Turning up the Heat on Hybridization: Implications for Cutthroat Trout Conservation

Hybridization between invasive rainbow (RBT) and native cutthroat trout (CT) 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 CT populations by facilitating nonnative RBT invasion across riverscapes.  We draw on results from multiple studies and diverse methodologies to describe our current understanding of how climatic variation and change influences introgression between RBT and CT.  Spatio-temporal patterns of introgression provide evidence that summer stream temperature and spring flow influence the prevalence of hybridization; introgression levels increase with warmer conditions and lower flow, both of which are predicted to be more common in coming decades.  However, intensive field-based studies demonstrate that natural selection against RBT introgression is strong, even in warm habitats.  Moreover, genome-wide patterns of introgression across thermal gradients in multiple watersheds suggest that natural selection against RBT hybridization may be widespread.  This paradox–pervasive hybridization between RBT and CT despite selection against RBT introgression–appears to be explained by RBT dispersal, which is overwhelming natural selection.   In other words, natural selection alone will not save CT from genomic extinction, especially in areas where source dynamics and climatic conditions can act synergistically.