Science study shows age, sex, weather, factors in fluctuating Soay Sheep population

May 24, 2001

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 influenced the probability of death.

Researchers then put their results in the form of mathematical equations, into a "matrix" model that could be used to predict future die-offs.

"Once we had pieced together the model, we ran it into the future and looked to see whether the models accurately predicted the size of the population for three years...and compared it with the number of sheep that were living in the study area. The model predicted what happened accurately," Coulson said.

"We have shown that to do this, one needs to consider how competition between animals, weather and the structure of the population are linked," Coulson said.

The model suggests the dynamics of animals are a mixture of several processes, which, Coulson says, shouldn't surprise many people.

"We have shown that it is possible to describe these interactions, and by so doing, one can predict, at least in the short term, how populations are going to change in size."
The 25 May issue of Science includes a related Perspective article, "Not All sheep Are Equal," by Jean-Michel Gaillard, Marco Festa-Bianchet, and Nigel G. Yoccoz.

American Association for the Advancement of Science

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