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

John Pickrell
for National Geographic News
May 2, 2003
Although most people in the United Kingdom have only known one species of squirrel to visit their back yard, the familiar gray squirrel is in fact a recent immigrant. As little as one hundred years ago, the typical visitor was the native red species cheerily depicted as Squirrel Nutkin in Beatrix Potter's well-loved 1903 children's book.

Though some naturalists believed a virus was introduced with the arrival of the gray, its role in contributing to the red squirrel's decline had been largely discounted. Now, according to new research, the deadly parapoxvirus may have had a much greater role in the century-long decline of the red, than previously supposed.

"Competition over food alone is not a mechanism of sufficient strength to account for the rate and pattern of red decline and gray squirrel expansion," said Daniel Tompkins an ecologist formerly at the University of Stirling in Scotland. "The virus may be the missing component from the explanation," he said.

Bolstering this argument, is evidence that a current outbreak of the same virus may be decimating small remaining strongholds of red squirrels in Mersyside, northwest England.

Relentless Spread

The mightily invasive North American gray squirrel (Sciuris carolinensis) was introduced into England in the late 19th Century from several sites such as Woburn Abbey, in Bedfordshire. From these few sites, the rodent has spread rapidly, replacing the native red squirrel (Sciuris vulgaris) in much of England, Wales, and the Scottish lowlands.

More than 2.5 million grays now call the British Isles home, almost 16 times the number of the 160,000 remaining reds. "The red squirrel generally disappeared from each locality…at the same time that the gray squirrel reached them," said Tompkins, now based at the University of Otago in Dunedin, New Zealand. Ecologists have mostly pinned the gray's success on its superior ability to compete for similar nuts, insects and berries. The competitive advantage may even hinge on its habit of eating acorns, which the red is unable to utilize—touted as one possible explanation for the red's slower rate of decline in the largely coniferous northern forests of Scotland.

Parapoxvirus has rarely been part of the traditional explanation for the decline because it is not easily noticeable in the wild, said Tompkins.

Even though sick animals suffer from lesions and scabs around the face, feet and genitals, these animals have been rarely observed, "due to a combination of low prevalence of infection and the fact that animal carcasses in the wild, are never found, or disappear quickly due to scavengers," he said.

Evidence suggests that gray squirrels carry the virus, but they don't appear to succumb to infection, unlike reds, which appear to suffer 100 percent mortality in the wild.

Number Crunching

Previous studies had suggested that competition over food alone could not account for the very rapid decline of the red. Assisted by his University of Sterling coworker, biologist Michael Boots, and mathematician Andy R. White of Herriot-Watt University in Edinburgh, Scotland, Tompkins decided to use computer simulations to test the idea that parapoxvirus was a factor.

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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