Density Variation In Common Weasels

Game records show enormous variations in the numbers of common weasels killed each year on English estates. Common weasels are capable of quite startling irruptions, up to a fivefold increase in numbers over the previous year— as illustrated for one estate in Figure 10.6. The best known of these irruptions was clearly related to the huge increase in food supplies for common weasels that followed the arrival of myxomatosis. When the rabbits had gone, grass and herbs previously nibbled constantly short flourished with unprecedented vigor (Sumption & Flowerdew 1985). The densities of small rodents reached record levels, followed by the common weasels. The contrast with the effect on stoats (Figure 10.2, from the same estate as Figure 10.6) is remarkable.

Since myxomatosis, lesser variations in the population densities of common weasels have become clearer. These are also linked to the availability of small rodents, especially voles. Early proof of the connection came from North Farm, the English game estate where Tapper (1979) worked. The numbers of common weasels caught followed the numbers of voles, delayed by a few months because voles and weasels were not counted at the same time of year (Figure 10.7). The same pattern was probably repeated all over England. For example, the records plotted in Figure 10.6 show an extra-large crop every third or fourth

Figure 10.6 The numbers of common weasels killed by gamekeepers increased suddenly after the arrival of myxomatosis on one game estate in England in 1953, and on average remained much higher than previously until the mid-1970s, when rabbit and stoat populations began to recover. (Data from the Game Conservancy, courtesy of S. Tapper.)

Figure 10.6 The numbers of common weasels killed by gamekeepers increased suddenly after the arrival of myxomatosis on one game estate in England in 1953, and on average remained much higher than previously until the mid-1970s, when rabbit and stoat populations began to recover. (Data from the Game Conservancy, courtesy of S. Tapper.)

Figure 10.7 The numbers of common weasels and their breeding effort were both closely related to the supply of field voles in Tapper's study area in England. The numbers of common weasels killed by game keepers has a solid line; vole density dashed line; percent voles in diet of weasels black in pie charts; percent adult female common weasels breeding bar graph. (Redrawn from Tapper 1979 and Anon. 1981.)

Figure 10.7 The numbers of common weasels and their breeding effort were both closely related to the supply of field voles in Tapper's study area in England. The numbers of common weasels killed by game keepers has a solid line; vole density dashed line; percent voles in diet of weasels black in pie charts; percent adult female common weasels breeding bar graph. (Redrawn from Tapper 1979 and Anon. 1981.)

year (up to twice the number caught the previous year) throughout most of the 1960s and 1970s, suggesting that those were the years when field voles were abundant. More recent data from Bialowieza Forest (described below) document similar relationships between common weasels and woodland rodents.

These variations in the densities of common weasels come about because shortage of food has the same controlling effect on their productivity as it has on stoats, although their reproductive physiologies are very different.

Breeding is a hugely expensive undertaking for the small weasels. The average British female common weasel, weighing about 60 g, has to find nearly 1 g of suitable food per hour, or 22 g per day every day, all the year round (Hay-ward 1983). Her needs increase only modestly (6% to 7%) during gestation, but soar to 80% to 100% extra during lactation, to about 70% of her own weight daily. In cold climates, and when the young are nearing weaning, the additional demand is even higher, reaching up to five to six times her own needs, or up to twice her own weight daily.

Because reproduction is so very energy intensive, there is a minimum density of available prey below which raising a litter is simply not possible. For common weasels, this minimum appears to be about 10 to 15 voles per ha, preferably Microtus (Henttonen et al. 1987). More precisely, common weasels need access to at least five reproducing female voles per hectare (Erlinge 1974; Tapper 1979; Delattre 1984).

If the main prey are wood mice, weasels seem to need even more prey to support breeding. For example, in an area of farmland in France where Delattre (1984) was trapping, the combined density of all small rodents was very low, and many of those that were there were yellow-necked and wood mice (see Table 7.5). In the first year, while voles were still in the majority, the common weasels in the area bred well. But in the second and third years, mice made up 80% of the rodents present and the weasels decreased. Delattre suggested that common weasels are so closely dependent on voles that they cannot breed or maintain their populations on mice alone, even if mice are relatively abundant. Later observations showing how vulnerable mice are to being caught within their burrows (Jgdrzejwski et al. 1992) casts some doubt on that interpretation, but the general point is valid.

In vole crash years, the breeding success of adult female common weasels is poor, and the few young females that are born do not breed themselves until the following year. For example, in Tapper's study area in Sussex (Figure 10.7), voles comprised between a quarter and a half of the weasels' diet in 1972 and 1974, and at least half the female weasels caught were breeding. In 1973 and 1976, the weasels had to turn to other foods, and few or none managed to produce young.

The total failure of an entire year's reproduction in a small, short-lived animal such as the common weasel has a serious effect on population density. When it happens on more than a very small scale, the result can be local extinction of that population. For example, in Bialowieza forest in Poland, in April and May of 1991, the density of female voles was 0 to 1.8 per ha. This density is well below the minimum needed to support breeding by the common weasels (Jgdrzejwski et al. 1995). Local extinction of weasels by spring 1992 was inevitable, since almost all the adult weasels present at that time were born in the previous year, and few weasels survive into their third year of life.

On the other hand, common weasels can respond to increases in voles much more rapidly than can stoats. Some idea of the speed of this reaction, and of the following decrease, can be gained from another study done by Delattre (1983) in the foothills on the French side of the Jura Mountains. At 600 to 1,000 m elevation near Levier, field voles staged a population irruption in the summers of 1979 and 1980. Delattre reckoned that in August 1980 the density of field voles was 100 to 200 per ha. The inevitable crash, over the winter of 1980-1981, was so complete that by May 1981 the density of field voles was down to less than a single vole per hectare.

The period of plenty was short, but while it was on, the response of the common weasels was spectacular. Delattre caught none on his 100-ha study area in August 1979, and only two outside it. At the end of 1979, weasels moved in and increased in numbers throughout 1980. In that season, two cohorts of young were produced; the first appeared from May to August, and the second from September to November. Some of the females were still suckling in October 1980. By May of 1981, 19 individuals were living in the area. Two months later, only one was left.

In a bumper season for small rodents, therefore, common weasels increase their reproductive output as do stoats, but by a different and far more effective means (Table 10.3). Potential litter size in common weasels in Britain is generally rather less than in stoats (see Table 9.2). But in good seasons, the best that the stoat can do is to decrease juvenile mortality, whereas the common weasel not only does that but also has the option to increase fertility. Increased fertility can produce far more rapid population increases in common weasels, even though their litters are smaller on average than those of stoats (McDonald & Harris 2002). The difference is due to the key part played by young females.

Young common weasels of both sexes can breed in the season of their birth, and delayed implantation does not restrict them to only one litter a year. In a vole peak year, the adults present in spring produce their first litters in about May. Abundant food eases the effort of providing for the litter, and fewer young are lost than usual. In August or September, each mother still alive can produce a second litter, which means that she has doubled her fertility for the season—something a stoat cannot do. This doubled fertility is not in itself, however,

Table 10.3 Effects of Delayed Implantation on the Response by Populations of Stoats and Longtails Versus Common/Least Weasels to Variation in Food Supplies

Stoat/longtail

Common/least weasel

Delayed implantation?

Yes

No

Fecundity

High (6-20)

Low (4-8)

Earliest possible age of

12 months

3-4 months

female at first littering

Life span

1-8 years

1-3 years

Response in good years

Much of the high potential

Additional summer litters

fecundity realized in a single

produced (second litter in

large litter; increase in fecundity

adults, first in early-born

impossible

young females)

Response in bad years

Increase in prenatal and nestling

Decrease in fertility of adults,

mortality

no summer litters

References

(King 1981a; King & Moody

(Tapper 1979; King 1980c;

1982; King 1983b; McDonald

Jgdrzejwski et al. 1995;

& Harris 2002)

McDonald & Harris 2002)

enough to permit the far greater numerical response of common weasels compared with stoats.

The really important difference is that by midsummer the early-born young female common weasels have several times the numbers and only half the mortality rate of the adult females. Yet these young females are fully mature and capable of producing young if well-enough fed. So, the breeding stock in late summer contains many more females 3 to 4 months old than 15 or more months old.

By the time of the autumn peak in weasel numbers in a year when voles are abundant, the majority of the populations of both stoats and common weasels are young animals. In common weasels the great majority of these young animals were produced in midsummer by the early-born young females of the same breeding season. By contrast, all young stoats are born in spring.

Under ideal conditions, a single adult female common weasel in spring can have 30 descendants by autumn, if she bears two litters of six herself (with equal sex ratio) and the three early-born females produce six each: (2x6) + (3x6) = 30. This astonishing reproductive capacity is more than sufficient to account for the sudden irruptions of common weasels reported during vole peak years (King 1980c; McDonald & Harris 2002). In fact, during one of the few irruptions actually measured, after a heavy seedfall in Bialowieza forest in Poland, the density of common weasels increased fivefold in less than 3 months (from 19 to 102 weasels per 10 km2) (Jgdrzejwski et al. 1995:189).

The Bialowieza study documented in great detail for common weasels the same three-stage seedfall-rodent-mustelid interaction as the one that produces massive variations in the densities of stoats in New Zealand southern beech forests. The stone-in-a-pond effect is strikingly similar: Both species of weasels adjust their reproductive effort to the spring density of rodents. The opposite reproductive physiologies of common weasels and stoats, however, make the mechanisms of their responses quite different (Table 10.3).

The Polish study also settled an old argument about whether the weasels' response to an increase in voles is delayed, as suggested by Tapper (1979) from gamekeepers' records (mainly from the spring trapping season), or immediate. The Polish team's intensive year-round monitoring of weasel numbers demonstrated that the effect is immediate. After a superabundant seed crop of oak, hornbeam, and maple in Bialowieza forest during the autumn of 1989, voles (Clethrionomysglareolus) and mice (Apodemus flavicollis) bred throughout the winter of 1989-1990 and rapidly increased in numbers (Jgdrzejwski et al. 1995). At their peak in late summer 1990, the forest rodents numbered almost 270 per ha, and supported a postbreeding population of 10 weasels per km2.

Over the winter of 1990-1991, the rodent and weasel populations crashed together (Figure 10.8). The critical ratio of rodents to weasels required for breeding, about 400 rodents per weasel, was maintained so far as possible by expansion of individual home ranges. Beyond the physiological limit of that adjustment, however, local failure in breeding became inevitable. In spring

1990 1991

Figure 10.8 Population fluctuations of common weasels (solid line) and rodents (dashed line) in Bialowieza Forest, Poland. Weasel abundance was indexed using live trapping (open circles) and direct observations and radiotelemetry (solid circles). (Redrawn from Jgdrzejewski et al. 1995.)

1990 1991

Figure 10.8 Population fluctuations of common weasels (solid line) and rodents (dashed line) in Bialowieza Forest, Poland. Weasel abundance was indexed using live trapping (open circles) and direct observations and radiotelemetry (solid circles). (Redrawn from Jgdrzejewski et al. 1995.)

1991 insufficient numbers of rodents were left to support weasel reproduction on even the largest possible home range. Since very few weasels survive to their third year of life, the local population of common weasels was extinct by spring 1992.

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