Ecosystem engineer restores trophic interactions: beavers alter landscape of risks and resources for megafauna

Ecosystem engineers hold potential to modify the environment and resource availability. The recovery of the Eurasian beaver reintroduces both a novel trophic component, as herbivore and prey, and an ecological driver that alters landscape of resources and risks. Together, the species comeback may trigger cascading effects on the ecosystem and shape relationships among megafauna, with contrasting effects on different trophic groups. We studied 30 beaver sites and paired reference sites in forests across Poland to assess how beaver presence affects habitat use and interactions among ungulates, large carnivores and mesocarnivores. With the same species composition and species richness, the number of detections of animals was lower at beaver sites, whereas detection duration was longer there. Structural equation modelling showed that beaver affects mammal community via a combination of direct and indirect pathways. The species presence had a positive direct effect on large carnivores, partly offsetting the negative indirect pathway mediated through reduced ungulate presence. These effects extended indirectly further to mesocarnivores, detection of which was positively related with increasing number of detections of ungulates and large carnivores. Beaver effects on habitat were spatially structured: canopy openness and deadwood abundance declined with increasing distance to water. Ungulates showed spatially diversified response at beaver sites, with the number of detections increasing away from the water, whereas large carnivores were detected most frequently near the water. Ungulates also changed seasonally their behaviour along water-land gradient, indicating a trade-off between resource acquisition and perceived predation risk. Our results show that beaver engineering reorganizes ecosystem through spatial and temporal changes in habitat use, behaviour and trophic interactions among large and medium-sized mammals. Beaver recovery may therefore restore ecological complexity by generating fine-scale mosaics of resources and risk that propagate across multiple trophic levels.