At one time the grey squirrel lived only in North America. Its exportation to Britain by the Victorians has had an effect of the survival of the red squirrel, a European native.
The red squirrel (Sciurus vulgaris) is native to Europe and Asia. Their population is declining in Britain, where hunting, disease and forest management through the years have affected their numbers.
The grey squirrel (Sciurus carolinensis) is native to the northeastern part of North America. They were first introduced to Britain in 1876, and have thrived, says Squirrels.Info, a website dedicated to British squirrels.
The grey squirrel, which can reach almost 2 feet long including the tail, is larger than the red squirrel, at 18 inches long. British grey squirrels on average weigh 553gr., while red squirrels in Britain generally weigh 7 to 298gr.
Gray squirrels spend more time foraging on the ground, and are adventurous. In contrast, the red squirrel is usually high in an evergreen tree, so its diet is more restricted.
One reason suggested for the increase of the grey squirrel in Britain, while the red squirrel declines, is diet. Gray squirrels are more adaptable and able to eat foods that are not digestible, or even toxic, to the red squirrel, according to Wildlife Online.