Why Are Gray Squirrels Invasive in the UK? The Full Story

How gray squirrels arrived in Britain

The eastern gray squirrel is native to the woods of eastern North America. In Victorian Britain, exotic animals were fashionable additions to country estates, and gray squirrels were seen as charming curiosities.

The first well-documented release was in 1876, when Thomas Brocklehurst let a pair go at Henbury Park in Cheshire. Over the following decades, gray squirrels were released at dozens of sites, including Woburn Abbey in Bedfordshire and Regent’s Park in London. Some squirrels from Woburn were later moved on to other places, helping the species spread.

By the time the dangers were recognized, it was too late. Importing and keeping gray squirrels was banned in 1937, and today the Wildlife and Countryside Act 1981 makes it an offence to release one into the wild.

How far have they spread?

Gray squirrels now occupy almost all of England and Wales and much of lowland Scotland. The Mammal Society’s 2018 review estimated the British population at about 2.7 million. The red squirrel, once found throughout Britain, has disappeared from most of the areas the grays have reached.

The species has also spread in Ireland, starting from a single release at Castle Forbes in County Longford in 1911, and in northern Italy, where squirrels were released in Piedmont in 1948 and later in Liguria and Lombardy. Scientists warn that from Italy gray squirrels could eventually spread into France and Switzerland.

Why do gray squirrels replace red squirrels?

The two species do not fight. Instead, gray squirrels win through a combination of advantages:

• Bigger and fatter. Gray squirrels weigh about 400–600 g (14–21 oz), compared with about 250–340 g (9–12 oz) for reds, and can store far more body fat for winter.
• Better acorn eaters. Grays digest acorns efficiently despite their tannins. Red squirrels cannot, so in oak woods the grays have a large food advantage.
• Higher densities. In broadleaf woods, gray squirrels live at much higher densities than reds and breed more successfully.
• Food theft. Grays dig up and eat nuts that red squirrels buried for spring, when food is shortest.

Under this pressure red squirrel numbers fall even without disease. Red squirrels hold out best in large conifer forests, where small conifer seeds suit them better than grays.

A side-by-side guide is in red squirrel vs gray squirrel.

Squirrelpox: the hidden weapon

The second big factor is squirrelpox virus. Many gray squirrels in Britain carry antibodies to it, showing they have been infected, but they rarely become ill. In red squirrels the disease causes sores and swelling around the eyes, mouth and feet, and it is usually fatal within about two weeks.

When squirrelpox reaches a red squirrel population, the decline is far faster. Modeling studies have suggested that red squirrels disappear around 17–25 times faster where the virus is present than through competition alone. In 2007–2008, an outbreak at Formby in northwest England killed roughly 80% of the local red squirrels, although the population later recovered.

The full list of threats is covered in why are red squirrels declining?

Damage to trees and woodland

Gray squirrels are also a problem for forests. In late spring and early summer, particularly from about May to July, they strip bark from the trunks and branches of young broadleaf trees to reach the sweet, sappy layer underneath. Beech, sycamore and oak are among the worst affected.

Bark stripping can deform trees, let in fungal disease and kill trees outright if the bark is removed all the way around the trunk. It reduces the value of timber and is one of the main obstacles to growing high-quality broadleaf woodland in Britain.

Gray squirrels also eat bird eggs and nestlings, and they take a large share of food from garden feeders. Their overall effect on bird populations appears to be small compared with other factors, but it is studied closely.

Pine martens and the fight back

One surprising ally of the red squirrel is the pine marten, a native tree-climbing predator related to weasels. Studies in Ireland and Scotland have found that where pine marten numbers recovered, gray squirrel numbers fell sharply and red squirrels returned. Gray squirrels spend more time on the ground and appear more vulnerable to martens, which they did not evolve alongside.

Other conservation work focuses on keeping gray squirrels out of red squirrel strongholds and stopping the spread of squirrelpox. The island of Anglesey in Wales was cleared of gray squirrels, and its red squirrel population has grown strongly since. The Isle of Wight and Brownsea Island have never been colonized by grays. To see red squirrels for yourself, try the places listed in where to see red squirrels in the UK.

Is the gray squirrel to blame?

Gray squirrels are not villains; they are simply a successful species placed by people in the wrong environment. In their native North America they are a valued part of the forest, planting oaks and hickories with their forgotten nuts. The problems in Britain, Ireland and Italy are the result of human introductions, which is why preventing new introductions of non-native animals is now a key principle of conservation.