The three species of northern weasels all have a strong family resemblance and all depend on more or less the same prey. At least two of them may be found in almost all countries north of 40°N, and in some places in North America all three species may be found together. Yet, it is one of the basic ideas of ecology that two or more similar species cannot coexist indefinitely.
The theory starts from the assumption that similar species tend to depend on the same resources, harvested in the same way, and then it predicts that, unless those resources are so abundant that there is always enough for both (in which case there is no competition), one species will eventually displace the other. Either one will be better at harvesting, so that the other starves, or one will aggressively drive the other away. The two forms of interaction are known as exploitation, or "scramble," competition and interference, or "contest," competition. The expectation is that, unless one of a pair of similar species develops some means of evasion (such as hunting at a different time of day or in different habitats), the other will exclude it from a habitat by one means or the other, or by a combination of both.
Where two or more weasel species coexist, they occupy slightly different niches: Members of the small species concentrate on small rodents, while members of the large species favor large rodents and rabbits (Chapter 5). In a diverse habitat with a variety of prey of different sizes, the overlap between their niches is minimal and the two can probably ignore each other for much of the year. But, whenever or wherever there is less choice of prey, both must search out anything available, and then competition is inevitable.
Because populations of small rodents are so unreliable, this must happen often enough to have a real effect on the distribution and population density of whichever weasel species is less well equipped to meet the conditions of the moment. Sooner or later one is likely to be eliminated, at least temporarily.
Mick Southern (a perceptive observer both of nature and of graduate students) always used to say that, although it is fun to discuss theoretical questions in the library at tea time (such as, what controls niche overlap?), if you really want to know, it is quicker and easier to go out and ask the animals themselves. This is true, provided first that the question is carefully framed, and second that the observations to be made are defined in terms that could provide a definite answer, one way or the other. This process has given us a preliminary idea of how stoats and common weasels coexist in
Britain, and how stoats and longtails or stoats and least weasels coexist in North America.
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