Variations In Weasel Density

The local distribution of weasels is closely related to that of their favorite prey. They haunt places where small rodents or rabbits may be found, such as hedgerows, stone walls, haystacks and brushland, thick forest, or old fields. They avoid places with little cover from hawks or owls for themselves or their quarry, such as ploughed fields and open-floored woodland. Their home ranges are small in places and habitats rich in food, and in these places their local density can be high (Table 10.2). For example, tracks of stoats in the snow can be found much more often in habitats rich in small mammals, such as among the cottonwoods or poplars and willows along river banks and in fields overgrown with scrub, than in open-floored conifer forests or open agricultural fields.

One of the most desirable places for a stoat to live is close to a large colony of breeding seabirds, where food is superabundant, even if only for a short period. Most such colonies are on steep cliffs or offshore islands, or they would not survive the attentions of predators for long, but New Zealand has two remaining very large colonies of burrowing Hutton's shearwaters located high above the treeline on the Kaikoura range of mountains facing the sea (Cuthbert & Sommer 2002). Stoats live in the colonies year round, and are ideal in both shape and behavior to raid the shearwater burrows both day and night, and to escape from the attentions of larger predators whenever necessary.

Surrounded by vastly more food (eggs, chicks, and adults) than they could possibly eat, these stoats can tolerate a local density (17 per km2) that is high by mustelid standards (Table 10.2)—and even this figure is probably an underestimate. The only higher density figures we could find were for Swedish stoats living in patches of marshland thick with water voles, but Swedish stoats are much smaller than stoats in New Zealand.

Weasel populations are linked to those of their prey in time as well as in space, and their variations in density with time are by far the more dramatic of the two. As in all animals that breed seasonally, including the small mammals on which they depend, the number of weasels in a given place is highest in mid- to

Table 10.2 Some Absolute Density Estimates for Weasel Populations (1 km2 = 100 ha)

Habitat and density

Species Country of weasels (n/km2) Reference

Stoat Holland, all year, 1960s

Southern Sweden, autumn 1974-1979

Ontario, all year 1973-1975

New Zealand, 1996

New Zealand, summer

1997-1999

New Zealand, winter

1999

Longtail Pennsylvania, January-March 1942

Michigan, January 1937 Kentucky, 1970-1975 Indiana, 1997-1999

Common Poland, all year 1971-1973 weasel Poland, all year 1985-1992

Least weasel

Finland, winter 1983-1987

Coastal dunes: Prebreeding 1.6, postbreeding 3.6 Average over rough pasture: 3-10; marshes with abundant water voles: up to 22 Average, including arable, short pasture, forest: 6; overgrown pasture and shrubby areas: 10 Beech forest, summer after seedfall: 4-10, over winter 0-2

In breeding petrel colonies: 17

On forested island: 3.1

Scrub oak-pitch pine forest: 12-15

Farmland: 3

Mixed farmland/forest: 2-18 Mixed farmland: 14

Mixed farmland: 1-7 Deciduous forest: summer 1.9-10.2; winter 0.5-2.7, depending on forest seeding

Farmland and forest: 2.413.0 depending on vole cycle van Soest & van Bree (1970)

Erlinge (1983) Simms (1979b)

Cuthbert & Sommer (2002)

M. Willans (unpubl.) Glover (1942b)

Allen (1938a) DeVan (1982) Gehring & Swihart

Goszczynski (1977) Jgdrzejwski et al. (1995)

Korpimäki & Norrdahl

(1989a)

late summer, at the end of the breeding season. At this time, the young leave their mother's protection and learn to fend for themselves. In a typical year, there will be easy prey available for the young weasels at first—young rabbits and rodents, which, like themselves, are new to independent life and inexperienced in avoiding danger. As the winter approaches and food becomes scarce for everyone, the seasonal crops of young animals—rodents, rabbits, and weasels alike— diminish. By late winter the twin scythes of starvation and predation have cut down the local populations of both predators and prey to their seasonal minimum.

Superimposed upon this regular variation between seasons, there may also be variations among years. Even in a favorable habitat, the local prey species have good seasons and bad ones, so the numbers of prey available in a given season are typically unstable from year to year. The numbers of weasels follow suit. Small prey species affect small weasel species, and larger prey affect larger weasels.

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