Understanding the dynamics of species distributions under climate change: linking empirical evidence, ecological theory, and statistical models

Climate change is threatening biota all over the world through dictating changes in species distributions. The ability to predict the effects of climate on species distributions requires an understanding of how climatic and disturbance factors relate to species distributions. I examined the relationship between climate, disturbance, and geographic distributions of vascular plants in the western United States. Within the mountain ranges of California, woody plants have largely shifted their distributions downhill during the 20th century. These shifts were strongly related to observed changes in climatic water balance, consistent with theoretical predictions. Downhill shifts were most prominent for species occurring at higher elevations where environmental conditions were marginal. Downhill shifts were observed at nested spatial scales where changes in climatic conditions were consistent, but at all scales were unrelated to species ecological traits. Niche models based entirely on climatic variables were highly accurate both within and across time periods. The inclusion of fire occurrence as an additional explanatory variable did not affect model likelihood or niche model transferability, due primarily to climatic influences on fire occurrence. Departure from historical return interval influenced total model likelihood when it was included as an additional explanatory variable for species adapted to high frequency return intervals, but did not improve model transferability for most species. Regeneration of tree species occurred within a narrower range of climatic conditions than adults. Species with greater niche breadth displayed greater differences between adult and regeneration niches. Both adults and juveniles displayed higher sensitivity to climatic variables representing mortality risk than to variables representing growth potential. Spatially explicit models of regeneration indicated overestimation of tree ranges based on niche models and the potential for future range contractions. These results were consistent across species