Grey Squirrels - an invasive species

The grey squirrel (Sciurus carolinensis) is a cute and charismatic species, commonly seen in rural areas and urban parks, but this species causes problems for native wildlife. Since being introduced from North America in the 19th century, the grey squirrel has successfully spread across almost the entirety of Britain, with some exceptions including northern Scotland, Anglesey and the Isle of Wight. 

A startled baby grey squirrel in a deciduous woodland, Hampshire

Larger than the red squirrel, grey squirrels weigh around 400-600g, are 24-29cm long, and have an impressive, fluffy tail around 20cm long. Other than the obvious difference in colour, the two species can also be distinguished by the grey's lack of ear tufts. Deciduous woodland is the primary habitat of grey squirrels, but they are also common in parks and gardens, often seen visiting bird feeders. Wild food sources include acorns, plant shoots, fungi, hazelnuts and seeds from pine cones, which they extract by nibbling around the cone, leaving the core. In autumn grey squirrels cache food to return to in winter, and will sometimes pretend to bury food in order to deceive onlookers who may steal it. Though these rodents don't hibernate in winter, they are less active. 

Camera trap footage of a grey squirrel visiting peanut butter bait.

Grey squirrels can have 2-3 litters of 3-4 young each year, starting in January to April and continuing through summer if food is abundant. The young are born in a drey built from twigs, leaves and bark, in a forked branch in the canopy. Squirrels are adapted to this arboreal lifestyle as they have very flexible ankles, allowing their feet to face forwards or backwards, improving dexterity when climbing. 

One issue caused by grey squirrels is damage to trees by bark stripping. This is a particular threat to thin-barked species such as beech and sycamore, and can kill or distort trees, leaving them vulnerable to wind snap and fungal invasion. The reason for this behaviour is unknown, though it seems to coincide with high numbers of young. Bark stripping by grey squirrels is damaging to ecosystems and and economic costs - in 2000 it was estimated to cost the UK timber industry £10 million. 

A grey squirrel alarm calling

As the range of the grey squirrel expanded it displaced the native red squirrel, Sciurus vulgaris, through interspecific competition for food and the transmission of squirrel parapoxvirus. Grey squirrels act as a reservoir host for this virus, and are unaffected by the disease which is fatal to reds. The colonization of new areas by greys is accelerated by the virus as it causes the red population to crash, reducing food competition and allowing the grey population to grow rapidly. Total eradication of grey squirrels from the UK is a distant prospect due to their large population (est. 2.7 million), wide range, and rapid recolonization rate - after removal from an area greys will recolonize within 6-8 weeks. Various methods have been trialled to cull grey squirrels, the most effective being warfarin poisoning, however this was banned by the EU in 2014. The efficacy of warfarin poisoning as a control method was limited as it couldn't be used in areas where red squirrels or pine martens also occurred. The next most effective method is trapping greys. Once caught it is an offence to release grey squirrels under the Wildlife and Countryside Act 1981. Shooting grey squirrels is also used as a control method, though this is less effective as fewer are caught and it is often not carried out in a systematic way. 

Current thinking poses reproductive inhibition by immuno-contraception as a potential method to control the grey squirrel population, though this is likely to be expensive and further research is needed. A recent discovery showed a correlation between increased pine marten presence and decreased grey squirrel populations. This is thought to be because greys lack the effective reactions and escape mechanisms of red squirrels, which have developed through their coevolution with pine martens. The higher predation of greys by pine martens has potential to be used in conservation of red squirrels, however, the restriction of pine martens to wooded areas mean they aren't the sole solution. Without simultaneous control in urban areas, towns and cities could be sources for grey squirrels which would then disperse into rural areas.