Another Approach

The King and Moors hypothesis on the coexistence of two or more weasel species was formulated at a time when competitive exclusion was considered to be a dominant ecological process. For example, most biologists generally accepted that the reason why the members of a group of similar-looking animals, such as the weasels, are graded by size is that size differences help to reduce competition.

It is true that small weasels specialize on small voles, whereas large ones kill prey the size of cotton rats, chipmunks, water voles, and rabbits (McDonald 2002; Sidorovich et al. unpublished). The trouble is, these are differences of degree, not of kind. The large weasels do eat a great many small voles when they are profitable, and the small weasels are still able, at times, to kill rabbits. Differences in size alone do not, in fact, ensure that the weasel species exploit different resources. There must be more to it than that.

Powell and Zielinski (1983) took a different approach to the question and came up with some suggestions as to what that extra factor might be. They added two twists to the argument put up by the simple basic hypothesis, derived from modeling a community with two weasel species, a range of prey from small to large, and other predators that compete with weasels for prey but also eat weasels on occasion (Figure 14.4).

The first twist is that, when both weasel species become locally extinct, the species that recolonizes first gains an advantage by building up a large population first. That large population increases its probability of survival locally during the next crash of the prey population. The second twist is that predation on weasels by other predators reduces the chances that their populations will grow to sizes large enough to lead to intense competition. Some field observations seem to support the hypothesis that predation facilitates coexistence, at least at landscape scale when populations of both species are present. In northeastern Canada, where longtails are extending their distribution into forests formerly occupied only by stoats and least weasels, the presence of larger fur-bearers seems beneficial to coexistence (St.-Pierre et al. in press-a).

Computer simulation models constructed to imitate the competitive interactions of weasels and analyses of the dynamics of communities including weasels confirm the core of the argument: Local coexistence of two weasel species depends on geographic scale. At the continental scale, two or more species of

deer mouse meadow vole cottontail rabbit

Figure 14.4 Powell and Zielinski (1983) modeled a community with two weasel species, a range of prey from small to large, and other predators that compete with weasels for prey but also eat weasels on occasion. The presence of large prey allowed the model population of large weasels to avoid extinction when small mammals disappeared. The ability to enter vole holes gave the small weasels an advantage in catching voles when they were rare. When both weasel species went extinct locally in the model, the species that recolonized first gained an advantage by building a large population first. Predation on weasels by other predators reduced both weasel populations and reduced competition between them. Thus, predation facilitated coexistence when populations of both weasels were present.

deer mouse meadow vole cottontail rabbit noncyclic 3-year cycles 10-year cycles

Figure 14.4 Powell and Zielinski (1983) modeled a community with two weasel species, a range of prey from small to large, and other predators that compete with weasels for prey but also eat weasels on occasion. The presence of large prey allowed the model population of large weasels to avoid extinction when small mammals disappeared. The ability to enter vole holes gave the small weasels an advantage in catching voles when they were rare. When both weasel species went extinct locally in the model, the species that recolonized first gained an advantage by building a large population first. Predation on weasels by other predators reduced both weasel populations and reduced competition between them. Thus, predation facilitated coexistence when populations of both weasels were present.

weasels have lived together in Eurasia and North America for thousands of years; at the local scale, living together is a dynamic matter of repeated local extinctions and recolonizations.

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