Climate‐Driven Habitat Loss and Natural Fragmentation Increase Extinction Risk and Compromise Population Viability in Freshwater Fish

Climate change is one of the major drivers of the current extinction crisis. In this context, determining the probability of species extinction is crucial. In aquatic ecosystems, basin area is a key factor influencing extinction risk. Habitat fragmentation can significantly reduce river basin size and lead to both deterministic and stochastic extinctions. Deterministic extinctions occur when climatic conditions become unfavorable across the entire area of occupancy of populations, while stochastic extinctions result from temporal variability in population size driven by stochastic events. In this study, we developed an extinction model incorporating species presence probability, basin area, and biological parameters. Using species distribution modeling to estimate current and future species distributions, we applied the extinction model to freshwater fish species in France to quantify deterministic and stochastic extinctions as well as population viability under future climatic conditions. Our results showed a significant effect of climate change on deterministic extinction rates, depending on the species considered and their spawning thermal preferences. We also observed spatial patterns in extinction rates, with small coastal rivers being particularly impacted. Furthermore, our results showed an effect of climate change on population viability, especially for cold-water species. While our results did not indicate high extinction rates for most species, they revealed that cold-water species could be severely affected by habitat loss due to climate change and natural fragmentation, with potentially greater impacts under anthropogenic fragmentation. The application of this model could be highly valuable for identifying both priority areas and species of concern in conservation planning.