Climate-related shifts in species ranges have been documented around the globe. Accurate prediction of future range shifts is important for natural resource management and conservation—and the maintenance of human welfare and well-being—in the 21st century. However, the extent to which temperature change drives range shifts remains unresolved and likely varies among systems, regions, and taxa. We used a spatiotemporal model to standardize three to five decades of annual survey data on over 100 marine fish and invertebrate range edges from the Northeast, the West Coast, and the Eastern Bering Sea. We characterized edge dynamics, quantified temperature extremes at species’ range edges (representing the realized thermal niche), and tested to what degree these “edge thermal niches” were conserved over time. We investigated three hypotheses for range edge changes: the thermal niche hypothesis that range edges track temperature change near-instantaneously; the partial tracking hypothesis that range edges track temperature change to a limited degree due to the influence of demographic and eco-evolutionary processes such as extinction debt, immigration credit, and adaptation; and the temperature-independent hypothesis that range edges are mediated by non-temperature processes such as biotic interactions and dispersal limitation and do not track temperature change.
Results/Conclusions
The majority of species’ range edges (82%) conformed to the thermal niche hypothesis, indicating that they were found at consistent temperatures over decades even as the oceans changed around them. These range edges typically maintained either a consistent warm or cold extreme temperature, supporting theoretical predictions that range edge positions are rarely simultaneously limited by both summer and winter temperatures. A higher proportion of cold range edges fell into this group than warm edges (97% versus 70%). Many of the warm range edges that were explained by the partial tracking or temperature-independent hypotheses actually contracted poleward into cooler waters, which may represent a response to climate change even if it is not classified as edge thermal niche conservatism. Testing for edge thermal niche conservatism rather than spatial shifts in edge position allowed us to detect temperature tracking in all regions despite their disparate historical temperature trends, and to categorize range edges by their degree of temperature tracking. This novel method for differentiating range edges that are tracking temperature from those that are not can be applied to refine species-specific predictions of future range shifts.