Riparian wetland creation using beaver dam analogues improves threatened salmon populations amidst severe drought

Climate impacts on biological systems are typically considered in isolation, but they are exacerbated by other human stressors. This suggests that alleviation of human stressors may recover lost biological climate resilience. However, real-world examples that document such interventions are rare. Coho salmon in California are an iconic species that requires cool, wetted habitat in a warm, drought-prone region. Extensive beaver trapping in previous centuries has left this landscape unnaturally lacking in wetlands that maintain cool, wetted habitats resilient to drought. Loss of climate resilience is especially concerning because California is both warming and experiencing severe droughts. Here, we restored wetlands using beaver dam analogues, which increased estimated coho salmon habitat capacity and coincided with increased juvenile survival and adult returns. Moreover, adult returns were less impacted by drought when juveniles used restored habitats. Remarkably, these responses coincided with a severe drought that collapsed coho salmon populations in neighboring, unrestored systems and the total adult returns of four monitored tributaries in the Klamath basin became dominated by the Scott River population. These data suggest that restoration of once abundant slow-water wetland habitat increased population performance overall and mitigated impacts of climate stress. Broadly, these results underscore the need to consider the land use history of ecosystems to recognize changes that have increased climate sensitivity to certain species and, by implication, paths to bolster systems against climate stress.