distributions of species. Spatial autocorrelation in both species distribution data and environments may inflate the strength of niche breadth-range size relationships, bringing into question the causal relationship between environmental tolerance and range size. Using null models, we aim to quantify the contribution of spatial autocorrelation to the frequently reported relationship between species’ range size and niche breadth.
Using phylogenetic least squares regression, we examined the extent to which variation in range size among species of Pelargonium from South Africa is statistically related to temperature and precipitation niche breadths. We developed null models that randomized the spatial distribution of the climatic variables, but retained their broad spatial autocorrelation structure. We then tested whether observed niche breadth-range size relationships were stronger than expected, given spatial autocorrelation in the climatic variables.
Results/Conclusions
We found the expected positive relationship between measures of niche breadth and range size, but the observed relationships were no stronger than expected based on our spatial null models. The influence of including spatial structure in simulations was to reduce expected niche breadths compared to simulations based on fully randomized environmental variables, resulting in steeper slopes for the simulated niche breadth – range size relationships.
Our results indicate that spatial autocorrelation may have a strong effect on niche breadth-range size relationships, suggesting that previously reported relationships between range size and niche breadth that have been derived from broad-scale distributional data may be, at least in part, artefactual. Future studies need to explicitly account for spatial autocorrelation, and inferences on the role of environmental tolerance in driving patterns of species range size variation should be derived in conjunction with laboratory and field-based experiments.