The importance of water voles in determining the population density of stoats in some parts of Europe was confirmed by the work of Claude Mermod and his students Sylvain Debrot and J.M. Weber at the University of Neuchatel in Switzerland. They live trapped stoats in two valleys in the Jura Mountains. The Brevine valley (at >1,000 m altitude) had 1,875 ha of pastures, open fields, and forest. Peat bogs in the valley bottom and 86 km of stone walls offered plenty of shelter for stoats. When Debrot's study began in the summer of 1977, the density of stoats in the surrounding area (estimated from regional hunting statistics) was high (6.8C per 100 TN), but over the following two summers the numbers of stoats in the region fell, reaching 0.7C per 100 TN by 1979.
The number of stoats live trapped on Debrot's study area dropped from more than 50 to only three over the same period (Debrot & Mermod 1983). At first, the resident stoats ranged almost everywhere except into the forest, but as numbers fell, the residents that remained became more or less restricted to the bogs. The reason for the crash was that 1975 had been an extraordinarily good year for water voles, which then disappeared over the following year and remained scarce for several more. When water voles were abundant, the Brevine stoats ate them almost exclusively, and thrived (probably reaching their highest numbers in 1976). When the water voles disappeared, the stoats followed suit. Debrot (1983) showed, from game and fur records covering the surrounding district and extending back to the 1950s, that the correlation between the numbers of stoats and water voles in that area was quite general. Each population peak of water voles (about every 5 years) was followed within a year by a peak in numbers of stoats.
Debrot also had a 616-ha second study area, the Val de Ruz, at 700 m elevation. Here, prey resources were more diverse and less variable, and the density of stoats remained fairly stable around 2.6C per 100 TN over 3 years beginning in April 1978. The stoats' diet in Val de Ruz included more wood mice and birds, and fewer water voles, than at Brevine (Debrot & Mermod 1981).
The study of the stoat population of the Val de Ruz was extended until early 1985 by Weber (1986), using the same "minimum number alive" method of counting resident stoats. The numbers of water voles were again high in 1982, but the response in the Val de Ruz stoats was far less drastic than had been observed at Brevine.
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