Red Alert ; with Their Tufty Ears and Bushy Tails, Red Squirrels Beat Their Grey Cousins in the Beauty Stakes - but Are Losing the Battle for England's Forests. Could a Mass Cull of Greys Save This English Icon from Extinction? Sanjida O'Connell Reports

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These days, a glimpse of a red squirrel is a rare treat. Numbers have declined so much that it is thought they could be extinct in the UK within 20 years, and a cull of grey squirrels has been announced to help protect the red's declining numbers. It's a policy that has divided environmentalists and nature lovers all over Britain. The biodiversity minister, Jim Knight, says: "Many people love grey squirrels, but the reality is that they are a real problem for some of our most threatened native species."

There are thought to be more than 2 million grey squirrels, which outnumber the reds by 66 to one. In a departure from normal planting methods, the Forestry Commission is not going to grow any more oak trees in Europe's biggest managed conifer forest after computer modelling carried out by Newcastle University and Queen Mary and Westfield College, London, shows how the grey squirrel is pushing the red to the brink of extinction.

The red squirrel migrated to Britain 10,000 years ago during the last Ice Age. Since the grey was brought over from America in the 19th century, the red has steadily declined. Explanations for this range from mating with the reds to produce infertile offspring to aggression. "These theories have been discounted," says Dr Peter Lurz, part of the team at Newcastle. "The greys don't beat up the reds. The two species ignore each other. And there is no influence on mating."

Research has shown that the main impact is during the juvenile stage. If reds live in an area that becomes infiltrated by greys, they normally disappear after about two years as the adults die and young reds are unable to colonise the area. One reason is because the grey is well suited to this country: they prefer broad-leafed trees, like oak, since they are similar to their native hickory habitat, and are better able to digest tannins in seeds like acorns. In contrast, the reds are better adapted to conifer forests containing trees like the Scots pine.

The reds have been reduced to between 20,000 and 30,000 animals in southern Britain. About 4,000 are found in small pockets in Wales, Thetford Forest in East Anglia and islands off the South Coast, such as the Isle of Wight. The only large remaining area suitable for red squirrels, which contains the remainder of the species, is Kielder Forest in Northumberland and north Cumbria However, grey squirrels are encroaching from the north, from the Border Counties as well as from the south. Kielder Forest, which spreads across 50,000 hectares, is likely to be one of the last strongholds of the red squirrel in England.

If the reds only faced competition for food at the juvenile stage, it is unlikely they would be at such a critical junction. Back in the late Seventies Dr Jonathan Reynolds, now a research scientist at the Game Conservancy Trust in Hampshire, was studying red squirrels at Thetford Forest, in an attempt to understand why the species was faring badly. He concentrated on their feeding behaviour: greys are 60 per cent bigger than reds and he thought this might increase the competition for food. His study population kept dying of a myxomatosis-like disease yet there was nothing obviously wrong with the animals. Dr Reynolds supplied carcasses to Norwich University's Veterinary Investigation Centre. After 20 routine post-mortems had been carried out, the vets realised that this was a far more complex situation and sent the next batch to Maff (now Defra), whose scientists discovered that the squirrels were infested with an unknown virus.

After 20 years we now know that the grey harbours the parapox virus, which is benign to this species. "It is most likely that the grey squirrels brought the virus with them from the US," says Tony Sainsbury, from the Institute of Zoology, Zoological Society of London. It is possible that the virus is carried by fleas that live on the grey, which red squirrels pick up when they use the dreys, or nests. …