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Squirrels & Wildlife

Which Squirrels Are Invasive? Four Species, Named

The Eastern grey in Britain and Italy, the Pallas's in Europe and Japan, the fox squirrel in the west, and the Finlayson's. Where and why.

By Zakaria 2 min read Save to Pinterest — opens Pinterest in a new tab
An Eastern grey squirrel hanging head-down from a post to reach a feeder
Photo: 15427268, Pixabay Content License, via Pixabay
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Four squirrel species cause real problems outside their native ranges, and one of them is on the IUCN’s list of the hundred worst invasive species on Earth.

The four

speciesnative toinvasive in
Eastern grey (Sciurus carolinensis)E. North AmericaBritain, Ireland, Italy
Pallas’s (Callosciurus erythraeus)SE AsiaFrance, Belgium, Netherlands, Italy, Argentina, Japan
Finlayson’s (Callosciurus finlaysonii)SE AsiaItaly, Singapore
Fox squirrel (Sciurus niger)E. United StatesW. United States, parts of Canada

Why the grey wins, specifically

mechanismeffect on red squirrels
Squirrelpoxgreys carry it unharmed; infected reds usually die in under two weeks
Digesting unripe acornsgreys feed earlier in the season; reds cannot
Larger body sizegreys dominate at concentrated food
Higher density tolerancemore greys per hectare than reds
Bark strippingdamages young broadleaf plantations

Same animal, opposite verdict

In Pennsylvania the Eastern grey is a keystone species that plants the forest. In Cumbria it is on a list of the hundred worst invasive organisms on the planet. Nothing about the squirrel changed — only which continent it is standing on.

In Britain it is an offence under the Wildlife and Countryside Act 1981, and subsequent invasive species regulations, to release a grey squirrel or to allow one to escape into the wild. A trapped grey therefore cannot lawfully be driven somewhere else and let go, which is what most people assume they will do. The rules differ in Ireland and elsewhere, so check locally before trapping anything.

Pine martens are the development worth watching

Where pine martens have recovered, red squirrel numbers have recovered with them. Martens appear to predate greys considerably more heavily than reds — greys are heavier, spend more time on the ground and did not evolve alongside the predator. It is the most promising thing in decades of grey control.

Common questions

Which squirrel species are invasive?

Four matter. The Eastern grey squirrel, native to eastern North America, is invasive in Britain, Ireland and parts of Italy. Pallas's squirrel, from south-east Asia, is established in parts of France, Belgium, the Netherlands, Italy, Argentina and Japan. The Finlayson's or variable squirrel is invasive in Italy and Singapore. And the fox squirrel, native to the eastern United States, is introduced and invasive in parts of the western United States and in Canada west of its natural range.

Why are grey squirrels such a problem in Britain?

Two mechanisms, and the second matters more. Greys out-compete red squirrels for food, being larger and able to digest unripe acorns reds cannot. More decisively, greys carry squirrelpox and are largely unaffected by it, while reds that catch it almost always die within two weeks. Where the virus is present, reds disappear many times faster than competition alone would explain.

Is it true the Eastern grey is one of the worst invasive species?

It is listed among the 100 worst invasive alien species by the IUCN's Invasive Species Specialist Group — a list that covers all taxa, from plants to mammals. That placement reflects the red squirrel decline across Britain and Ireland and the bark-stripping damage to broadleaf forestry, which runs to real money in young plantations.

Is the grey squirrel invasive in North America?

Not in its native range, which is the eastern United States and south-eastern Canada, where it is a keystone seed disperser and plants much of the oak regeneration. It has been introduced west of its natural range — in parts of the western United States, British Columbia and elsewhere — and in those places it behaves as an introduced species, competing with native western squirrels.

Can you legally release a grey squirrel in the UK?

No. Under the Wildlife and Countryside Act 1981 and subsequent invasive species regulations it is an offence to release a grey squirrel or allow one to escape into the wild in Britain. That means a trapped grey cannot legally be relocated, which is the detail most people are unaware of when they trap one. The rules differ in Ireland and elsewhere, so check locally.

What is being done about them?

In Britain, a mix of local control around remaining red populations, pine marten recovery — martens predate greys far more heavily than reds and their spread correlates with red recovery — and research into fertility control delivered through feed hoppers. Eradication from a large landmass is not considered realistic; holding refuges for reds is.

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