For the small stoats living in open habitats across North America, the most important prey are usually the local species of voles. Changes in the numbers of the small northern stoats are usually linked to widespread changes in abundance of voles, from great scarcity one year through enormous abundance and back again 3 to 5 years later. In high arctic Greenland, the only rodents available are collared lemmings, and there the interactions between lemmings, stoats, arctic foxes, snowy owls, and long-tailed skuas have been worked out in great detail (Gilg et al. 2003). The peaks in abundance of lemmings recur every 4 years, driven by the 1-year delay in numerical response of stoats (see Figure 7.2). In the Kluane area of the Canadian Yukon Territory, Krebs and his team (2001) documented the changes in numbers of predators through the 10 years of a typical snowshoe hare cycle. Systematic counts of snow tracks showed that weasels (mostly the tiny northern stoats, 45 to 106 g) began to increase after a 1991 peak in red-backed voles, and reached high numbers (34 tracks per 100 km per night) in 1994, the year after an irruption of Microtus in 1992-1993.
In southern Quebec, a 3-year live-trapping study attempted to document this relationship, on two 90-ha areas of farmland about a kilometer apart. Raymond and Bergeron (1982) estimated the densities of meadow voles and other small rodents, and caught, marked, and released 94 stoats in the years 1978-1980. In each area, changes in the numbers of voles were reflected in the numbers of resident stoats caught in autumn, particularly in the numbers of young. Most of the females had at least attempted to raise a litter every year, because 18 of 21 females showed signs of lactation in early summer. The close link between vole density and the number of young caught in autumn, therefore, suggests that litters did not survive well unless voles were abundant. Other experiments by the same team have shown that the profitability of meadow voles to a hunting female stoat is higher than that of any other prey species (Raymond et al. 1990).
The long-term records of the Hudson's Bay Company fur-trading posts have often been used to ask questions about the population dynamics of northern species, a practice going back to Charles Elton in the 1920s. As mathematical tools have become more sophisticated, analyses can eliminate more of the inevitable "noise" in the data and come to more useful conclusions from historical material. A recent example is the reexamination of records from 45 fur-trading posts in eastern Canada from 1915 to 1940 by Johnson et al. (2000b). The trading posts sampled the populations of ermine (stoats) across a region of comparable habitats occupied by fluctuating species of voles. Such data inevitably are contaminated by unknowable errors, such as whether all the furs counted were the same species (the sample area was north of the range of the long-tailed weasel, but some of the pelts counted could have been least weasels) and how (not whether) pelt price influenced trapping effort. Nevertheless, comparisons of fur returns with data on the local vole populations confirmed that voles and stoat populations fluctuated together in a near-cyclic pattern that was stronger toward the north (Chapter 7).
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