The Interactive Threats of Climate Change and Fragment Size to Native Cutthroat Trout Conservation

Impending changes in climate will combine with other stressors to threaten aquatic ecosystems and biota.  Native Colorado River cutthroat trout (CRCT; Oncorhynchus clarkii pleuriticus) are now relegated to 309 isolated high-elevation (>1700 m) headwater stream fragments owing to nonnative trout invasions and habitat loss.  Predicted changes in climate (i.e., air temperature and precipitation) and resulting changes in physical processes (i.e., wildfire, debris flow, and channel drying/freezing) could further exacerbate threats to CRCT populations.  We use spatially explicit predictions of stream temperature for the remaining fragments of CRCT populations and a Bayesian Network (BN) model to examine future population persistence across the upper Colorado River basin.  Using our BN to integrate the combined threats of changing lotic thermal conditions, stochastic events, and small population size we find that decreases in the viability of these CRCT populations varies across the landscape.  Overall, the interactions of stochastic events with fragmentation are projected to be greater threats than warming for CRCT populations.  These threats mainly stem from past nonnative trout invasions which have restricted CRCT to high elevation stream fragments that are less susceptible to warming, but at risk of headwater freezing or drying.  These tools hold promise for planning more effective conservation strategies.