Why do some animal populations fluctuate--abundant at times, or rare at others? Moreover, why would one group of animals fare differently from a similar group, under identical weather conditions? Those are questions ecologists constantly attempt to answer.

One way to do that is by using a census method, although it can be difficult to test whether yearly counts of wild animals can accurately determine the factors that cause fluctuations in population numbers.

Tim Coulson and his Cambridge University research team were up to the challenge. The results of their 11-year study of wild Soay sheep will be published in the 25 May issue of the international journal, Science.

Even though the Soay live in relative isolation, they are a good model for ungulates, or animals with hooves, and the researchers say the findings should be applicable to other ungulates, like white-tailed deer in the United States, and red deer in the United Kingdom. It is essential for determining how a certain management strategy can affect the future population of an animal. For example, knowing how many fish or deer can be removed from a population and what the consequences would be.

The findings show for the first time, by figuring out how competition for food, weather, and population structure interact to cause population changes, it's possible to predict what the population will do in the future.

The subjects of the decade-long study are Soay sheep, a rare breed living in the wet and windy climate of St. Kilda, a remote, uninhabited island off the coast of Scotland. The researchers studied 3000 wild sheep from 1986 to 1996.

Using a statistical analysis, they found population size and weather were strongly associated with the proportion of animals that die. In adults, the inclement weather had a strong influence on mortality rate. They also found weather at different times of the year influenc
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Contact: Cherita Gonzales
cgonzale@aaas.org
202-326-6414
American Association for the Advancement of Science
24-May-2001