The British case

The grey squirrel was introduced in Britain much earlier than in Italy. From 1876 to 1930 there were 31 cases of introduction, some with animals coming directly from North America, in other cases following the capture of animals in England and their release in other areas. An introduction was made in Ireland in 1913. On both islands, the grey squirrel has soon started its expansion, followed by the disappearance of the red squirrel almost everywhere. Currently the grey squirrel has colonized much of England and Wales and is expanding in Scotland. In the meantime, the red squirrel has become one of the species most at risk of extinction.

Decline of the red squirrel and spread of the grey squirrel in Great Britain and Ireland from 1945 to 2010; orange areas of overlap of the two species (taken from the website of the Red Squirrel Survival Trust: www.rsst.

The replacement of the red squirrel by the geay squirrel is based on competition for food between the two species. Unfortunately, the presence of a virus (called Squirrel Poxvirus) facilitates and accelerates this process. The Grey squirrel, in fact, is resistant to the virus and take it with them. In the areas of red squirrel presence, the greys can pass the virus to the European species that has no defenses against the virus and is destined to die within a couple of weeks.

In Britain, the grey squirrel also causes damage to forests and plantations of trees and shrubs, removing the bark of trees to access the sap below. This type of barking facilitates the penetration of insects and fungi in the trunk, putting at risk the survival of the trees. In Italy damages to hornbeam, poplar and also in cereal crops have been recorded. The damage occur more easily in areas with high density of young squirrels, probably favored by competitive or exploratory behavior. There is also evidence that squirrels prefer to attack trees with more sap production. In England it has been estimated that grey squirrels alone reduce to about 25% (10 million pounds) the economic value of trees grown for timber production, while the expenditure for forests and wood plantations management is worth 3 million pounds.