The classification of squirrels as an invasive species depends on the specific squirrel species and geographic location. While many squirrel species are native to their habitats and play roles in local ecosystems, certain species have become invasive in regions where they were introduced by humans. This means a squirrel can be a natural part of one environment and a disruptive force in another.
Defining an Invasive Species
An invasive species is a non-native organism that causes, or is likely to cause, harm to the economy, environment, or human health. These species are also called alien species, meaning they are outside their historical range. Not all non-native species are invasive; for a non-native species to be classified as invasive, its introduction must lead to demonstrable harm.
Introductions can occur accidentally, such as organisms hitchhiking on goods or vehicles during shipping or travel. Intentional introductions happen when species are brought for specific purposes, like pets, ornamental plants, or pest control, and then escape or are released into the wild.
When Squirrel Species Become Invasive
Some squirrel species have become invasive when introduced to new environments beyond their native ranges. The Eastern Gray Squirrel (Sciurus carolinensis) is a prime example. Native to eastern North America, this species has been widely introduced to other parts of the world, including Europe. In countries like the United Kingdom and Italy, the Eastern Gray Squirrel is considered highly invasive.
The same species can be native in one region and invasive in another. The Eastern Gray Squirrel is a natural inhabitant of North American forests, contributing to seed dispersal and forest regeneration. However, its introduction to Europe, often as an ornamental species, led to its classification as an invasive alien species of Union concern by 2016. Similarly, the Fox Squirrel (Sciurus niger), native to much of the eastern and central United States, can become invasive in parts of western North America where it competes with native squirrel populations.
Consequences of Invasive Squirrel Populations
Invasive squirrel populations can negatively impact the ecosystems they colonize. They directly compete with native species for resources like food and nesting sites. For example, the larger Eastern Gray Squirrel outcompetes the native Eurasian Red Squirrel (Sciurus vulgaris) in Europe, contributing to the red squirrel’s decline.
Invasive squirrels can also spread diseases that harm native wildlife. The squirrel poxvirus, carried by Eastern Gray Squirrels, is largely harmless to them but can be fatal to European Red Squirrels. Invasive squirrels also damage trees by stripping bark, particularly from younger trees, to access inner layers. This bark stripping can injure or kill trees, affecting forest health.
Invasive squirrel populations can prey on bird eggs and nestlings, disrupting avian breeding success and biodiversity. Economically, these squirrels damage agricultural crops by consuming nuts and fruits. They also inflict damage on infrastructure by gnawing through electrical wires or other materials in homes and gardens. These ecological and economic impacts highlight the threats posed by invasive squirrel species.