Identifying the drivers of aquatic ecosystem vulnerability to wildfires in the Pacific Northwest - AGU 2023

Wildfires can have complex effects on aquatic ecosystems that vary widely depending on the characteristics of the fire and the ecological context of the watershed, making predictions of fire effects on species of social and conservation interest, like salmonid fishes (Oncorhynchus spp.), difficult. As fire regimes shift, resource managers want to predict where on the landscape wildfires pose a risk to fish and aquatic habitats. However, before we can effectively predict where aquatic systems will be vulnerable to wildfires and shifting fire regimes, we first need to understand how and why fires influence aquatic ecosystems. As a result, new approaches are needed that can synthesize aquatic ecosystem responses to fire and identify the mechanisms driving those responses. Here we applied food web system-dynamics models to explore how wildfires influence aquatic ecosystems across multiple trophic levels via the physical and biological processes that support them. To illustrate these concepts, in this analysis we ran model simulations to explore how variation in fire severity influenced aquatic ecosystem responses in headwater streams of western forests of the Pacific Northwest (USA) and the associated pathways driving those responses. Model simulations indicated that wildfires had diverse effects on aquatic ecosystems that varied extensively through time, with fire severity, and across trophic levels. Sensitivity analyses identified the individual pathways leading to collective behavior as predicted by the model. Stream temperature more so than any other individual variable considered in the model appeared to drive fish vulnerability to fire. Collectively, these simulations highlight the utility of whole system approaches like food web modeling to understand the mechanisms linking fire and fish. Such approaches can lend unique insights that can be applied to update our conceptual models of how fires can influence aquatic ecosystems, direct future empirical studies, as well as act as decision support tools for resource managers to help guide pre- and post-fire management actions on the landscape.

Impact/Purpose

Fish, as cold-blooded organisms, are strongly influenced by the temperature of the water they swim in.  Wildfires can affect fish and other aquatic life in many ways, but one of the most important mechanisms is through increased water temperatures following loss of shade following fire.  Food availability for aquatic life can also change after a fire, and can mediate the adverse effects of warmer water temperatures in some situations.  For this reason, it is important to consider not just the physical effects of fire, but also the larger ecosystem effects that influence the entire aquatic food web, when evaluating fire effects on fish.  This presentation describes an objective and transparent approach for doing this.

Citation

Roon, D., J. Benjamin, J. Bellmore, F. Robinne, K. Bladon, R. Flitcroft, J. Compton, J. Ebersole, AND J. Dunham. Identifying the drivers of aquatic ecosystem vulnerability to wildfires in the Pacific Northwest - AGU 2023. American Geophysical Union, San Francisco, CA, December 11 - 15, 2023.