U.K. Red Squirrel Drop: Are Gray Squirrels to Blame?

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As summarized in the March issue of the journal Ecology Letters, the team plugged in data on competition, population sizes, breeding, and death rates and infection scenarios for both species.

The researchers found that simulations based on competition over food alone didn't closely match the real speed with which the gray squirrel replaced the red in the wild, although these models did predict that competition alone could kill off red squirrels over longer time spans.

However, when the team included data on the effects of the virus, the result was quite different. "The output is a near perfect match to the replacement pattern that has been observed [in the wild]," said Tompkins. "We are not proposing the virus as a stand-alone explanation for the red squirrel decline," said Tompkins, who suggested that the gray squirrel's phenomenal success has been due to an interaction between the disease and competition.

"The concept of disease-mediated competition is a new and important consideration in problems with introduced species," said Robert Kenward of the Centre for Ecology and Hydrology at Dorchester in England. This research, "is the best evidence yet that disease aided red squirrel replacement" in some parts of England, he said.

Wild Experiment

The idea that parapoxvirus lies behind the decline may be about to get a dramatic test in the natural environment. Throughout the last century, rates of red decline in both Scotland and Ireland have been markedly lower than in England and Wales. Tompkins believes that this could be due to a lack of the virus in the grays in these areas, which were introduced from different sources to those carrying the virus in England.

Now however, expanding gray populations from uninfected Scotland and infected England are on a collision course. Compelling evidence for the role of the virus would be if the replacement by gray squirrels does indeed accelerate, when the populations begin to mix, and the virus gets into Scotland, said Tompkins.

Wildlife Trusts, a partnership of conservation nonprofits based in Nottinghamshire, England, announced earlier this month that a parapoxvirus outbreak has been found in England's most southerly remaining red squirrel population, found in Sefton, Mersyside relatively close to the Scottish border.

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