The visual spectacle of a small, russet-colored red squirrel chasing a much larger grey squirrel is a common, yet often misinterpreted, observation. This aggressive pursuit is a snapshot of a decades-long ecological conflict between the native Eurasian red squirrel (Sciurus vulgaris) and the invasive North American grey squirrel (Sciurus carolinensis). Introduced to Europe in the late 19th and early 20th centuries, the grey squirrel has expanded its range across the UK and the continent. Today, the native species is largely confined to isolated strongholds, primarily in Scotland, northern England, and a few protected island populations. The chasing behavior is a direct expression of the intense competition for survival between the two species.
The Competitive Advantage of Grey Squirrels
The grey squirrel’s dominance is explained by biological adaptations that grant it a superior competitive edge. Grey squirrels are significantly larger than their red counterparts, weighing approximately 90% more (575 grams versus 305 grams). This larger size translates to higher energy expenditure, meaning they must consume more food, intensifying pressure on shared resources.
The grey squirrel also possesses a digestive advantage concerning acorns. Acorns contain tannins, which are difficult for the red squirrel’s digestive system to process and can be toxic. The grey squirrel can neutralize these chemical defenses, allowing it to consume a food source unavailable to the native species.
This dietary flexibility allows the grey squirrel to forage earlier and sustain a higher population density in broadleaf woodlands. Its robust build enables it to lay down greater fat reserves, increasing its chances of surviving harsh winters and breeding earlier. These combined factors reduce the red squirrel’s survival and reproductive rates, leading to competitive exclusion.
Direct Resource Competition
The act of chasing is a visible manifestation of immediate resource defense, often initiated by the smaller red squirrel. Red squirrels are naturally more territorial and vigorously defend their foraging areas and nesting sites (dreys) to exclude intruders from high-value resources.
When both species converge on a limited food source, the interaction escalates to a chase. This pursuit demonstrates dominance, aiming to drive the competitor away. The larger, more dominant grey squirrel often wins these encounters, monopolizing the food source.
Competition also extends to cached food, essential for winter survival. Grey squirrels are effective at locating and stealing cached nuts the red squirrel has buried. This exploitation of stored resources diminishes the red squirrel’s ability to survive lean months, making territory defense a critical strategy.
The Role of Squirrel Pox in Displacement
While resource competition is a significant factor, the most devastating mechanism of displacement is the Squirrel Parapoxvirus (SQPV). Grey squirrels act as asymptomatic carriers, harboring the pathogen without suffering ill effects.
When the virus is transmitted to a red squirrel, it is almost always fatal, with mortality rates approaching 100%. The disease causes severe skin lesions, scabs, and swelling around the eyes, mouth, and feet. These symptoms prevent the infected animal from seeing, feeding, or moving properly, leading rapidly to dehydration and starvation.
The introduction of SQPV dramatically accelerates population decline, with extinction rates up to 25 times greater than those caused by resource competition alone. The virus spreads through direct contact or indirectly via contaminated environments, such as shared feeding stations. This biological factor prevents the two species from coexisting long term.
Conservation and Management Efforts
Conservation efforts focus on creating and maintaining isolated, safe environments for the native red squirrel. A primary strategy involves targeted grey squirrel control programs within designated strongholds and strategic buffer zones. These measures aim to keep grey squirrel populations at extremely low densities to prevent the spread of the fatal parapoxvirus.
Habitat management involves planting and maintaining conifer-dominated woodlands, which favor the red squirrel’s diet and foraging habits. Isolated geographical features, such as islands like the Isle of Wight and Brownsea Island, serve as natural safe havens. These locations provide protected environments where red squirrel populations can thrive without the threat of competition or disease.
Conservationists also engage in monitoring programs and reintroduction projects, using red squirrels from captive breeding programs to boost small, vulnerable populations. This multi-faceted approach combines landscape-level grey squirrel management with habitat enhancement and population support.