Context-dependent effects of beavers (Castor spp.) on freshwater pearl mussels (Margaritifera margaritifera) in European rivers

Recovery of the Eurasian beaver across Europe has highlighted questions about its potential influence on the endangered freshwater pearl mussel. Both are protected ecosystem engineers. Their ecological niches overlap only minimally, intersecting geographically but diverging functionally with their specific habitat requirements opposed. This study explored the professional experiences of experts working with one or both species to explore examples of these species interacting. Small, fragmented mussel populations in low-gradient channels appear most vulnerable to negative impacts of beaver dam building, for example, sediment accumulation, reduced flow, and altered fish passage. Larger, better connected mussel populations appear more resilient, and the longer term benefits for mussels through stabilized substrates, improved flows, and resilience to climate change are explored. Tools such as the Beaver Habitat Index and Beaver Dam Capacity modeling could identify areas of potential distribution overlap and guide targeted interventions. Adaptive management can minimize short-term risks while promoting natural ecosystem processes that support both species.