Beyond beaver wetlands: The engineering activities of a semi-aquatic mammal mediate the species richness and abundance of terrestrial birds wintering in a temperate forest

Abstract

The engineering activities of the Eurasian beaver Castor fiber have far-reaching effects on the components of an environment and therefore modify the functioning of the ecosystem. The wetlands thereby created are the most conspicuous effect of beaver activity and attract water-related species. However, there is some evidence suggesting that beavers influence not only aquatic ecosystems but also the terrestrial habitats adjacent to these wetlands and the organisms occurring there. Because the impact of beavers on terrestrial birds is still poorly understood, this study evaluates the assemblage of birds wintering on beaver sites (N = 65) and paired reference sites (N = 65) in temperate forests of central Europe. We investigated the correlations between beaver presence, parameters of wetland areas, terrestrial vegetation characteristics, distance from the water’s edge and bird species richness and abundance. We found a greater species richness and abundance of wintering birds on beaver sites than on watercourses unmodified by this ecosystem engineer (by 38% and 61%, respectively). Species richness and abundance were higher in the terrestrial habitats near the edges of beaver ponds, but for some species this tendency also held in forests growing at some distance from beaver wetlands. Greater species richness was related to beaver presence, but also increased with a more open canopy and greater forest floor diversity, whereas bird abundance was correlated only with canopy openness. The beaver sites attracted primary cavity nesters, secondary cavity nesters and frugivorous species. This study provides evidence that the engineering activities of beavers during the growing season have a delayed cascading effect on the richness and abundance of the bird assemblage in terrestrial habitats in winter. This indicates that beaver ecosystem engineering should be seen as having a potential for carry-over effects, in which the consequences of beaver activity become apparent in subsequent seasons. Birds are considered to be ecological indicators, so our results highlight the importance of beavers for the distribution of terrestrial organisms at the local scale, and therefore the functioning of ecosystems beyond the immediate wetland area.