Some ecologists now see the theory of competitive exclusion as not very useful in explaining the real world, because its assumptions are too restrictive to be realistic outside the laboratory and its reasoning is suspected of being circular. After all, every species is by definition slightly different from every other, every patch of habitat is in practice unique in some respect, and very few environments are truly in stable equilibrium.

On the other hand, opportunistic species such as the weasels never do live in equilibrium conditions, so any theory based on that assumption simply cannot be applied to them. Instead, we emphasize that the coexistence of weasel species actually depends on lack of equilibrium. Weasels simply use the fluctuations in rodent populations as a resource (Levins 1979; Powell & Zielinski 1983).

Fluctuations in food supply and the patchy distribution of habitats orchestrate the frequent local extinctions and recolonizations of weasel populations. Throughout this book we have discovered very many costs attached to being a small, thin specialist rodent hunter, and the problem of unstable numbers is one of them. Our guess is that, given enough space and time, these in turn permit the long-term survival in one place of more than one variation on the weasel way of life. Confirmation of this idea awaits some ingenious future project. We are glad that there are still plenty of puzzles left for future weasel biologists to study.

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