Stoats in Southern Sweden

In the area where Sam Erlinge and his students (Erlinge et al. 1983, 1984) had observed stoats for years, the field voles showed a fairly predictable seasonal variation in numbers, but little variation across years (Chapter 7). Field voles, wood mice, water voles, and rabbits were the main prey available in the 40-km2 study area of open fields with interspersed woodlots and marshes. The predators hunting them were stoats and common weasels, foxes, feral cats, badgers, polecats, common buzzards, tawny owls, long-eared owls, and kestrels. Through a gargantuan effort of teamwork, Erlinge et al. (1983, 1984) documented the numbers and annual breeding success of most of these predators. They also estimated the densities of prey available to the predators and, roughly, how many of each prey were taken by which predators (see Table 7.4).

The part of this census work concerned with stoats was done by live trapping in March and April (to estimate the numbers of adults present before each breeding season) and from August to October (to estimate the numbers of young produced). Over the 6 years of the study, 142 individual stoats (75 males, 67 females) were marked and released (Erlinge 1981, 1983). Snow tracking in winter confirmed that the trapping data did reflect the real distributions and numbers of the stoats, even though not all individuals were caught. Erlinge estimated the numbers of stoats present in various ways (not all of which agreed), but the general pattern was very clear. The density of stoats was higher at the beginning of the study than at the end (Figure 10.3). The figures suggested that about 45 to 50 individuals were present in the autumns of 1974 and 1975, and about 35 in 1976. After 1977, the data became too few to calculate actual numbers but, by autumn 1978, there were probably fewer than half the number of individuals that had been present in 1974-1975.

Two important prey were also censused twice a year throughout the study. The numbers of field voles drifted slowly upward, in almost direct opposition to those for stoats. The numbers of rabbits decreased sharply, along with those for stoats. Rabbits were counted because they are a key resource for the larger, generalist predators living in the same area. Rabbits were not often eaten by the stoats, which are rather small in Sweden (though not as small as in Canada) and are rodent specialists. The third most important prey, the water vole, apparently remained fairly stable in population size throughout the study.

Erlinge (1983) calculated the breeding success of the stoat population from his live-trapping records. He could not estimate fecundity, but that did not really

Figure 10.3 The total number of stoats present in Erlinge's study area each year was closely linked with the breeding success of the previous summer, as reflected in the proportion of young in the autumn population, which, in turn, is linked to the abundance of prey. When the rabbit population (open squares and dashed line) decreased, the number of young stoats 3 to 5 months old (white bars) decreased immediately. The number of adult stoats over 15 months old (grey bars) decreased a year later. (Redrawn from Erlinge 1983.)

Figure 10.3 The total number of stoats present in Erlinge's study area each year was closely linked with the breeding success of the previous summer, as reflected in the proportion of young in the autumn population, which, in turn, is linked to the abundance of prey. When the rabbit population (open squares and dashed line) decreased, the number of young stoats 3 to 5 months old (white bars) decreased immediately. The number of adult stoats over 15 months old (grey bars) decreased a year later. (Redrawn from Erlinge 1983.)

matter. The corpora lutea counts that were available showed that potential litter size in Swedish stoats is close to the average elsewhere (see Table 9.2), and it is probably safe to assume that mean fecundity is as constant from year to year in southern Sweden as anywhere else. So the start of each year's reproduction saga is almost certainly the same in Sweden as it is everywhere.

The following stages are presumably also the same, because by the time the young stoats appeared in the traps, their numbers had been drastically cut down compared with what was theoretically possible. The vital difference was that there was no positive relationship between the numbers of field voles and the production of young stoats each season. In 4 of the 5 years observed, the stoats produced more young in seasons when the field voles started breeding early and fewer when they started late; in the fifth year the voles started early but the stoats had their worst season. If these data are representative, the surprising conclusion must be that field voles in southern Sweden did not influence the breeding success of stoats in those years.

On the other hand, Erlinge did find a relationship between rodent numbers and breeding success, demonstrated in space rather than in time and involving water voles rather than field voles. The isolated water meadows and marshes scattered through the study area, somewhat like islands in a grassy sea, harbored more voles of both kinds than did the open pastures surround ing them. Moreover, the marshes were not all the same. In some, only field voles were common, while others supported water voles too, which were sometimes even more abundant than field voles. Both kinds of voles are among the stoat's favorite prey, but water voles provide larger packets of food for the effort of making a kill. So, given a choice between them, a stoat would probably always prefer a water vole.

Erlinge found that the breeding female stoats living in the marshes with abundant water voles produced more young females (27 young to 11 adults caught, mean 2.45 per adult) than those that had only field voles available (14 young to 16 adults caught, mean 0.88). The difference between these figures was not significant overall, but the trend suggests that breeding success was generally higher in the marshes with plenty of water voles. In the hedgerows on nearby farmland, breeding success was about the same as in marshes without water voles (seven young to seven adults, mean 1:1).

As in all weasels, the age structure of the population in the autumn closely reflected the success of the preceding breeding season. Erlinge's data (Figure 10.3) show that the years when stoat numbers were high were also the years with the highest proportion of young in the autumn catch. The years 1976, 1977, and 1978 were poor years for recruitment, and in fact the population was by then not replacing itself. Decline was inevitable; yet the numbers of field voles, usually regarded as the stoat's most important resource, were if anything increasing. Why didn't the rising numbers of field voles benefit the stoats, as one would expect?

The Swedish stoats lived in a diverse community including many alternative prey, but they had to share their main foods, small rodents, with many other predators. Erlinge's team concluded that the pressure exerted by the whole community of predators on the small rodents was so great throughout the winter, when the rodents were not being replaced by breeding, that by spring there was often little food left for the female stoats. Only in places with augmented supply, such as in the marshes with water voles, were a female's chances significantly improved.

Paradoxically, the general increase in the numbers of field voles through the study was not large enough at the critical times to make any difference. The main reason for this was, they suggested, that as the numbers of rabbits decreased, the other predators had to hunt more intensively for field voles. The decline in rabbits was not itself a crucial problem for the stoats, since the small Swedish female stoats seldom killed rabbits. But other predators sometimes also directly affected the stoat's breeding success (see Figure 2.8). Remains of stoats were found in the pellets of raptors, and two dens of breeding female stoats in meadows were dug up by some predator. Erlinge concluded that the increase in competition for voles, caused by the decrease in rabbits, was the reason for the poor breeding success and, in due course, the decrease in stoat numbers, despite the increase in density of field voles.

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