Ashley Cowell, Kyle Leathers, Guillermo de Mendoza, Grace Dwyer, David Herbst, Albert Ruhi

Climate change is projected to decrease snowpack and advance snowmelt in mountain ranges globally–including in California’s Sierra Nevada, where streamflow in snow-dominated rivers is expected to peak up to 2 months earlier, and remain in baseflow conditions for extended periods of time. Predators may be particularly sensitive to low flows, owing to their larger body sizes, higher metabolic demands, and longer lifespans. Experimental manipulations can help understand mechanistic links between future hydroclimates and stream predator persistence, fitness, or behavior–allowing us to anticipate how whole food webs are likely to respond to snow droughts. Here, we experimentally examined how the trophic ecology of predatory macroinvertebrates will respond to extended low flows expected under climate change. To this end, we manipulated flow regimes in a set of outdoor, large-scale artificial streams in the Sierra Nevada to produce a current flow regime and two future regimes that advanced low-flow conditions by three and six weeks, respectively. We sampled and performed stable isotope analysis on the macroinvertebrate community, focusing on three predatory taxa, i.e., Perlidae stoneflies (Plecoptera), Rhyacophila caddisflies (Trichoptera), and Turbellaria flatworms, as well as non-predatory invertebrates (n = 310 samples across eight taxa). We examined the isotopic signals (δ15N and δ13C) of these predators, and quantified how trophic niche areas, trophic positions, and energy sources (i.e., algal vs. detrital-derived energy) varied seasonally and across low-flow treatments. We found no change in reliance on algal (‘green’) vs. detrital (‘brown’) energy pathways following extended low flows. However, low flows did compress the trophic niches of predatory taxa, with variable responses across taxa and treatments. Predatory taxa exhibited changes in niche areas similar in magnitude and direction to other feeding groups, but they were the only group that was severely impacted by the most extreme low-flow treatment (62% niche compression). Further, gut content analysis showed that Perlidae stoneflies had fewer prey items in their guts under extended low flows. Our experiment suggests that climate-driven low flows will likely shift invertebrate predators’ diets, and thus food-web structure, in Sierra Nevada streams. However, species-specific dietary changes illustrate the importance of response diversity, and thus the need to preserve diverse alpine stream assemblages in the face of future hydroclimates.