The problem of studying the impact of predation by weasels on their prey has been tackled in four ways. All are inaccurate and imprecise to some extent, but all provide important information.

The first is an indirect method. If the daily food requirements of individual weasels are known, the researcher can either estimate how much of a known loss could be accounted for by weasels eating prey at that rate, or else compare how much loss could be inflicted by weasels compared with the amount needed to reduce prey of a known density by a given amount. Which of these two estimates is made depends largely on what sort of information there is about the prey. It sounds logical, but there is a snag. This method has to assume that weasels kill a certain, average number of prey per day, but they do not. Weasels are adaptable and intelligent predators that kill as many as they can catch, which is sometimes more and sometimes less than they need, depending entirely on the circumstances (Chapter 6).

The second is the direct method of counting the number of prey alive and then counting the number of them that have been removed by weasels. This method can be sabotaged by spectacular errors of assumption or census. One early study used unrealistically high figures for the density and productivity of least weasels, and produced the calculation that the weasels had eaten over three times more voles than were present (Golley 1960). The opposite error can be equally large if the number of voles removed is estimated from analyses of the weasels' diet, since weasels may kill and cache many more voles than they eat.

The third is the simplest to do and most difficult to interpret: Remove the weasels and watch what happens to the prey populations. The simple underlying assumption is that whatever changes in the numbers of prey might follow must have been caused by the change in weasel numbers. Unless the experiment is carefully controlled, however, it is usually impossible to eliminate any of umpteen other possible explanations. For example, if predation reduces only the production of young, but the density of adults is controlled by something else, removal of weasels and other predators may or may not increase the density of the population. After stoats and other predators were removed from sites in northern and south ern Finland, the brood size and survival of young grouse improved (Kauhala et al. 2000), but the benefit to the adult grouse was not so clear.

The fourth is the theoretical approach. Mathematical, graphical, and verbal models of the dynamics of predators and prey can be very powerful aids to thinking, but are helpful and realistic only in direct proportion to the amount of information about real animals they incorporate.

All of these methods have different combinations of advantages and disadvantages, and all have been tried somewhere. The most progress in understanding weasels and their prey comes when different approaches are combined. Many examples of this in the recent literature show that the effects of predation by weasels are not the same in all situations. Weasels that have, for most of the year, almost exclusive access to small rodents in their burrows and under prolonged snow cover in boreal forests and grasslands can have a substantial impact on numbers of their prey. Weasels that are part of a wider community of predators hunting a variety of prey in temperate forests and farmlands typically have much less impact.

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