WEASEL POPULATION RESPONSE, HOME-RANGE, AND PREDATION ON RODENTS IN ADECIDUOUS FOREST IN POLAND

Numerical responses of the weasel Mustela nivalis to the changes in po pulation density of forest rodents (bank vole Clethrionomys glareolus and yellow-necked mouse Apodemus flavicollis) and predation by weasels were studied in the pristine deciduous forests of Bialowieza National Park, eastern Poland, in 1985 through 1992. The rodents experienced 5 yr of noncyclic (seasonal) fluctuations (autumn density 23-74 rodents /ha) and 2 yr of outbreak and crash (triggered by synchronous heavy cr op of oak, hornbeam, and maple seeds). Autumn numbers of rodents excee ded 300 individuals/ha during the outbreak and dropped to 8 individual s/ha in the following autumn. Weasels were censused by livetrapping in summer and by snowtracking on an 11.2 km(2) grid of transects in wint er. Radiotracking of 12 weasels in 1990 and 1991 yielded estimates of home ranges and daily movement distances, which were combined with sno wtracking and livetrapping data to estimate densities during 7 yr. Est imated winter density of weasels varied from 5.2 to 27.3 individuals/1 0 km(2) in December and declined to 0-19.1 individuals/10 km(2) by ear ly spring (March). Midsummer (July/August) indices of weasel numbers w ere extremely variable and corresponded to 41.9-47.6 individuals/10 km (2) in years with moderate density of rodents, 101.7 individuals/10 km (2) during the rodent outbreak and 19.1 individuals/10 km(2) during th e crash. Increase of weasel numbers from spring to midsummer was posit ively related to the spring numbers of rodents. Autumn and winter decl ine of weasel numbers was not related to rodent density changes. Durin g the outbreak and crash of the rodent population, the numbers of weas els and rodents (both sampled at 2-3 mo intervals) were positively cor related (P < 0.0005) with no time lag. Home ranges of male weasels rad iotracked during the rodent outbreak were 11-37 ha (minimum convex pol ygon), compared to 117-216 ha during the crash year. The predator/prey ratio varied from 0 to 2.5 weasels/1000 rodents. The ratio was highes t at low densities of rodents. With increasing numbers of rodents, the ratio declined, since rodent population growth was overwhelmingly fas ter than weasel population growth. In the 7 autumn-winter seasons (1 O ctober-15 April), weasels removed, on average, 1.6 to 9.5 rodents from each hectare, i.e., from 2 to 28% of autumn numbers of rodents. Winte r predation by weasels was heaviest at rodent density of approximate t o 20 individuals/ha. At lower densities of rodents, the number of weas els was restricted by food shortage and the role of their predation ra pidly declined. At high rodent densities, the rodent numbers by far ex ceeded the predatory capacity of weasels and predation percentage decl ined again. Weasel predation in relation to rodent density has the sam e pattern in geographic zones ranging from Turkmen deserts to European farmlands and forests.