Ecosystem-engineered infections: beaver-modified wetlands are associated with conflicting drivers of amphibian pathogen prevalence.

Beavers are ecosystem engineers and keystone species that protect freshwater resources and increase biodiversity. Beaver reintroductions are promoted for amphibian conservation, yet their impact on (Bd), a pathogen linked with amphibian population declines worldwide, remains unclear. We investigated the abiotic and biotic drivers of Bd prevalence in Columbia spotted frogs () and western toads () in 20 beaver-modified and 23 non-beaver wetlands in Glacier National Park, USA. We found that beavers increased wetland hydroperiod, which was associated with higher Bd prevalence. However, beavers also reduced wetland canopy cover, which was associated with lower Bd prevalence. Our models also predicted higher Bd prevalence associated with higher adult density of both species of amphibians, although species' densities were similar in beaver-modified and non-beaver wetlands. These results suggest that beavers have a cumulatively negligible net effect on Bd prevalence owing to their effects on both hydroperiod and canopy cover, which is encouraging for amphibian conservation. Our findings also suggest that decreasing canopy cover may be a potential management option to reduce Bd prevalence. In addition, these findings indicate that beaver-mimicking restoration projects may harm amphibian populations if they increase wetland hydroperiods without reducing canopy cover.