The Grey Squirrel is a non native species introduced in the 1870s to the UK from North America. The Grey Squirrel has had a devastating population impact on the native red squirrel due to a virus the Grey Squirrel carries called the Squirrel Parapox Virus. This virus is lethal to the red squirrel but rarely effects the grey squirrel.
Squirrels can occasionally become a pest when they invade roof spaces to nest. Squirrels have also been know to start fires due to chewing through electrical cables. They also have an impact on trees on which they gnaw to get to the sweet sap within resulting in tree succumbing to fungi infections and also death.