WINTER DISTRIBUTION AND ABUNDANCE OF MUSTELIDS AND BEAVERS IN THE RIVER VALLEYS OF BIALOWIEZA-PRIMEVAL-FOREST

Lutra lutra, Mustela vison, M. putorius, M. erminea, M. nivalis, and t he settlements of Castor fiber were surveyed along 170 km of rivers in Bialowieza Primeval Forest (Poland and Belarus), the best preserved t emperate lowland forest in Europe. The censused rivers varied from ver y small (1-5 m wide, < 1 m deep) to medium-sized (11-15 m wide, up to 3 m deep). Mustelids were counted by tracks left in snow. Mean index o f abundance of otters was 2.2 inds/10 km of the river bank (range 0-5) and that of mink 4.6 inds/10 km (range 0-7.5). On average, 1.4 poleca ts/10 km were recorded (range 0-5). Otters and mink were most abundant on the medium-sized rivers and least numerous on very small ones. Pol ecats lived predominantly on very small rivers. Species structure of a predator guild varied with river size. On average, 5.1 stoats and 4.0 weasels were counted per 10 km of river bank. Stoats were twice as co mmon along rivers with open marshy flood-plain as along rivers with fo rested valleys. On average, 2.9 beaver settlements were recorded per 1 0 km of river bank (range 0-5). Habitat niche overlaps were highest be tween otter and mink, and stoat and mink. The smallest overlaps were b etween the polecat and all other predators. Densities of mustelid pred ators and beavers in Bialowieza Primeval Forest were similar to those in other fairly well preserved woodlands in Europe.