Linking fire and fish: the importance of a whole-ecosystem perspective
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Wildfires can have complex and varying effects on aquatic ecosystems depending on fire characteristics and the ecological context of watersheds. This variability creates challenges for predicting fire effects on species of social and conservation interest, like salmonid fishes (Oncorhynchus spp.). As fire regimes shift, resource managers need to identify where wildfires pose risks to aquatic habitats and fish. To predict these vulnerabilities effectively, a deeper understanding of how and why fires influence aquatic ecosystems can be informative. For example, light availability and temperature in streams often increase after fires, accompanied by increases in sediment, carbon and nutrient transport, but the nature of these changes and how they interact depends on context and can have varied effects on aquatic biota. Whole-ecosystem perspectives are essential to synthesize aquatic responses to fire and identify the mechanisms driving those responses. In our presentation, we will describe two approaches using whole-ecosystem perspectives to better understand wildfire effects on aquatic systems and species in the Pacific Northwest. First, we applied food web simulation models to explore how wildfires influence aquatic ecosystems across multiple trophic levels. Our model simulations enabled us to explore how variation in fire severity can influence aquatic ecosystem responses in forested headwater streams of the Pacific Northwest, highlighting the pathways driving those responses. Preliminary results from model simulations revealed the importance of stream temperature as a major driver, and indicated that wildfires can have diverse effects on aquatic ecosystems, varying extensively over time, with fire severity, and across trophic levels. Second, we conducted a post-fire, watershed-scale field study in the Hinkle Creek watershed in western Oregon—an area intensively monitored in the early 2000s that burned at high severity as part of the Archie Creek Fire in 2020. Preliminary data from this unique pre- and post-fire comparison have revealed some surprising responses. For example, post-fire reductions in riparian canopy cover by 48-63% are associated with increases in post-fire summer stream temperatures by 6°C, yet salmonid populations have shown resilience, not only surviving but increasing in abundance by 1.5-2x relative to pre-fire levels. Additional post-fire data suggest potential mechanisms driving these fish responses, including spatial variation in stream temperatures and post-fire increases in aquatic productivity. Collectively, these studies highlight the value of whole-system approaches, such as food web modeling and post-fire watershed-scale studies, to better understand the mechanisms linking fire and fish.