Linking fire, food webs, and fish in stream ecosystems
As wildfire regimes shift, resource managers are concerned about potential threats to aquatic ecosystems and the species they support, especially fishes. However, predicting fish responses can be challenging because wildfires affect aquatic ecosystems via a myriad of different pathways, requiring whole-ecosystem approaches that account for these different mechanisms. We applied a dynamic food web simulation model that mechanistically linked stream trophic dynamics to the multiple effects that wildfires can have on aquatic and riparian ecosystems. We simulated how different fire severities may influence short- (months to years) and long-term (years to decades) periphyton, aquatic invertebrate, and fish biomass responses to wildfires in headwater streams of western Pacific Northwest (USA) forests. Model simulations suggest that wildfires can have diverse effects on temporal patterns of aquatic productivity, with response trajectories that vary extensively in their direction (i.e., positive or negative), magnitude, and duration depending on fire severity, time since fire, and trophic level. In many cases, wildfire increased modeled periphyton, invertebrate, and fish biomass over both short (months-years) and long (years to decades) time periods. The shape of these response trajectories, however, were strongly influenced by a variety of wildfire effects, with water temperature, vegetation cover, and water turbidity all identified as drivers that negatively affect fish biomass. Our analyses suggest a single fire could result in a range of different aquatic ecosystem responses, especially in watersheds with mixed burn severity. Collectively, our simulations highlight the utility of whole-ecosystem approaches, like food web modeling, for improving our understanding of the mechanisms linking fire and food webs and for identifying contexts where fires could have deleterious impacts on fishes.