Trout and invertebrate assemblages in stream pools through wildfire and drought

Climate change is increasing the frequency, severity, and extent of wildfires and drought in many parts of the world, with numerous repercussions for the physical, chemical, and biological characteristics of streams. Yet information on how these perturbations affect top predators and their impacts on lower trophic levels in streams is limited.

The top aquatic predator in southern California streams is native Oncorhynchus mykiss, the endangered southern California steelhead trout (trout). To examine relationships among the distribution of trout, environmental factors, and stream invertebrate resources and assemblages, we sampled pools in 25 stream reaches that differed in the presence (9 reaches) or absence (16 reaches) of trout over 12 years, including 8 reaches where trout were extirpated during the study period by drought or post-fire flood disturbances.

Trout were present in deep pools with high water and habitat quality. Invertebrate communities in trout pools were dominated by a variety of medium-sized collector-gatherer and shredder invertebrate taxa with non-seasonal life cycles, whereas tadpoles and large, predatory invertebrates (Odonata, Coleoptera, Hemiptera (OCH)), often with atmospheric breather traits, were more abundant in troutless than trout pools.

Structural equation modeling (SEM) of the algal-based food web indicated a trophic cascade from trout to predatory invertebrates to collector-gatherer taxa and weaker direct negative trout effects on grazers; however, both grazers and collector-gatherers also were positively related to macroalgal biomass. SEM also suggested that bottom-up interactions and abiotic factors drove the detritus-based food web, with shredder abundance being positively related to leaf litter (CPOM) levels, which, in turn, were positively related to canopy cover and negatively related to flow. When compared to the literature, these results emphasize the context dependency of trout effects on prey communities and the relative importance of top-down versus bottom-up interactions on food webs, contingent on environmental conditions (flow, light, nutrients, disturbances) and the abundances and traits of component taxa.

Invertebrate assemblage structure changed from a trout to a troutless configuration within a year or two after trout were lost owing to post-fire scouring flows or drought. Increases in OCH abundance after trout were lost were much more variable after drought than after fire. The reappearance of trout in one stream where they were lost resulted in quick, severe reductions in OCH abundance.

These results indicate that climate-change induced disturbances can result in the extirpation of a top predator, with cascading repercussions for stream communities and food webs.  This study also emphasizes the importance of preserving or restoring refuge habitats, such as deep, shaded, perennial, cool stream pools with high habitat and water quality, to prevent the extirpation of sensitive species and preserve native biodiversity during a time of climate change. 

Methods
Sampling of stream invertebrates, observations of trout, and measurements of stream abiotic factors.