Clearly, weasels and other predators can kill significant numbers of small rodents, and they do increase their kill rate with rodent numbers. The consequence for the community depends on a combination of two rather different processes, which operate at the level of the individual (the functional response) and at the level of the population (the numerical response) (Table 7.1). Weasels do not hunt more often when rodents are abundant (rather the opposite), but they can make more kills per hunting expedition. To demonstrate this, Sundell et al. (2000) held least weasels and sibling voles in 0.5-ha enclosures. The numbers of voles killed by least weasels increased quickly with increasing vole densities, but only over the range from 4 to 16 voles per ha. The weasels' interest in hunting leveled off at around three voles a day (10 voles caught per 72 hours), even when vole density was increased to 100 voles per ha. In other words, as rodent numbers increase, an individual weasel can increase its kill rate (by a functional response), but only up to the limit set by its willingness to continue investing energy in hunting.
When vole numbers increase beyond that limit, the kill rate can increase further only if populations of weasels increase their own numbers, that is, by making a numerical response. When food is abundant, this numerical response of weasels can be impressive. For example, in the first half of 1990 in Bialowieza when the vole population was high, Jgdrzejwski et al. (1995) watched the density of a population of common weasels grow from a low of about 19 individuals per
10 km2 at the end of winter (when they were not yet breeding) to 102 per 10 km2 less than 3 months later.
All species of weasels are capable of making functional and numerical responses, although the results are not always the same. In some places, populations of rodents periodically escape the effects of predation and reach very high numbers, at least for a while, before they crash; in other places, vole populations tend to fluctuate annually over a more modest range. Two somewhat different but not mutually exclusive hypotheses have been proposed to explain this difference.
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