Wildfires are a significant disturbance affecting aquatic ecosystems in the western United States, with an estimated 11% of streams impacted within the past 30 years (1). Despite extensive research on wildfire effects, the specific mechanisms by which fires influence aquatic systems, such as increased sedimentation and reduced riparian cover, remain highly system-dependent and poorly understood. As wildfire size and severity are projected to increase under anthropogenic climate change (2), improving mechanistic understanding of fire effects on aquatic ecosystems is increasingly important. Aquatic macroinvertebrates play a key role in stream ecosystem function and often respond to wildfire in predictable ways, making them an effective indicator of wildfire disturbance (3).
To address this knowledge gap, we combined macroinvertebrate data from the Bureau of Land Management’s (BLM) Assessment, Inventory, and Monitoring (AIM) program with spatial fire datasets to construct a large (N > 600) dataset of stream reaches across BLM managed land in Idaho, including standardized community data, local environmental variables, and fire disturbance metrics. We used this dataset to explore relationships between wildfire disturbance and macroinvertebrate community composition.
Because our ultimate goal was to develop a Structural Equation Model (SEM) describing causal pathways of fire effects, we first identified broad-scale drivers of community structure independent of fire using multivariate analyses. PERMANOVA indicated that geographic location, represented by field office, explained approximately 15% of variation in community composition (p < 0.001). Distance-based redundancy analysis (dbRDA) then identified percent overhead cover and stream temperature as the strongest predictors of community composition. These variables will be incorporated into an SEM to evaluate whether higher fire disturbance (characterized by greater percent of watershed burned, closer proximity to fire, and more recent fires) indirectly influenced macroinvertebrate communities through changes in overhead cover and sedimentation, measured as a decrease in median particle size (D50). Given the complex and indirect pathways through which wildfires affect aquatic ecosystems, this study highlights the value of spatially extensive, long-term monitoring datasets for disentangling fire-driven mechanisms and guiding more targeted hypothesis development.