Red squirrels (Sciurus vulgaris) and grey squirrels (Sciurus carolinensis) are two distinct species. Red squirrels are native to Europe and parts of Asia, while grey squirrels originate from North America. In regions where their habitats overlap, such as the UK and parts of Europe, observing red squirrels chasing grey squirrels is a common sight. This behavior often stems from competition and threats that significantly impact native red squirrel populations.
The Arrival of the Grey Squirrel
Grey squirrels were introduced to the UK and much of Europe from North America starting in the late 19th century. The first recorded introduction in England occurred in 1876, with around 30 separate introductions taking place until 1930. Wealthy landowners brought them over as ornamental species for estates.
Their introduction led to a rapid spread across the British mainland, displacing native red squirrels. For example, grey squirrels introduced to Ireland in 1911 have since colonized much of the suitable countryside. This proliferation established the conflict between the two species.
Competition for Food and Habitat
Grey squirrels outcompete red squirrels for essential resources, often leading to chasing behavior. Grey squirrels are larger, weighing between 400-600 grams, almost double the size of red squirrels (250-330 grams). This size advantage contributes to their dominance.
Grey squirrels also possess a dietary advantage, digesting a wider variety of foods, including unripe acorns. Their digestive system can process tannins in unripe acorns, which red squirrels cannot. This allows them to access and deplete food sources earlier in the season. Their broader diet and efficient digestion enable grey squirrels to thrive in diverse woodland environments, displacing red squirrels from prime feeding grounds. Both species compete for nesting sites, known as dreys, and suitable territories.
The Threat of Squirrel Pox
The squirrel poxvirus (SQPV) poses a fatal threat to red squirrels. Grey squirrels carry the virus asymptomatically, acting as carriers and spreading it without showing symptoms. The virus can transmit through direct contact, such as fighting, or indirectly through contaminated environments like shared feeding sites, urine, faeces, or saliva.
Once a red squirrel contracts squirrel pox, the mortality rate is nearly 100%, with death usually occurring within 4-15 days. Symptoms include lethargy, panting, and severe lesions around the eyes, mouth, feet, and genitalia. The presence of squirrel pox can accelerate the decline of red squirrel populations by 20 to 25 times faster than in areas without the virus.
Ecological Consequences and Conservation Efforts
The dominance of the grey squirrel has led to the decline of red squirrel populations. Red squirrels have retreated to isolated strongholds, primarily in Scotland, northern England, Wales, and certain islands. Beyond competing with red squirrels, grey squirrels also damage trees by stripping bark, harming timber production and affecting woodland ecosystems.
Conservation efforts focus on protecting remaining red squirrel populations and mitigating grey squirrel impact. These efforts include grey squirrel control programs to manage their numbers. Habitat management, such as planting red squirrel-friendly vegetation and preserving coniferous forests, also plays a role. The establishment of red squirrel refuges aims to create safe havens where native populations can thrive without grey squirrel pressures.