Beaver impact on water coverage of forest-steppe territories (Penza Region, European Russia)

Abstract

The impact of the European beaver (Castor fiber) on the water coverage of natural territories was assessed by analysing public available satellite images. Four model squares with a total area of 900 km2 were chosen, with coverage of the vicinity of four sectors of the Privolzhskaya Lesostep’ State Nature Reserve (Penza region). This study showed that modern public satellite images can clearly distinguish beaver ponds even of small sizes, as well as elements of their activity, namely beaver dams, canals and tracks. We identified 373 water bodies that were made by beavers, with an average area of 1040 ± 4843 m2, 107 man-made ponds (average area 24 272 ± 65 977 m2), and 67 lakes and oxbows (average area 8127 ± 17 946 m2). The proportion of zoogenic ponds was 11% of the total area and 68% of the total number. In certain study sites, these values varied from 7% to 18% (for the area) and from 44% to 84% (for the number). But within the boundaries of the Privolzhskaya Lesostep’ State Nature Reserve, where there were no man-made water bodies, the proportion of beaver ponds reached 100% in three sectors. The total water body’ area percentage of the studied territories was 0.39%. In different sites, beavers increased the proportion of water bodies by 0.02-0.10%, in total by 0.04%. If we consider only the area of natural water bodies, then beavers increased it in 1.7 times, while within the boundaries of the Privolzhskaya Lesostep’ State Nature Reserve this increase was 3.6 times. Since the water coverage of the central regions of European Russia was considerably increased due to the construction of man-made ponds, this could affect the structure of habitats in the floodplains of small rivers, and favour the activity of beavers in the forest-steppe regions. Therefore, there was a slight correlation between the number and area of beaver ponds and the size of downstream man-made water bodies. Our data have shown that the zoogenic contribution to water resources is comparable to the contribution of natural lakes and ponds. In some areas with low anthropogenic impact, water bodies created by beavers are the only lentic habitats in the landscape. However, the obtained results do not completely cover the real beaver impact, since narrow channel-like ponds hidden by the canopy of floodplain trees fall out of analysis. It is impossible to take into account the length of the backwater zone with a cascade of zoogenic water bodies, neither is it possible to remotely assess the contribution of animals to the retention capacity of old man-made ponds, and the role of the ecological engineer in maintaining and increasing the surface area of pre-existing lakes. Therefore, the contribution of beavers to the water coverage of the territory is actually more remarkable.