Ice Age Biogeography Corresponds With Current Climate Vulnerability of Freshwater Fishes

Both local environmental factors and historical biogeography shape ecological communities, but determining which historical biogeographical patterns correspond with contemporary climate vulnerability is an underused conservation method. The historical colonisation patterns of freshwater fishes following the Pleistocene (‘Ice Age’) glaciations offer an ideal model for comparing historical biogeography and climate‐change vulnerability. We used current thermal niches and future stream‐temperature projections to estimate the climate vulnerability of 29 Great Plains and Rocky Mountain fishes that we classified as either early or late colonists of the region in the wake of glacial retreat (~19,000 years ago). Ninety‐three percent of the most vulnerable species were amongst the earliest colonists of the region and we consider ‘postglacial‐pioneer species’. Median predicted site loss (number of historically occupied sites predicted to become too warm by end‐of‐century) was 0% for late colonising species and 33% for early colonising species. We provide empirical evidence that postglacial‐pioneer fishes are uniquely vulnerable to climate change, and we suggest this may apply to many taxa from formerly glaciated regions. More broadly, we demonstrate that evaluating the relationship between current species‐environment patterns and historical biogeography may be a fruitful avenue for future climate change and conservation research.