Density Variation In Least Weasels

The least weasel is often considered rather a rare species, since it is usually less well represented in museum collections than the two larger American weasels. Nevertheless, in some years least weasels suddenly become very abundant, only to disappear again just as quickly. The earliest evidence for this erratic variation in numbers came from fur buyers' returns.

Mr. Fryklund, a fur buyer from Minnesota interested in natural history, recorded the species of weasels he handled every year. He offered a better than average price to trappers for whole weasel carcasses, and was well known in Roseau County, so local trappers brought to him all the weasels they caught. From 1895 to 1927, he handled only seven least weasels. In winter 1927-1928, he handled three. But from November 1928 to April 1929, he got 59; from August 1929 to May 1930, 84; then from 1930 to 1935, only three. In his 40 years as a fur buyer, Mr. Fryklund handled 166 least weasels, 143 of them in the 2 years 1928 to 1930 (Swanson & Fryklund 1935).

In North Dakota in 1969-1970, the same thing happened. Wildlife biologists working on the food habits of red foxes noticed that least weasels suddenly began to turn up in the fox dens they excavated. Foxes kill least weasels but seldom eat them, so most of those collected by a hunting fox are left at its den. Also, fur buyers reported more least weasels than usual in those 2 years, and several least weasels were found dead on the roads. Altogether, biologists accounted for six least weasels from November 1968 to June 1969, 54 in the same period in 1969-1970, and eight in 1970-1971 (Lokemoen & Higgins 1972). This irruption of least weasels apparently followed a population increase in meadow voles.

Likewise, no least weasels had ever been recorded in Missouri and southwestern Iowa until 1963. Small rodents were abundant in 1966-1968, and then, in 2 months of intensive trapping in early 1968, Easterla (1970) caught eight least weasels in one 6-ha field.

During these periods of high density, least weasels disperse into new areas. For example, they were unknown in Kansas until 1965, but since then have been dispersing south, presumably stimulated by periodic surges in numbers (Choate et al. 1979). In the late 1980s they reached eastern Oklahoma, some 300 miles further south (Clark & Clark 1988). On the other hand, when the local population of small rodents crashes, least weasels simply disappear. Years of scarcity outnumber years of abundance for least weasels, which explains why sudden changes in the distribution or numbers of these normally seldom-seen little animals are so remarkable.

At Barrow, on the north coast of Alaska, the population fluctuations of the brown lemming have been observed since the early 1950s. Of 66 least weasels collected over the 20 years after 1953, 48 came from lemming peak years (MacLean et al. 1974). The response of least weasels to a lemming peak is presumably the same as that of common weasels to a vole peak, but the results can be even more startling. Not only can least weasels have a postpartum estrus, and produce several litters in very quick succession (Chapter 9), but they are also capable of producing extra large litters (of up to 19 young!) in lemming years (see Table 9.2).

Population fluctuations of least weasels, in relation to the density and distribution of rodents, have also been observed in northern and eastern Eurasia (Nasimovich 1949; Rubina 1960). The estimates of two to 13 least weasels per km2 over 4 years, calculated by Korpimaki and Nordahl (1989a) from snow tracking, may be a bit rough, but they certainly reflect the way that numbers of weasels follow changes in the numbers of small rodents in the far north.

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