Habitat fragmentation drives divergent survival strategies of a cold-water fish in a warm landscape

Climate change is a global phenomenon, but natural selection occurs within landscapes. Many global analyses predict how climate change will shape behavior and physiology, but few incorporate information from the landscape scales at which animals actually respond to selective pressure. We compared cold-water fish (redband trout Oncorhynchus mykiss newberrii) from neighboring habitats in a naturally warm, recently fragmented basin to understand how different responses to warming may arise from landscape constraints. Trout in warm, hydrologically connected Upper Klamath Lake fled summer temperatures and sought refuge in cool tributaries, while trout in an equally warm but fragmented reach of the Klamath River endured summer conditions. Trout in the river were more physiologically tolerant of high temperatures than trout in the lake across multiple metrics, including capacity for aerobic activity, recovery from exertion, and loss of equilibrium. Two independent metrics of energetic condition indicated that the behavioral strategy of trout in the lake came at a substantial energetic cost, while the physiological strategy of trout in the river was able to mitigate most energetic consequences of high temperatures. No clear genetic basis for increased tolerance was found in trout from the river, which may suggest tolerance was derived from plasticity, although our analysis could not rule out genetic adaptation. Our results show that landscape processes such as fragmentation can cause different climate survival strategies to emerge in neighboring populations. Connecting the mechanisms that favor similar survival strategies among related organisms at broad scales with mechanisms that drive landscape-scale variability within taxa should be a major goal for future predictions of biological responses to climate change.