Grey squirrels are recognized as an invasive species in many regions outside their native North American range. An invasive species is a non-native organism whose introduction to a new environment causes or is likely to cause harm to the environment, economy, or human health. These squirrels pose significant threats to local ecosystems and native wildlife where they have been introduced.
The Grey Squirrel’s Global Journey
Grey squirrels, Sciurus carolinensis, are native to the eastern and midwestern United States and southern Canada. Their journey to other continents began in the late 19th and early 20th centuries.
The first recorded introduction to the United Kingdom occurred in 1876, when they were brought from North America as an ornamental species for stately homes. Multiple introductions continued until around 1930, after which the damage they caused was recognized. The species spread rapidly across much of mainland England, Wales, and parts of Scotland. In Ireland, grey squirrels were introduced in 1911, with genetic analysis suggesting additional introductions occurred later. On continental Europe, grey squirrels were introduced to Italy starting in 1948, with notable releases in Turin and Genoa. These deliberate introductions, combined with accidental escapes, allowed the species to establish populations far from their original habitat.
Competitive Advantages of Grey Squirrels
Grey squirrels possess several biological and behavioral traits that enable them to outcompete native species, particularly the European red squirrel. They are considerably larger and heavier than red squirrels, averaging around 48 centimeters in length and weighing about 575 grams, almost double the red squirrel’s average weight of 305 grams. This size advantage allows them to dominate food sources.
Their dietary flexibility is another significant advantage; grey squirrels can digest a wider variety of foods, including unripe acorns, which are toxic to red squirrels. They are also more efficient at foraging and caching food, providing a survival advantage during periods of scarcity. Grey squirrels exhibit higher reproductive rates, typically having 3 to 6 young per litter compared to the red squirrel’s 2 to 4, contributing to faster population growth.
A major factor in their competitive success is their asymptomatic carriage of the squirrelpox virus. This virus is harmless to grey squirrels but is often fatal to red squirrels, causing lesions, blindness, and ultimately death within weeks. The presence of this disease can accelerate the decline of red squirrel populations significantly. Their adaptability to various habitats, including urban and suburban environments, further contributes to their ability to thrive in new regions.
Ecological and Economic Impacts
The introduction of grey squirrels has led to severe ecological consequences, primarily impacting native red squirrel populations. Grey squirrels displace native red squirrels by competing for food and habitat. When grey squirrels colonize an area, red squirrel populations can disappear within approximately 15 years due to this intense competition and disease transmission.
Beyond their impact on native squirrels, grey squirrels cause extensive damage to broadleaf trees through bark stripping. They strip bark from the main stems and branches of trees, particularly those aged between 10 and 40 years. This damage disrupts the tree’s vascular system, making it vulnerable to fungal infections, weakening its structure, and often leading to its death.
The economic cost of this forestry damage is substantial. In England and Wales alone, the loss in timber value, reduced carbon capture, and the need for replacement trees due to grey squirrel bark stripping is estimated to be at least £37 million annually. Some estimates project the cost over 40 years to exceed £1.1 billion. Grey squirrels also prey on the eggs and nestlings of birds, adding another layer of ecological disruption.
Population Management Efforts
Various strategies are employed to manage grey squirrel populations in areas where they are invasive. Trapping and culling are common methods, with live traps used where native red squirrels are present. Shooting is also used for control.
Research is ongoing into non-lethal methods for population control, such as fertility control through immunocontraception. Habitat management can also discourage grey squirrels while favoring native species. Despite these efforts, widespread eradication remains a significant challenge due to the grey squirrel’s adaptability and high reproductive capacity.