In Natural Populations Sweden and Switzerland

The first method of estimating survival, by following cohorts of living animals, was applied by Erlinge (1983) to 6 years of data on stoats in southern Sweden (Table 11.1). In this type of table, the data are set out as a declining total: so many live animals of age x, of which so many are still alive at age y, so many at age z, and so on. Erlinge had no way to estimate whether any stoats lived for more than 6 years, or to distinguish between death and emigration as reasons for losses of marked stoats. The study area was very large (40 km2), however, with traps distributed throughout, and the stoat population was undisturbed. Consequently, the turnover of resident stoats documented by Erlinge was at least natural, even though he did not know that every stoat that disappeared had died.

The great advantage of this method is that the calculations are unaffected by changes from year to year in the age structure and rate of increase of the population. The disadvantages are that the result is a life table for only a few cohorts in given years, not the population as a whole in all years; that collecting long-term data from marked live animals of known age is a prolonged and time-consuming business; and finally, that it is extremely hard to meet the minimum sample size for a useful table, about 150 animals (Caughley 1977: 95).

Erlinge (1983) started live trapping in the autumn of 1973, and captured, marked, and recaptured 75 males a total of 232 times, and 67 females 171 times, over the years 1974-1979. Those first marked as young born in the years 1973— 1976 inclusive, whose ages were known with certainty, were followed for as long as they lived on the study area. Of 47 males present in the study area in their first autumn, 28 were still present a year later, and of these, nine another year later, and so on (Table 11.1). The oldest animals were a male that stayed for 4.5 years and a female that stayed for 3.5 years. Both were born in 1975, the year of highest numbers and most successful reproduction in that population; the longevity of those born in the last 3, lower density years was unknown. The average expectation of further life in this study area for a newly independent stoat aged 3 to 4 months was 1.4 years for males and 1.1 years for females.

The mortality rate among first-year males was 40% (4 out of10 disappeared between the ages of 0.25—0.5 years and 1.25—1.5 years), and significantly higher (68%) a year later. Male stoats live in the fast lane, so it is not surprising that one male that had been radio tracked through two breeding seasons looked "shabby and worn out" well before his second birthday (Sandell 1988).

Table 11.1 Life Table for Stoats in Sweden from Live Trapping of Marked Individuals

Number Proportion surviving at Proportion dying Age class1 alive start of age class (lx) at that age Mortality rate qx%

Table 11.1 Life Table for Stoats in Sweden from Live Trapping of Marked Individuals

Number Proportion surviving at Proportion dying Age class1 alive start of age class (lx) at that age Mortality rate qx%

Males

3-6 months

47

1.00

0.40

40

1.25-1.5 years

28

0.60

0.41

68

2.25-2.5 years

9

0.19

0.15

78

3.25-3.5 years

2

0.04

0.02

50

4.25-4.5 years

1

0.02

0.02

100

Females

3-6 months

48

1.00

0.54

54

1.25-1.5 years

22

0.46

0.33

73

2.25-2.5 years

6

0.13

0.11

83

3.25-3.5 years

1

0.02

0.02

100

1. All animals born in the cohorts of 1973, 1974, 1975, and 1976 are considered together. They are grouped as if they had all been born at the same time, and the survivors counted at the end of every year. (From Erlinge 1983.)

1. All animals born in the cohorts of 1973, 1974, 1975, and 1976 are considered together. They are grouped as if they had all been born at the same time, and the survivors counted at the end of every year. (From Erlinge 1983.)

For females, the mortality rates were even higher in both their first (54%) and second (73%) years. These figures mean that about half the stoats present in autumn will disappear before spring every year, regardless of the autumn density. There will also be additional losses over the summer.

Another live population was observed in some detail at the Val de Ruz, in Switzerland. The study area was rather small, but the population was quite stable. Debrot (1984) did not calculate a conventional life table, but used a different means of arriving at a similar conclusion. He counted all young of the previous year as adults on January 1, and in the three Januaries of 1979-1981 the total number of newly recruited plus older adults averaged 6.2 on the 616-ha study area. They disappeared at an average rate of 68% a year, which means that two out of every three adults present on any given day will have gone a year later. Conversely, the annual replacement rate of adults was 93%.

Such a high turnover, Debrot admitted, reflects the large dispersal range of the adults compared with the size of the study area. The rapid replacement rate also explains why most of the marked stoats were recaptured infrequently (average 2.3 times each), and why only 7% of adults lived on the study area for more than a year. Some individuals lived (not necessarily only on the study area) to over 4 years old, but the average age of the resident animals was 14.4 months.

This is not the same statistic as expectation of life at independence, but it emphasizes the same point, that most stoats do not live long. In particular, the average life spans of stoats in both populations observed by live trapping were not noticeably longer than in the populations observed by removal sampling, described below.

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