Beaver Dams Are Associated With Enhanced Amphibian Diversity Via Lengthened Hydroperiods And Increased Representation Of Slow-Developing Species

Land managers are increasingly using beavers to restore hydrological function, provide wildlife habitat, and mitigate the effects of climate extremes on water balances and ecosystems. Although North American beavers (Castor canadensis) and Eurasian beavers (Castor fiber) both hold great potential for landscape-scale benefits, more information about the interactions between beavers and wildlife is necessary to maximise the ecological benefits and minimise the social and ecological costs of beaver-centred management. Beaver dams create large, deep pools with long hydroperiods, which could benefit aquatic and semi-aquatic species, especially pond-breeding amphibians, which breed in still and slow-moving water.We studied the relationship between beaver dams and pond-breeding amphibians in the southern Washington Cascade Range of the north-west U.S.A. by surveying 29 beaver-dammed and 20 undammed lentic sites in three mid-elevation (560-1,010 m) spatial blocks.We found that mean amphibian species richness was 2.7 times higher in dammed sites than in undammed sites (2.4 vs. 0.9 species). This increase in species richness was driven by increased occupancy of slow-developing species-red-legged frogs (Rana aurora) and northwestern salamanders (Ambystoma gracile)-which were also more abundant at sites with dams. These two species were detected almost exclusively in beaver-dammed sites, suggesting that some amphibians rely heavily on beaver-dammed sites for successful reproduction in areas such as our study blocks where ponds and wetlands with long hydroperiods are otherwise scarce. Species with highly variable development periods-long-toed salamanders (Ambystoma macrodactylum) and rough-skinned newts (Taricha granulosa)-showed nonsignificant trends of higher site occupancy and, in the case of newts, higher number of mating events per hectare in dammed sites compared to undammed sites.Compared to undammed sites, dammed ponds were consistently deeper and had longer hydroperiods, consistent with beavers benefitting slow-developing amphibians primarily by increasing the quality of lentic breeding habitat. We suggest that slow-developing amphibians and some variable-rate developers might benefit greatly from beaver restoration, especially in areas where climate change is predicted to reduce summertime water levels. Beavers could therefore be useful and important components of ecosystem-based restoration, management, and climate adaption, especially in parts of their native ranges in North America or Eurasia predicted to undergo climatic drying.