Populations Density and Breeding Success

because weasels are predators, they are necessarily rareā€”much more so than the voles, mice, and rabbits on which they prey. In a meadow or old field, there are from hundreds to thousands of grass plants for every vole, and from scores to hundreds of voles for every weasel. On the other hand, because weasels are the smallest of the warm-blooded predators, they can be much more common than the foxes, feral cats, hawks, and owls that also eat voles. It is therefore often possible to collect large samples from populations of weasels.

One might expect from this that the population dynamics of weasels would be among the best studied of all carnivores, but that is not so. Weasels are too small and secretive to be observed directly (except in occasional lucky glimpses), so systematic study of them has to be done indirectly, by routine trapping or footprint tracking. Suitable field techniques were worked out in the nineteenth century by fur trappers, naturalists, and gamekeepers, but were not applied to scientific studies until the 1960s.

More important, many people do not see weasels as charismatic animals (Chapter 1), and they are unreliable. They are not evenly distributed in all habitats, and even where they do live, they may be abundant one year and scarce the next. These variations make researchers nervous, because it is hard to plan a study on such a slippery, moving target. Many hopeful researchers and students have put in weeks or months of fruitless fieldwork, often in places where weasels were known to have been present at some previous time, before giving up disappointed.

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