Survival In Common Weasels

Populations of common weasels are unpredictable, so no one has attempted to tabulate their age structures by following marked live individuals. The only thing that can be said with certainty is that there is generally a new set of resident common weasels on a study area each season, and that no individual has been known to hold a home range for more than 3 years (Chapter 8). This implies that most common weasels live only for a year or less.

Confirmation of this guess comes from the only three attempts made to construct a frequency distribution of annual age classes from collections of carcasses, one in Denmark and two in Britain (Figure 11.5). The huge preponderance of first-year animals in all three samples confirms our expectations. Various other studies have used age classes defined in different ways, which give results of the same general order but usually are impossible to compare in detail.

The first attempt to construct a life table for common weasels on British game estates, by King (1980c), was based on a sample of 455 dead common weasels, collected all the year round (Table 11.4). All the data were pooled, since there were no significant differences between the age distributions for the five sample areas or for the two sexes. The first-year mortality rate was very high (80% in males, 75% in females), which is not surprising for small predators that live life at such a hectic pace. The expectation of further life for a young weasel at independence was about 10 months in both sexes. The density of a population will therefore depend heavily on the production and survival of young; since this varies drastically according to food supplies, the instability of populations of common weasels needs no further explanation.

As in stoats, and for the same reason (both samples were collected from gamekeepers), there was a well-defined peak in the risk of mortality every year in spring (Table 11.3). On the other hand, spring is a stressful time for weasels anyway, especially if food is short. In Wytham, a protected population never subject to regular trapping, several residents that had been watched for months

11.5 The vast majority of common weasels are under a year old, and the of them reach only 3 years of age. England, 1968-1970 (King 1980c); , 1995-1997 (McDonald & Harris 2002); Denmark, 1969-1970 (Jensen

Figure oldest Britain 1978).

11.5 The vast majority of common weasels are under a year old, and the of them reach only 3 years of age. England, 1968-1970 (King 1980c); , 1995-1997 (McDonald & Harris 2002); Denmark, 1969-1970 (Jensen

Table 11.4 Life Table for Common Weasels in Britain in the 1970s and the 1990s from Age Determination of Carcasses

Age class

Number alive

Proportion surviving at start of age class (lx)

Mortality rate at that age (qx%)

Survival rate during age class (px)

1970s (King 1980c) Males

0.25-1 year 339 1.00

1-2 years 69 0.20

2-3 years 8 0.02 Females

0.25-1 year 116 1.00

1-2 years 29 0.25

1990s (McDonald & Harris 2002) Males

344 1.00

50 0.15

0.25-1 year 1-2 years Females 0.25-1 year 1-2 years

59 0

0.85

0.97

0.15

0.03

died or disappeared in spring, often after drastic loss of weight (King 1975c), and the age distribution of weasels from Wytham was not different from those on the game estates.

When McDonald and Harris (2002) made a second collection of common weasel carcasses from British gamekeepers, numbering about the same (n = 458) but sampling a larger selection of estates, they found similar general patterns (e.g., first-year mortality rate 85% and 97%). But in addition, they contributed two important new details to the story. Their two-species collection strategy enabled them to compare the effects of culling on stoat and common weasel populations sampled in the same areas and by the same means, and their more advanced data analysis techniques allowed a first test of the importance of the late summer litters to the population biology of common weasels.

McDonald and Harris applied to common weasels the same population model that they had developed for stoats (with a slight variation to allow for the different reproductive cycles of the two species). They included the same necessary assumptions (survival independent of density, population closed), except that young common weasels were considered fully independent by the age of 7 weeks.

Again, the model proved extraordinarily useful despite its limitations, because it demonstrated both the similarities and the differences between these two coexisting species. In both, the single factor most critically influencing r was the survival of the first-year class, but the difference was that, for the British population of common weasels during the years they sampled, r averaged a positive figure, 0.30.

To reduce this positive figure to a negative, that is, to make a breeding population of weasels decline, the survival rate of newborn weasels in the first 3 months of their lives set in the model had to be cut from 0.88 to under 0.56, which might well mimic what happens in less favored habitats when food is very short (Chapter 9). An alternative means of inducing a decline was to reduce the probability of late summer litters (Chapter 10) from 1.0 to 0.4. Conversely, if the survival of newborns and the probability of second litters were both set at high values, as during a rodent irruption, the weasel population growth rate soared.

In reality, both these parameters are strongly affected by food supplies and are probably never the same for 2 years in a row, so the actual numbers are less important than what they tell us about the great capacity of common weasels to respond quickly when rodent numbers increase, and their equally sudden disappearance afterward. Like stoats, common weasel populations are unstable, but they can compensate for the annual harvest imposed by gamekeepers more easily than can stoats.

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