Least and common weasels are the smallest and probably the most numerous carnivores in the Holarctic and, weight for weight, they eat the most. The daily food requirement of a biomass of, say, 50 kg of least/common weasels would be much greater than that of an equivalent weight of foxes: 50 kg of least/common weasels eating 35% of their body weight in food per day (see Table 2.1) would need 1,750 g of food (about 88 mice), whereas 50 kg of foxes, at 8% per day, would need only 400 g (20 mice).
Better still, when rodents are abundant, weasels do not kill only what they need for the day: They kill as many as they can and store up the surplus for future use. If the prey were, say, voles causing damage to crops, the weasel would be a much better friend to the farmer than the fox. It is therefore assumed by some that weasels in general have the power to "keep the rodents down" (plus any other small mammals that can cause a nuisance, such as rats and rabbits) and that therefore farmers and foresters should actively encourage weasels to live on their land.
Norman and Stuart Criddle (1925) were among the earliest North American champions of weasels, arguing against the common perception, picked up from the folk lore of Europe and "the game mentality," of the weasel as a savage killer of game birds and poultry. They listed many field observations of all three species of weasels in Canada in the early years of the twentieth century, mostly describing weasels hunting rodents or rabbits. They pointed out that, in 9 years out of 10, a longtail will find sufficient food in the fields and woods, and only on the tenth turn temporarily to domestic fowls. A thousand mice may have been killed in the meantime, but the destruction of half a dozen hens is enough to label the weasel a pest. Surely, they add, this is a remarkably small payment for the great good done by weasels in killing rodents and ample evidence that weasels should be protected.
Field observers can easily find evidence to support this attitude. Lippincott (1940) described how he had tracked a weasel through the snowy woods and farmlands around his home near Philadelphia. All along the two-mile trail, the weasel took no notice of rabbit sign or gray squirrel tracks or roosting pheasants. It stopped only to flush out a rat from a pile of corn shocks and two meadow voles from under a low tree, and to try to get into two other mouse holes, which were too frozen for even its slim body to enter.
On another occasion Lippincott was surprised to see meadow voles running across the lawn beside his house, until he found a small weasel busy flushing them out of nearby weeds. Later the same day he saw three meadow voles scurry across the road, with a brown streak in furious pursuit. He concluded that, even if a weasel does take a few ducklings in the springtime, he still preferred to have a weasel about the place than hordes of rodents.
Other North American authors have also pointed out the absurdity of killing weasels on sight even where there is no bounty on them (Hamilton Jr. 1933; Quick 1944). Weasels probably kill hens only when barnyard rats are scarce, and the rats may well have killed more hens themselves. These authors supported the same idea: Weasels perform a valuable service to farmers.
Whether or not this argument is actually correct has never been tested. In logic and in fact, it is quite untenable, for the same reason that no one attempts to judge the financial position of a company only from the debit columns of its balance sheet. The effect of predation by weasels on rodents is the difference between the rate at which weasels remove rodents and the rate at which rodents are replaced (see Table 7.1), and this is impossible to figure out from casual field observations.
Nevertheless, the idea that weasels "keep the rodents down" remains strong in the public imagination. The logical conclusion is that it should be possible to enlist the help of weasels to reduce high numbers of pest mammals in field crops and forest plantations, or at least to control the damage done by pests. For this purpose, weasels have been released in various places, with mixed results.
In Canada, deer mice and Oregon voles are considered serious pests of forestry, because they eat great quantities of conifer seed and debark young trees, thereby interfering with the regeneration of logged forests. In one short experiment at Maple Ridge, British Columbia, in the autumn of 1978, seven of the small Canadian stoats were captured and released on a 1-ha experimental plot, while the numbers of mice and voles were monitored both there and on a control plot (Sullivan & Sullivan 1980). The hope was that stoats at such a high local density would reduce the number of rodents on the experimental area.
Unfortunately, they did not. Numbers of deer mice remained high in both areas, while numbers of voles decreased in both. On the other hand, there was no evidence that the stoats had stayed where they had been released; seven could not live together on 1 ha at any season, especially not in the fall when the young are normally dispersing. In fact, two of the stoats returned to their original home ranges, up to 4 km away. It is clearly unrealistic to expect predators to stay put and clear rodents from an unfenced area of mainland, just to oblige the owners of the forest.
Weasels seldom persist in hunting prey at low densities unless they have no alternative. The only temperate habitats where the choice is severely limited and emigration impossible are small islands, and weasels arriving on islands can have a substantial impact. One of the earliest known deliberate introductions was in the Shetlands, off the north coast of Scotland, to which stoats were taken in the seventeenth century or earlier (Venables & Venables 1955), but the consequences for the local fauna went unrecorded.
Much more is known about another introduction, on the island of Terschelling off the coast of the Netherlands (680 km2). During an afforestation program in 1910-1930, pines, oaks, and alders were planted on 600 ha of open sand country. The ditches dug to drain the plantations provided ideal conditions for water voles, and after about 1920 the high numbers of voles began to damage trees and gardens. Bounties, mouse typhoid, and poison failed to control them so, in 1931, stoats and common weasels were introduced. By 1937, the stoats had established a fluctuating but substantial population; the water voles (and the common weasels) were extinct (van Wijngaarden & MSrzer Bruijns 1961). Without the water voles, the stoats could not survive either and they, too, are now extinct on Terschelling (Mulder 1990).
This success was repeated in even shorter time on the much smaller Danish island of Stryno Kalv (46 ha), south of Fyn (Kildemoes 1985). High numbers of water voles were destroying the dikes and damaging crops. Beginning in October 1979, the numbers of water voles were assessed by trapping sessions of 5 to 6 days in October, March, May, June, and August for 2 years. In May 1980, six male stoats were released, evenly distributed along a 2-km dike. The numbers of both water voles and of ground-nesting birds on the island were regularly monitored until October 1981. In the first breeding season, 1980, there was no effect on the birds and not much on the voles. In the following year, water voles were very scarce, even though breeding conditions elsewhere in Denmark were favorable in 1981. Some birds were fewer than usual, but could soon be replaced from neighboring islands.
More often, however, weasels fail to do what is expected of them by those who believed that they would "keep the rodents down." The best-documented case is that of New Zealand, where ferrets, stoats, and common weasels were deliberately introduced in the mid-1880s, in the misplaced hope that they would control rabbits (King 1984b). They failed (Chapter 13).
With hindsight, we can see now that the New Zealand experiment could never have worked on the scale required (Trout & Tittensor 1989), but at the time it seemed to be the only way to save the sheep farmers from the ruinous plagues of rabbits. It so happened that one of the most famous examples of successful biological control of pests by introduced predators, the immediate repression of scale insects by the vedalia beetle in the citrus orchards of California, achieved spectacular success almost overnight in 1889. It must have greatly encouraged the continued importation of mustelids into New Zealand, which was going on at the same time. Vedalia beetles saved the citrus farmers, so why did mustelids fail to save the sheep farmers?
The answer is that vedalia beetles have advantages over their prey not shared by mustelids. These advantages include a reproductive rate nearly matching that of the scale insects, efficient aerial searching, no territorial restrictions, and absolutely no risk from killing prey because they are immobile and conveniently clumped. When the scale insects increase in numbers, the vedalia beetles can respond immediately, and when a beetle is stuffed full it simply produces more mouths to continue feeding. These characters make the beetles superefficient shoppers (see Table 7.1).
By contrast, there is a huge disparity in the reproductive rates of stoats and rabbits; searching on and under the ground is energy sapping, inefficient, and confined to the stoat's own home range, and adult rabbits run and hide, and when found are risky prey for stoats (see Figure 6.3). Even when a stoat is stuffed full it cannot produce any more mouths to feed on rabbits until the following year, so it usually goes to sleep. These differences help to explain why stoats never had any hope of controlling the numbers of rabbits over an area as large as the two main islands of New Zealand (114,000 km2 and 157,000 km2). Rather, as the history of myxomatosis shows, the converse is true: Rabbits control stoats (see Figure 10.2).
Natural predators are not normally geared to control the numbers of their prey, only to harvest them. A predator can control its prey only if it eats more prey animals than are produced as they increase, and fewer than are still surviving as they decrease (see Table 7.1). This is not what weasels do.
The combined force of all the local predators may have a substantial effect on rodents or birds (Chapter 7) but, in natural ecological communities, weasels usually make only a modest contribution to the total effect. The rate of reproduction of all weasel species is slower than that of any rodent species, and the number of rodents an individual weasel can kill in a day has a practical limit. Consequently, weasels cannot hope to match their consumption to the numbers of prey, so they cannot prevent an increase, nor start a decrease, unless the rodents have already ceased to add recruits to their population. During and after the decrease phase, the "overshoot" of predators caused by the lag of their response ensures that their predation is disproportionately heavy just at the time it should be slackening. This is why predation by weasels has such a destabiliz ing effect on populations of lemmings and voles such as those described in Chapter 7.
In the real world, the population dynamics of weasels are almost entirely controlled by the density of prey, whereas the density of prey is affected by many other things besides predation—particularly food supplies and social behavior. So, contrary to appearances, weasels do not have the whip hand: Rather, it is the other way about. The very mechanics of weasel predation make it impossible for them to "keep the rodents down," and this is why attempts to use weasels as a control agent on mainland areas have failed.
The only way to get around this is to remove their power of choice. Hunting gets more difficult as the prey become scarce, and the last few are the hardest of all to catch. The weasels' natural inclination to move away when hunting is unrewarding can be frustrated only on small islands. There, it is a matter of hunting the last few or starving; weasels have the same instinct for self-preservation as the rest of us, so, not surprisingly, they oblige.
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