Hybridization between invasive rainbow and native cutthroat trout is a conservation problem across the western United States. There is growing concern that climate change, particularly increasing summer stream temperatures, will exacerbate loss of remaining native cutthroat trout populations by facilitating non-native rainbow trout invasion across connected riverscapes. We draw on results from multiple studies and diverse methodologies to describe our current understanding of how climatic variation and change influences genetic admixture between rainbow and cutthroat trout.
Spatiotemporal patterns of admixture provide evidence that summer stream temperature and spring flow influence hybridization dynamics; the prevalence of rainbow trout hybridization increases with warmer conditions and lower stream flow, both environmental outcomes of climate change. However, intensive field studies demonstrate that natural selection against rainbow trout admixture is strong, even in warm habitats (mean selection coefficient against rainbow trout = 0.6). Genome-wide patterns of introgression spanning thermal gradients in multiple watersheds further suggest that natural selection against rainbow trout is widespread across populations and throughout the genome. Ultimately, climate change likely mediates but does not eliminate selection against rainbow trout; pervasive hybridization between rainbow and cutthroat trout appears to be explained by rainbow trout dispersal, which is overwhelming strong natural selection. Thus, climate change appears to be simultaneously eroding fitness in, and facilitating genomic extinction of, native cutthroat trout.