Impact assessment of European beaver reintroduction on amphibians of small rivers.
A study aimed at the assessment of the effect of European beaver reintroduction in the valleys of small rivers on amphibians was carried out. The research covered the areas of the state nature reserves, Rdeiskii, Prioksko-Terrasny, and Privolzhskaya Lesostep. Small rivers of the area under study were inhabited by European beavers in different years, so the degree of European beavers' impact on the valley ecosystems varied. This fact allowed us to study the amphibian species reactions in different conditions. The modifications of lighting and water regimes were the main factors of European beavers' impact on amphibians. Loss of trees due to European beaver activities led to the increase in solar radiation, which, in turn, sped up the water heating in spring. Because of the damming by the European beavers, a lot of standing water bodies appeared, and the diversity of habitats increased. These conditions promoted the spawning and early metamorphosis of the common amphibian species, Rana spp. and Bufo bufo. After a long-term influence of European beaver activities in river valleys, the majority of suitable habitats for amphibians became necessarily associated with the presence of European beavers, and so the biodiversity increased here. For example, Pelophylax spp. appeared in the European beaver ponds. If the European beaver populations were new to the area or unstable, the reintroduction of European beavers had an insignificant or even negative effect on amphibian populations. The water regime was the limiting factor only in the large European beaver ponds, where the European beaver dams maintained a stable water level. When the European beavers left the ponds, especially in springtime, this resulted in mass death of the amphibian eggs and larvae.