Counting sheep may settle old score

ISLAND sheep may help to resolve a fierce debate in ecology about population crashes. British researchers are using sheep populations on St Kilda, a remote archipelago off the west coast of Scotland, to tease apart the relative importance of climate and local factors such as the availability of food. Ecologists usually do this by comparing populations living near each other. But animals migrating from one population to the other can confound the results. Now Bryan Grenfell of the University of Cambridge and his colleagues have analysed 40 years of sheep population data from the islands of Boreray and Hirta, between which migration is impossible. “Sheep don’t swim,” Grenfell says. They report in this week’s Nature (vol 394, p 674) that the numbers of sheep on the two islands change in synchrony,
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