PREDATION ON RODENTS IN BIALOWIEZA PRIMEVAL FOREST, POLAND

Autumn and winter predation on bank vole Clethrionomys glareolus and yellow-necked mouse Apodemus flavicollis was studied in 1985/86-88/89 in an 11.2 km2 area of the Bialowieza National Park. Rodents regularly increased in numbers from spring to autumn and decreased throughout winter. Out of 23 species of predators, the most common were: tawny owl Strix aluco (43-57 adult ind. 10 km-2), weasel Mustela nivalis (17-27 ind.), buzzard Buteo buteo (12-16), and pine marten Martes martes (5-8). Voles and mice were the staple food for two specialists, the stoat Mustela erminea and the weasel, and two generalists, the tawny owl and the pine marten. The generalists exploited different alternative prey when rodents were scarce: tawny owl - amphibians, marten - small mammals, and the red fox Vulpes vulpes - ungulate carcasses and hares. The depth of snow and abundance of voles were two major factors shaping the contribution of voles to tawny owl and marten diets. No such relationships were found for mice and generalist predators. The predation by 8 species (tawny owl, buzzard, marten, weasel, stoat, polecat Mustela putorius, fox, and raccoon dog Nyctereutes procyonoides) from 1 October to 15 April in 1986/87. 87/88, and 88/89 was estimated to be on average 28-35 voles and 14-17 mice ha-1. Estimates suggested that three species were responsible for 86-95% of the total predation impact: tawny owl (56-71%), weasel (11-21%), and marten (10-15%). The predation impact was similar to the annual decrease in rodent numbers from autumn till spring: voles 35 (SD 11.1) inds ha-1 in autumn and 8 (SD 2.6) inds ha-1 in spring, and mice 24 (SD 16.3) inds ha-1 in autumn and 3 (SD 2.5) in spring. Predation, therefore, was regarded as the main agent of rodent mortality throughout autumn and winter.