When beavers get burned, do fish get fried? The role of beavers to mediate wildfire effects on arctic grayling in boreal Alaska

Abstract

Wildfire is a dominant natural disturbance process throughout boreal North America and fires are increasing in size, frequency, and severity. However, little is known about how wildfire affects boreal fish populations and aquatic habitat despite the substantial impacts of fire on ecosystem processes, and even less is known about how fire effects are mediated by species interactions. For example, North American Beavers (Castor canadensis) are affected by and can influence wildfire dynamics, and their engineering has complex effects on aquatic habitats. North American Beavers therefore have the potential to mediate wildfire effects on aquatic ecosystems and fish populations. Here I investigated relationships between wildfire and the distribution of beavers and a common fish species across a fire-dominated riverscape in Interior Alaska. First, I used satellite imagery to locate and enumerate beaver ponds throughout five large watersheds (total area: 20,711 km²) and modeled the relationship of beaver pond density (ponds per km²) as a function of wildfire history, stream geomorphology, hydrology, and vegetation composition. I then used a simulation to conceptualize the impacts of wildfire and beaver dams on Arctic Grayling (Thymallus arcticus) habitat availability under variable hydrologic conditions. Next, I sampled 62 streams for Arctic Grayling environmental DNA (eDNA) and sampled 10 of those streams for Arctic Grayling abundance. I used a generalized linear model (GLM) and N-mixture model to understand the relationship between eDNA concentration and Arctic Grayling abundance and distribution throughout the study area. I found that wildfire metrics explained most variation in beaver pond density (pseudo R² = 0.75) across the landscape and were positively associated with beaver pond density, although geomorphological and hydrological parameters were also important. My simulations indicated that beaver dams can create substantial barriers to fish dispersal during low water conditions (up to 20% reduced habitat availability in some river basins) and can severely reduce (up to 65%) habitat availability in some tributary streams. I found that eDNA concentration was moderately correlated with Arctic Grayling abundance (GLM: pseudo R² = 0.45) and unexplained variation was likely due to the spatial mismatch between fish sampling and scale of eDNA representation. However, I estimated eDNA residence time of about 6.7 hours in one stream, and eDNA appeared to accumulate longitudinally throughout the tributary, indicating that an eDNA sample near the downstream end was likely a good relative representation of Arctic Grayling abundance in a tributary. Results from the N-mixture model indicated that stream geomorphology and hydrology were the most important predictors for Arctic Grayling abundance (eDNA concentration), wildfires had a negative effect, and beaver dam density had a mixed effect on Arctic Grayling abundance. Overall, this study illustrated that beaver densities can increase after wildfires in Interior Alaska stream networks, which could result in negative impacts on Arctic Grayling habitat availability if beavers impair fish passage; however, these effects are dependent on the environmental context and suggest beaver-fish interactions may be best managed on a case-by-case basis.