Weasels In Tundra And Boreal Forests

Weasels were entirely at home in the cold, windswept open spaces of Pleistocene times, and they still occupy the tundra and conifer forests of the far north today. Their speciality is hunting the various species of voles and lemmings that are also well adapted to living in those chilly habitats. In summer, these prolific little rodents make networks of runways through the matted felt of dead grass stems on the surface of the ground, and pull the green stems down through the tangle from below. Voles and lemmings are the favorite prey of all the cold-climate predators: hawks, owls, foxes, martens, and weasels. During summer, these other predators have to hunt voles and lemmings through the curtain of grass, while the small, northern weasels can follow them along their runways and into their nests. In winter, the migratory raptors move south, and deep snow protects the small rodents (at least to some extent) from foxes. Martens can get under the snow, but are too large to use vole and lemming tunnels.

Many northern populations of voles and lemmings display spectacular fluctuations in abundance every 3 to 4 years. In the summers that the rodents are increasing and at high density, raptors flock to kill huge numbers of them, and fledge large broods of hungry young. Predatory mammals reproduce well too, including weasels. The combined functional and numerical responses (defined in Table 7.1) of all local predators increases the toll on small mammals many-fold. But no predators can outbreed voles and lemmings at that stage, so the rodents can more than replace the losses.

The crunch comes in the following winters, when the permanent snow cover is established, the rodents are no longer breeding, and their food is getting scarce.

When the large predators have gone, the weasels carry on alone, searching out the diminishing numbers of rodents with deadly determination. The rodents have few defenses against predators that can follow them right into their last refuges. The weasels' relentless pursuit prevents the remaining rodents from rebuilding their populations, until in their turn, weasel numbers are cut down by starvation. When weasels become scarce, the surviving rodents have a breathing space to build up their numbers again, especially as, while rodents were few, their food supplies were able to recover. Improved food supplies and fewer weasels set the stage for the whole process to begin again. These repeated chains of events used to be described as "cycles," but there is important variation between places and years, so we prefer to use the less elegant but more accurate term "population fluctuations."

The most authoritative predator biologist of the 1930s to 1950s, Paul Errington (1963), did not believe that rodent populations in temperate habitats could be controlled by predation. His opinions were held to apply generally and were not disputed for a long time. In temperate habitats, Errington's judgment was more often right than wrong. But by the 1980s, a strong body of opinion had developed, pioneered by Pearson (1966, 1985) and Fitzgerald (1977) and developed especially by the Scandinavian school of ecologists (Hansson & Hent-tonen 1985; Henttonen et al. 1987; Sonerud 1988), holding that the northern voles and lemmings are different: Their population fluctuations are caused largely by continued heavy predation during the winters of the decline phase, due mainly to weasels. To test this hypothesis, one must count the numbers of both rodents and weasels in winter, just when fieldwork is most difficult. Fortunately, there is another way.

The nesting behavior of the small northern mammals offers a particularly useful opportunity to estimate predation rates. To survive the winter, each vole or lemming constructs a substantial nest of shredded grass stems under the snow. Weasels use these nests as temporary headquarters for several to many days, and they usually leave visible evidence of their stay in the form of scats and leftovers. Raiding nests can be very profitable for weasels, especially the nests of sociable voles that huddle together for warmth, and the large overwintering nests made by breeding female lemmings. The nests are well protected by snow during the winter, but on the arctic tundra in the first few days after the thaw they are clearly exposed to human view.

As the snow melted on Banks Island, in the Canadian Northwest Territories, Maher (1967) found 153 winter nests of lemmings, of which he reckoned 20% had been occupied by stoats. He believed that the stoats were responsible for the low density of lemmings in 1962-1963. At Barrow, Alaska, in 1968-1969, nearly twice as many lemming nests (35%) were raided by least weasels as on Banks Island (MacLean et al. 1974), and in the following year lemmings were so scarce that the number of nests found dropped from 770 to 0.

In northeast Greenland, numbers of lemming nests counted by Sittler (1995) on his 10-km2 study area varied from a high of nearly 3,700 in 1989-1990 to a low of 105 in winter 1991-1992. Stoats occupied the most nests during winter 1990-1991, when large numbers of newly independent young hunters helped to push down a crashing population of collared lemmings. By 1992-1993 the stoat population had disappeared. Where do they go? Presumably, anywhere that there are lemmings. Sittler reports (personal communication) that he has found lemming nests taken over by stoats at 83° 40', in the far north of Greenland less than 500 km from the North Pole.

In the high Sierra Nevada of California, Fitzgerald (1977) worked in alpine meadows buried under 1 to 3 m of snow from late November to mid-April. The small weasels he studied habitually carried voles they had killed back to eat in the safety of their dens, but they usually ignored the front of a vole's skull, including the large incisor teeth. Therefore, Fitzgerald could count the pairs of incisors left by weasels in occupied nests and estimate the numbers of voles killed. He concluded that the stoats and longtails living on the meadows removed up to half the overwintering population of montane voles in the winters 1965-1966 to 1968-1969, and after the winter of heaviest losses the voles were reduced to very low numbers (Table 7.2).

In Canada and Fennoscandia, all the species of small rodents living in one locality generally reach low density at the same time every 3 to 4 years, along with the shrews (Henttonen et al. 1987; Sonerud 1988). Intense searching by hungry weasels willing to kill any small animal they meet in that vast network of subnivean tunnels seems the obvious cause. What else could synchronize the

Table 7.2 Impact of Predation by Stoats on Overwintering Montane Voles in the Station Meadows (14 ha), Sierra Nevada

1966-1967

1967-1968

1968-1969

Mean no. voles/ha in autumn

25

83

127

No. vole nests/ha

23

65

85

Total no. nests examined

292

783

793

No. stoats resident

3

1

4

% nests occupied by

stoats

28

5

13

longtails

2

2

4

No. voles killed by

stoats

159

46

225

longtails

5

13

91

Mean no. voles/ha next spring

10

23

<2

% total losses attributed to weasels

>80

13

54

(From Fitzgerald 1977.)

(From Fitzgerald 1977.)

fluctuations of a whole community of unrelated small rodents with different population dynamics? Therefore, the population fluctuations of weasels and their prey in boreal regions have been studied extensively during the 1990s. Before we discuss the dynamics of these communities, though, we must first look at how weasels interact with rodents in the very different habitats of more temperate climates.

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