Understanding the responses of plants and animals to ongoing climate change is one of the most pressing questions for a practicing ecologist today. Over the past century, global climates have warmed and climatic models predict continued warming in the near future. Warmer temperatures will directly impact organisms given the thermal sensitivity of numerous physiological traits. One strategy for persistence, then, is for organisms to shift their geographic ranges by tracking climates to which their physiological processes are adapted. Here, we examine how thermal reaction norms for energy assimilation, which have been optimized along a latitudinal gradient, might contribute to future shifts in the geographical range of a species. Because daily, seasonal, and annual temperatures vary with latitude, we expect that selective pressures have generated distinct reaction norms within populations along a latitudinal gradient. Using 50 years of historical climate data, we modeled the optimal thermal reaction norms for energy assimilation for hypothetical populations that range over a latitudinal gradient in North America. We then used these locally adaptive reaction norms to predict the fundamental niches and potential ranges.
Resultant reaction norms varied in both their thermal optima and breadth, and thus their contribution to fitness over a particular range of temperatures. Thermal reaction norms become generalized to a broader range of temperatures at higher latitudes but confer reduced fitness at their thermal optima compared to those of individuals at lower latitudes. Such local adaptation of thermal reaction norms will limit the genotypes that will be able to either persist or redistribute to new geographic locations. We illustrate this concept by contrasting the potential ranges of genotypes from latitudinal extremes in light of both contemporary and warming climates.