Mammals of the British Isles - The Squirrel

Grey Squirrel

Strangely enough, the cute and lovable grey squirrels (Sciurus carolinensis) we see playing in the grounds of Stover School are in fact rodents – the same group of mammals as rats. Since their introduction from North America, they have had a catastrophic effect on the native red squirrel (Sciurus vulgaris) throughout the British Isles. Grey squirrels also cause significant damage to woodlands through ‘bark stripping’ and reduce density and diversity in populations of woodland birds.

Wild grey squirrels first appeared in Britain in the late 19th Century, with some possibly escaping from London Zoo. In Italy two pairs escaped from an ambassador's garden in Turin in 1948, although some sources suggest that pet grey squirrels had already escaped in Piedmont by the late 1800s. The spread of grey squirrels, at the expense of native red squirrels, appears to have had a more marked effect in the British Isles than on mainland Europe – so far at least.

Red Squirrel

There are now thought to be just 140,000 red squirrels in Britain, whilst there are over 2.5 million greys, and the future of the red squirrel is becoming increasingly uncertain as they are now extinct in southern England (except for a few on the Isle of Wight). Red Squirrels are still widespread in the North of England and Scotland, but even here their range is contracting. The grey squirrel is having such a profound impact on British wildlife that the International Union for the Conservation of Nature has now listed it on their global list of the 100 worst invasive species. The UK Forestry Commission is working with partners in projects across Britain to develop a long-term conservation strategy that deters greys and encourages reds.

The Grey Squirrel is larger and more aggressive than its European cousin, and greys out-compete reds for food, feeding more on the ground and being able to digest acorns, which reds can’t. Contrary to popular opinion, red squirrels do not hibernate – but as greys can build up and store more fat they are better at surviving cold winters. Grey squirrels are also more generalist feeders eating: nuts, flowers, fruits, seeds, tree bark, fungi, bird eggs, nestlings and frogs - whilst reds prefer the seeds of coniferous forests, although they will diversify under pressure. Grey squirrels are also thought to have introduced the squirrelpox virus from North America which is deadly to red squirrels, although greys have a natural immunity.

Distribution of Grey Squirrel

Distribution of Red Squirrel

Grey squirrels, which can live at high population densities in broadleaved woodland, cause significant damage to trees such as sycamore, beech, oak, sweet chestnut, pine, Norway spruce and larch, by bark-stripping. This dramatically reduces the economic value of woodland – costing more than £10 million per year. They may also be partly responsible for recent declines in many woodland bird species - through predation of eggs and young chicks, competing for nest sites or because they consume food which would otherwise be available for birds.

Squirrels build large nests, called dreys, often in the forks of tree trunks. They can breed twice in a season, in spring and in late summer, but usually only breed once. There are between one and six young in a litter. The young are born naked and blind. Young squirrels may stay with their mother until she has her next litter. Mortality is quite high, with only one in five surviving to their first winter. Adult squirrels have few natural predators, as they are far too quick and agile for most.

this article is based on work by Jane Chen, Year 10