Numerical responses of the weasel Mustela nivalis to the changes in population 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 crop of oak, hornbeam, and maple seeds). Autumn numbers of rodents exceeded 300 individuals/ha during the outbreak and dropped to 8 individuals/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 winter. Radiotracking of 12 weasels in 1990 and 1991 yielded estimates of home ranges and daily movement distances, which were combined with snowtracking and livetrapping data to estimate densities during 7 yr. Estimated winter density of weasels varied from 5.2 to 27.3 individuals/10 km(2) in December and declined to 0-19.1 individuals/10 km(2) by early spring (March). Midsummer (July/August) indices of weasel numbers were 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 the crash. Increase of weasel numbers from spring to midsummer was positively related to the spring numbers of rodents. Autumn and winter decline of weasel numbers was not related to rodent density changes. During the outbreak and crash of the rodent population, the numbers of weasels and rodents (both sampled at 2-3 mo intervals) were positively correlated (P < 0.0005) with no time lag. Home ranges of male weasels radiotracked during the rodent outbreak were 11-37 ha (minimum convex polygon), 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 highest at low densities of rodents. With increasing numbers of rodents, the ratio declined, since rodent population growth was overwhelmingly faster than weasel population growth. In the 7 autumn-winter seasons (1 October-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. Winter predation by weasels was heaviest at rodent density of approximate to 20 individuals/ha. At lower densities of rodents, the number of weasels was restricted by food shortage and the role of their predation rapidly declined. At high rodent densities, the rodent numbers by far exceeded the predatory capacity of weasels and predation percentage declined again. Weasel predation in relation to rodent density has the same pattern in geographic zones ranging from Turkmen deserts to European farmlands and forests.
