In Aberdeen in 1977, Chris Pounds chose to study the coexistence of common weasels and stoats for his doctoral thesis. In Ontario in 1973-1976, another graduate student, Dave Simms, was doing field research on stoats and longtails and thinking along the same lines. Unaware of either, King and Moors (1979a) proposed a verbal hypothesis on how stoats and common weasels might coexist, hoping that someone would undertake the fieldwork necessary to see whether or not it was right. Pounds (1981) was able to test parts of the hypothesis before he finished fieldwork, and the rest of it was tested by Erlinge and Sandell (1988).

The original hypothesis was developed with common weasels and stoats in mind, but is easily generalized for any pair of weasel species. It was based on the idea of an unstable balance of opposing advantages. Because common weasels are smaller than stoats, they are better able to reach small rodents in their burrows and nests; they can survive in a smaller area, and for long after the rodents have become too scarce to support stoats. They can also respond more rapidly to a glut of rodents. Common weasels therefore have an advantage over stoats in exploitation competition, although their specialization also makes them vulnerable to local extinction if the rodents disappear altogether. This certainly happens occasionally, but the vacated area can be recolonized from elsewhere in the next few years (Chapter 9).

On the other hand, because stoats are larger than common weasels, they can turn to larger prey when rodents become scarce (Chapter 5), and they will also always win in direct confrontations with common weasels, so are better able to evict common weasels from a choice area or to steal their catch. Stoats, therefore, have an advantage in interference competition, but only so long as they have access to larger prey. Delayed implantation means they cannot take immediate advantage of a rodent peak, but their wider choice of prey and longer average life span means that individual stoats have more chance of surviving to the next breeding season.

Hence, each species has a different combination of advantages in foraging and reproduction, and each is best adapted to exploit slightly different conditions. In any one place and time, one may be present and the other not, but over the long term the two coexist because in a patchy environment there will always be places and times where common weasels can avoid confrontations with stoats and stoats can avoid overdependence on rodents. The hypothesis predicts that, when they are forced to face up to each other, one or the other always wins, depending on the circumstances. A variable environment permits coexistence by constantly changing the balance of the different advantages of each.

As examples of how these ideas might work, King and Moors (1979a) cited two cases in which coexistence broke down. In one the outcome favored common weasels, and in the other it favored stoats.

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