Assessing the Impacts of Beavers on Biodiversity and Water Quality in Class II Trout Streams in Northwest Wisconsin

Abstract

North American beavers (Castor canadensis) are a much maligned and aggressively managed species in Wisconsin, particularly on classified trout streams. However, there is a substantial lack of data and research in the region to support current management practices. Furthermore, there has been no consideration of beaver impacts to species other than trout. I assessed biodiversity and water quality at six pairs of beaver and control stream sites in western Wisconsin between May and October 2024. I used bioacoustic recording units to assess both bat and avian diversity and camera traps to capture species utilization of beaver ponds, dams, and control stream reaches. I collected water depth, dam length, water temperature, turbidity, and conductivity readings for all sites on four separate collections across the study period. Overall biodiversity measures were not statistically significantly different between beaver and control sites, but bat activity was 73% (27%, 119%) higher at beaver pond locations (𝑡𝑡 = 4.0, 𝑑𝑑𝑑𝑑 = 5, 𝑝𝑝 = 0.005). Based on linear mixed effect models, temperatures in beaver ponds and downstream of ponds were not significantly different than control streams, and precipitation was the only statistically significant factor impacting water turbidity values (𝑝𝑝 = 0.009). Evidence that beaver activity does not significantly increase water temperatures, particularly in reference to trout thermal tolerances, indicates that perceived negative impacts of beavers on stream temperatures are not supported. Furthermore, novel evidence of a link between beaver ponded locations and increased bat activity suggests that beavers can be leveraged as tools to support bat conservation efforts.