The 3- to 4-year interactions between weasels and voles have a close parallel, played out over a longer term, in the 10-year cycle of snowshoe hares in Canada. The hare cycle involves a regular fluctuation in abundance of vegetation, snow-shoe hares, and lynxes and other predators (Stenseth 1995; Krebs et al. 2001). When snowshoe hares are abundant, predator populations increase, especially the populations of lynxes, which specialize on snowshoe hares. When hare populations begin to decrease because of food shortage, pressure by abundant predators on the remaining hares is intense. Hare populations are reduced and kept at low levels until predators starve. During that period, plants recover and can support high hare populations again. And so the cycle continues.
By contrast, predation by weasels on small rodents, and by lynxes on hares, differs in many ways from predation by wolves on deer. Some ideas developed from studies of larger carnivores simply cannot be applied to weasels. For example, like all large predators, wolves prey as much as possible on prey that are easy to catch. For wolves, the easiest targets tend to be old, sick, disabled, or temporarily disadvantaged deer, because for wolves the risk of injury in attack is very real. Therefore, over the long term the wolves tend to cull unfit deer from a population. But the average mouse is no match for most weasels, so there is less need for weasels to select between individual prey. As Pearson (1985) put it, weasels do not wait until the meadows are overflowing with insecure or maladjusted voles; they can kill almost any vole with ease, and search them out even when vole populations are low.
It is true that a small female least weasel approaching a large, female vole must be cautious (Chapter 6), and so is any weasel attacking a well-grown rabbit. Nonetheless, on average, weasels can kill voles more easily than wolves can kill deer, and that makes a great difference to the economics of hunting by large versus small vertebrate predators. The difference between easy targets for weasels (such as nestlings and newly independent rodents, lagomorphs, and birds) and less easy ones is not obvious, but natural selection needs only a small differential to produce a real effect over many generations. To the extent weasels do select some prey rather than others when they can, they constitute an agent of natural selection favoring bright, alert prey just as wolves do, and it would be interesting to examine this question in the field.
Other ideas on predation developed from studies of invertebrates are not applicable either. Invertebrate predators and their prey both produce large numbers of young with only a very short time between generations. The rate of increase of populations of invertebrate predators is much closer to that of their prey than is that of any of the weasels to theirs (Chapter 12).
Clearly, the old idea of the "balance of nature," of predators and prey living in a dynamic balance, is irrelevant to weasels. Weasel populations are seldom stable, and no balance can be established, let alone maintained. Not only is the concept of any balance in nature difficult to define, but most ecologists now agree that no such balance exists. Perhaps better than any other example, ecological communities containing weasels illustrate that natural populations of predators and prey can fluctuate wildly. Those fluctuations are normal, natural, and just fine.
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