Insatiable Appetites — the grey squirrel’s damaging desire for tree bark
Bushy tailed and with frosted grey fur, this creature’s staccato jumps, tail-flicks and gravity-defying treeward trails are unmistakable. They have a penchant for nuts, but are often nearby when bird-feeders mysteriously empty, or when a fat-ball or two disappear. Their scratchy baritone chirps are almost crow-like, belying the more endearing fuzzy form of this rodent. In the United Kingdom, the grey squirrel is seen everywhere from the towering canopies of rich woodland, to the open lawns of suburban parks and gardens. If, like me, you enjoy cycling through urban leafy streets, it is not uncommon to have to make a last minute veer or brake as one dashes between the verdant verges.
In fact, two species of squirrel call the UK home: the Eurasian red squirrel (Sciurus vulgaris) and the grey squirrel (Sciurus carolinensis). But throughout much of the country you are very unlikely to come across a red scampering in the leaf piles or treating your bird-feeders like climbing frames. Red squirrels have suffered a dramatic decline in numbers since the introduction of grey squirrels, and are now listed as endangered in the UK. The Wildlife Trusts estimate there to be around 2.5 million grey squirrels living in the UK, far outweighing the remaining 140,000 red squirrels. Your best chance to spot one is in Scotland, which is home to around 75% of the remaining red squirrel population, while in England there are strongholds in northern England in the pine forests in Northumberland and the Lake District, and on the Isle of Wight, Brownsea Island, and Formby. In Wales they are mostly restricted to Anglesey in the northwest.
You have probably heard that the grey squirrel has not in fact always been within British woods and gardens. An arrival from native to eastern North America, it is an invasive species, and a notorious one at that, being on the list of Invasive Alien Species of European Union concern. Grey squirrels were introduced to a number of sites in England and Ireland between from the late 1800s, and although this importation has now been banned, they have subsequently largely pushed out the red. They directly compete for food and nesting spots, and with greys being bigger and more aggressive, they gobble up larger shares of available food, raiding the stores of reds. They can also carry disease, with one particular ailment, the squirrelpox virus, having the biggest impact thanks to natural resistance in greys, while it can readily infect and kill red squirrels.
However, you may not have heard so much about the greys’ impacts on the very woods they inhabit, and the serious impacts that can result. Grey squirrels strip the bark from broad leaf trees to feed on the nutritious sap underneath, negatively impacting the health of the UK’s trees and woods by creating open wounds, leading to weakened, stressed trees that are more vulnerable to infection by diseases. If the bark is stripped from right around the tree’s circumference, it can also lead to something called girdling, where the tree’s nutrient supply from its roots is cut off, killing the tree.
High numbers of young trouble-making squirrels appear in spring and summer, leading to peak bark-stripping occurring between April and September, often targeting young broadleaved trees around 10–40 years of age. The damage can be repeated and compounded every year if these numbers of toothsome squirrel teenagers remain high and unmanaged, causing serious problems for the future of UK tree and woodland cover. Tree species that are particularly susceptible to damage include oak, beech, hornbeam and sweet chestnut, which are important for supporting biodiverse species-rich woodlands. Species such as lime, horse chestnut and wild cherry are far less affected, which can over time change the groups of species of trees making up UK woodlands. In 1985 a Broadleaves Policy aimed to increase tree species that support native biodiversity, while also providing hardwood timber for manufacturing and construction, but there has been an ongoing reluctance to plant broadleaves for timber purposes due to the impact of this bark stripping by the large grey squirrel populations. A Royal Forestry Society survey of landowners and managers showed that grey squirrels are still seen as the greatest threat to broadleaf trees — above deer and diseases.
As grey squirrels cause this loss and damage, trees are prevented from reaching a healthy maturity that provides ecosystem services such as capturing and storing atmospheric carbon, cleaning air and water, improving soil quality and function, and preventing flooding during heavy rainfall. The lesions, callus growth and dysfunctional shape means that grey squirrels cause £37m worth of damage a year to trees in England and Wales due to the lost timber value, reduced carbon capture, and costs for damage mitigation and tree replacement.
UK Squirrel Accord, a UK-wide partnership of 41 leading conservation and forestry organisations, government agencies and companies, is exploring methods to humanely manage grey squirrels to help lessen these impacts. Research is being undertaken on an oral contraceptive that would reduce grey squirrel numbers. This aims to provide an effective, species-specific, non-lethal method of supporting red squirrel conservation and protecting the UK’s biodiverse broadleaf woodlands.
Squirrels are a welcome sight for many, and it can be a begrudging notion to acknowledge the damage caused by those familiar swishing tail brushes and buck-toothed faces. However, hoards of bark-nibbling grey squirrels are not sustainable for the health of our woodlands or our broader wildlife environment, or for the fate of our native red squirrel. With current policies and methods to control grey squirrels not working, novel approaches to squirrel and tree management are needed to support a rich, dynamic landscape for wildlife, ecosystems and their services.
