Habitat heterogeneity and food availability in beaver-engineered streams foster bat richness, activity and feeding.

As ecosystem engineers, Eurasian beavers (Castor fiber) modify aquatic and terrestrial ecosystems, which can benefit the biodiversity and community composition of plant and animal species. However, in contrast to aquatic taxa, beaver engineering impacts on terrestrial taxa, like bats, are so far largely overlooked. While it has been shown that bats prefer beaver-engineered ecosystems, the reason for this choice is poorly understood. We hypothesized that this preference may be associated with beaver-related changes in habitat characteristics and food availability. To address this knowledge gap, we recorded bat species richness, activity and feeding activity in eight beaver-engineered ecosystems (pool) with paired control sites without beavers (control) along the same stream in Switzerland. In addition, we collected data on food availability (arthropods) with arthropod flight interception traps and characterized habitat suitability with deadwood volume and vegetation surveys, as well as assessing canopy heterogeneity based on different digital height models. The nighly bat species richness increased from four to five species between control and pool sites. Bat activity increased 1.6 times and bat feeding activity 2.3 times in beaver-engineered systems compared to controls. These increases in richness and activity were explained by higher volumes of standing deadwood, higher canopy heterogeneity and higher arthropod abundance in beaver systems compared to controls. Overall, the volume of standing deadwood, a critical resource for bat roosting and foraging, had a stronger effect on bat species richness than canopy heterogeneity or arthropod availability. Bat feeding guilds (short-, mid-, long-range echolocators) responded differently to beaver-engineered habitat changes, with edge-hunting mid-range species benefiting the most. Our findings suggest that beaver engineering created structurally diverse habitats that supported a broader range of bat species. By modifying both habitat structure and prey abundance, beaver engineering affected bat activity, richness, and feeding activity directly and indirectly. These changes operated across aquatic-terrestrial boundaries, highlighting the cross-ecosystem influence and ecological complexity of ecosystem engineering.