Ecosystem engineers cause biodiversity spill-over: beavers affect breeding bird assemblages on both wetlands and adjacent terrestrial habitats

Abstract

The influence of ecosystem engineers on habitats and communities is commonly acknowledged in a site-bounded context, i.e. in places directly affected by the presence of the focal species. However, the spatial extent of the effects of such engineering is poorly understood, raising the question as to what impact they have on ecosystems situated beyond the species` direct influence. Beavers Castor spp., iconic ecosystem engineers, are capable of significantly transforming aquatic ecosystems. Their presence boosts biodiversity in near-water habitats, but as a result of cascading processes, beavers may affect terrestrial habitats situated beyond the range of their immediate activity. Our study investigates the breeding bird assemblage along a spatial gradient from the water to the forest interior on central European watercourses modified and unmodified by beavers. The results show that beaver sites are characterized by a higher species richness and abundance of breeding birds than unmodified watercourses. Such sites also host a different species pool, as 27% of the recorded bird species occurred exclusively on the beaver sites. The effect of the beaver`s presence on the bird assemblage caused this to spill over from the wetlands to adjacent terrestrial habitats located up to 100 m from the water`s edge, where the species richness and abundance was higher and the species composition was substantially modified. We also found a positive correlation between the total area of beaver wetland and the numbers of bird species and individuals recorded. Our study indicates that beaver activity has major consequences for ecosystem functioning that extend beyond the wetland, spilling over on to adjacent terrestrial ecosystems. These findings add to the general understanding of the spatial context of the ecosystem engineering concept, as the changes brought about by engineers have an influence beyond the area of their immediate occurrence. Our work also has implications for landscape planning and management, where existing beaver sites with terrestrial buffer zones may constitute a network of biodiversity hot-spots.