Testing Assumptions about the Relationship between Geographic Range and Climatic Tolerance using In-Situ Observations, Ex-Situ Experiments, and Species Distribution Modeling

Climate change is predicted to result in large losses in biodiversity, igniting a flurry of studies on how and where species ranges will change. At highest perceived risk for extinction are species with small geographic ranges. This is because it is assumed that species with small geographic ranges have narrower climatic tolerances than species with large geographic ranges. This assumption fails to acknowledge the influence of factors other than climate on determining the range limit of a species. Species interactions, soil specificity, and dispersal to name a few, may play significant roles in limiting range sizes. If a species is thought to have a smaller climatic tolerance than is true, we may underestimate future potential suitable habitat and overestimate extinction risk. I used three methods to test the hypothesis that species with smaller ranges have narrower climatic tolerances than species with larger ranges: an ex-situ experiment that tested biomass and reproductive success responses of species to water and temperature treatments, an in-situ observational study where I monitored changes in population density and reproductive success between years, and a modeling study that examined the relative effect of including temporal climatic variation on projected suitable habitat. When I subjected species with small and large ranges to a range of water and temperature treatments, trends in biomass and reproductive fitness responses across treatments were not significantly different between species of differing range size. Population measures of density and reproductive success in the field provided similar results. However, inferences of relative climatic tolerances based on results had less evidentiary support. Finally, using species distribution models, I projected potential suitable habitat based on changes in precipitation and temperature values. This approach is meant to illustrate how including climatic variation would differentially impact resulting projections for small-ranged and large-ranged species. I found that small-ranged species have a greater proportional difference between projections that include examples of climatic variation and projections that do not, relative to large-ranged species. Each of these three studies contribute to a novel approach in examining the climatic tolerances of species. It is my hope, that the outcome of studies such as these three, contribute vital information that may improve efforts for biodiversity conservation.