Effects of niche properties on variation in geographic range size among species of western North American monkeyflowers

Background/Question/Methods

Though every species has a limited geographic distribution, range size can vary among species by several orders of magnitude. Properties of species’ ecological niches, defined here as the set of environmental conditions under which births exceed deaths, may explain differences in geographic range size among species. For example, if a species can persist under a broader range of environmental conditions, then it should be able to occupy a greater geographic area than a species with a narrower environmental tolerance. This hypothesis predicts a positive relationship between niche breadth and range size.

We conducted a comparative study across a group of closely related species of western North American monkeyflower (genus Mimulus) to test the prediction of a positive relationship between niche breadth and range size. We compiled ~17,000 georeferenced occurrence records for Mimulus species that occur in western North America. We then used these locality data along with climatic variables regarded as important to the survival and reproduction of Mimulus species to model the climatic niche of each species. For each species, we quantified niche breadth based on climatically suitable habitat and estimated various metrics of range size.  

Results/Conclusions

Regardless of how range size is quantified, our results strongly supported the hypothesis that range size increases with climatic niche breadth across species, even after correcting for phylogenetic non-independence and sampling effort. To experimentally test these results, we are now quantifying niche breadth in terms of survival and growth of individuals across a range of controlled environments in six pairs of closely related Mimulus species that differ in range size. This will allow for a mechanistic understanding of how broader niches may lead to larger ranges. Species with restricted distributions are thought to be more prone to chance extinctions than widely distributed species. Further, species with small ranges and/or narrow niche breadth may be more sensitive to climate change. Thus, understanding the relationship between physiology, niche characteristics, and range size will allow for better predictions of species’ responses to changing climate.