Ecosystem engineers cause biodiversity spill-over: Beavers are associated with breeding bird assemblages on both wetlands and adjacent terrestrial habitats

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 adjacent aquatic and riparian 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 extended 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 adds 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 hotspots.

Beaver-related restoration: An opportunity for sandy lowland streams in a human-dominated landscape

The expansion of beavers into human-dominated landscapes can help improve the ecological status of degraded streams. This study aimed to assess the changes in the ecological status of a degraded sandy-bottom stream under the influence of processes generated by the activity of a European beaver. We assumed that the processes in the beaver dam-and-pond complex significantly improved the physical, chemical, and bacteriological properties of water and improved the ecological status of a small lowland stream in a human-dominated landscape. To achieve these objectives, water for quality assessment, benthic invertebrates, and fish samples were collected, and the forms of land use and the natural values of the stream were analysed. In the upstream zone of the sandy-bottom stream flowing in a human-dominated landscape, the inflow of pollutants caused a significant reduction in water quality. The amount of nutrients was high and the amount of dissolved oxygen was low. Similarly, the number of meso- and psychrophilic bacteria was significantly higher than in the reference stream. The building activity of beavers initiated a series of positive environmental changes, and beaver ponds improved the self-purification of water in a polluted stream. In the lower part of the studied stream, after the stream passed through the beaver wetland system, the water quality improved, which was indicated by a significant improvement in physicochemical, microbiological, and biotic indicators, that is based on macrozoobenthos and fish. By constructing dams, beavers flooded large areas and created habitats for plant and animal species associated with wetlands, including amphibians, birds, and mammals. The improvement of the ecological status of the stream, combined with the preservation and restoration of wetland habitats in the catchment area, enables the use of ecosystem services provided by the streams in the human-dominated landscape and their surroundings.

The positive response of small terrestrial and semi-aquatic mammals to beaver damming

Ecosystem engineers, such as the Eurasian beaver, Castor fiber, transform habitats, thereby creating favourable conditions for other species and increasing biodiversity. Multiple studies have revealed that beaver ponds are valuable habitats for invertebrates and vertebrates, including other mammals, but the impact of watercourse damming on the fauna of small terrestrial rodents and shrews has not yet been documented. We tested the hypothesis that the presence of beaver dams and consequent flooding enrich the small mammal assemblage both quantitatively and qualitatively. We live-trapped small mammals at nine beaver-modified sites on 300-metre transects alongside dammed watercourses, starting from the dam through to the pond to the sections with unmodified lotic conditions. The abundance and species richness of trapped small mammals were highest near the dams and declined with distance. Additionally, five out of 12 trapped species significantly decreased their abundance with linear distance along the shoreline from the dam and none revealed the opposite trend. Four species were more abundant on plots subjected to beaver-related inundation (especially Sorex minutus and Micromys minutus), while none were present solely on uninundated plots. Among the semi-aquatic species, two water shrews benefited from beaver activity in different ways. Neomys milleri occurred only in flooded sections, while N. fodiens preferred unmodified sections, but was the most numerous species closer to the dams, as per known patterns of competitive displacement observed in Central Europe. An important factor affecting small mammals, the herbaceous layer cover, appeared to be interdependent with damming. We provide the first unequivocal evidence that the presence of beaver dams facilitate the abundance and diversity of small mammals, presumably due to increased food abundance, availability of shelter and habitat connectivity. Beaver-created wetlands may act as potential refuges for species most susceptible to the consequences of anthropogenic climate change.

Can reintroduction of beavers improve insect biodiversity?

Ecosystem engineering species, such as beavers, may help the restoration of biodiversity. Through the building of dams and lodges and altering the natural hydrology, beavers change the habitat structure and create multiple habitats that facilitate a wide variety of other organisms including terrestrial invertebrate communities. Here we study the effect of beaver reintroduction in Klosterheden in Denmark on biomass of flying invertebrates and diversity of moths. Further, aerial photos were used to assess riparian structure and productivity using the normalized difference vegetation index (NDVI). Our findings show that the presence of beavers affected flying invertebrate biomass, but that this was dependent on time of the year. Further, a strong effect of presence of beavers was found on diversity of moths. The results also show an increase in vegetation productivity and structural heterogeneity at sites with presence of beavers. Overall, our results demonstrate the importance of beavers as important ecosystem engineers that affect invertebrate species composition and abundance, as well as riparian structure and productivity.

Artificial Waterholes for European Bison as Biodiversity Hotspots in Forest Ecosystems: Ecological Effects of Species Reintroduction Activities

Despite the growing population of European bison (Bison bonasus), it is necessary to plan the reintroduction of these animals to new areas. Reintroduction of European bison often requires the improvement of natural conditions. Such preparatory activities allow European bison to more easily adapt to new places, but also impact the functioning of animals from other taxa. The aim of the presented study was to examine the impact of waterholes for European bison on the development of local populations of amphibians and dragonflies (Odonata), as well as the creation of new feeding grounds for bats. We examined 15 reservoirs in the Augustów Forest District located in northeastern Poland, of which five were waterholes for European bison built in 2013–2014, four were semi-natural reservoirs transformed into waterholes for European bison in 2018, and six were natural reservoirs. Dragonflies were studied in 2021–2022; amphibians in 2018 and 2020; and bats in 2018, 2019, and 2020. In total, 24 species of dragonflies (Odonata), 10 species of amphibians, and 13 species of bats were found. The results of the inventory of three taxonomic groups using different comparative variants indicate a significant impact of the construction of waterholes for European bison on the biodiversity of the forest ecosystem. We concluded that the waterholes for European bison present better resistance to drying out than natural reservoirs. In addition, waterholes warm up more quickly, supporting better conditions for amphibians. The surface of the reservoirs and their exposed surroundings are favorable for insects (including dragonflies), and these are a source of food for bats, becoming attractive feeding grounds for them.