Grey Squirrels

An invasive, non-native mammal species in the British Isles whose bark-stripping behaviour causes extensive structural damage, timber degradation, and mortality in broadleaved woodlands.

The Grey Squirrel, introduced to Britain from North America in the late 19th century, represents one of the most severe biotic threats to the health, economic viability, and ecological sustainability of broadleaved woodlands across the British Isles.

The primary silvicultural challenge stems from their destructive bark-stripping behaviour, which peaks during the active growing season between May and late July. Grey squirrels target young, vigorous trees—typically aged between ten and forty years—gnawing away the outer bark to feed on the sugary, nutrient-rich phloem sap running through the vascular cambium layer underneath. If the bark stripping extends completely around the circumference of the stem, the tree is effectively girdled, cutting off the flow of nutrients and causing immediate mortality to the crown above the wound. Even when the damage is partial, it creates jagged, permanent scars that structurally weaken the trunk, making the tree highly vulnerable to wind-snap and providing an easy entry point for wood-rotting fungal pathogens. This damage completely ruins the internal wood grain, downgrading premium timber crops to low-value firewood.

Beyond physical timber damage, grey squirrels pose a catastrophic ecological threat; they act as asymptomatic carriers of the deadly squirrel pox virus. While harmless to greys, this pathogen is fatal to native red squirrels, meaning their impact is far more devastating than simple resource competition. Consequently, active population management through landscape-scale trapping and control programs is a vital operational necessity for protecting both resilient broadleaved forests and native wildlife. Researchers are working on a contraceptive which could help reduce populations of grey squirrel in Britain.