The general conclusion that the population density of stoats is controlled by variations in the density of their prey is inescapable. Exactly how the ups and downs of vole numbers induced matching effects in stoat numbers was, however, unknown until recently. Popov (1943) reported an early clue, which went unnoticed for decades. In 1937-1938, when stoats were numerous in the Tatar Republic (in the former USSR), the proportion of young in the fur trapper's catch rose to 65%, but when stoats were scarce (1939-1940), it dropped to 19%. The reasons for this shift in age structure remained unexplored at the time.
Since then, long-term attempts to answer that question have been made in two very different environments, using two quite different methods. In Sweden, Erlinge (1983) used live trapping and radio tracking over 6 years to follow the fortunes of the members of an undisturbed population of stoats living on a large area of pastures and marshes. In New Zealand, regular kill trapping was used over 8 years to sample stoats living in three simple forest communities (King 1983b; Powell & King 1997). Radio tracking in these and other study areas added further information later (Murphy & Dowding 1995; Dowding & Elliott 2003; Purdey et al. 2004).
The habitats occupied by the stoats observed, their average body sizes, and the prey resources available to them were all wildly different in the two countries. The most significant contrast, though, is between kill trapping and live trapping as methods of counting the numbers of stoats present. These differences in methodology are also an advantage, because they produce complementary kinds of information. Some data (e.g., on the home ranges and behavior of individuals) can be gained only by watching undisturbed, live animals. Equally important data (e.g., on the age structure, fecundity, and pregnancy rates of populations) can be obtained only from systematic examination of large samples of carcasses. The impressive thing is that the results support each other, and can be integrated to provide a fascinating general view of the way stoats adapt their lives to the resources at hand.
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