The striking appearance of the black squirrel often leads to questions about its place in the ecosystem, particularly whether it is an invasive species. This curiosity stems from the noticeable presence of these dark-furred animals in areas where they were previously uncommon. However, the designation of a species as ecologically invasive is not based on local population spikes or unusual coloration. Addressing the black squirrel’s status requires examining its genetics, native range, and the regulatory definitions for invasive organisms.
Defining the Black Squirrel
The black squirrel is not a separate species but a color variant, known as a melanistic morph, of the Eastern Gray Squirrel (Sciurus carolinensis) or, less commonly, the Fox Squirrel (Sciurus niger). This deep, dark coloration is the result of a genetic mutation that causes an overproduction of the pigment melanin. For Eastern Gray Squirrels, this is often linked to a 24 base-pair deletion in the melanocortin 1 receptor (\(MC1R\)) gene.
This genetic change is responsible for the coat color, which can manifest as jet black or a brown-black shade. Since the black squirrel shares the exact same species identity, behaviors, and diet as its gray counterparts, it is considered a naturally occurring part of the native wildlife.
Criteria for Invasive Species Status
To be classified as an invasive species, an organism must meet two strict criteria established by ecological and governmental bodies. First, the species must be non-native, or alien, to the ecosystem under consideration. Second, its introduction must cause, or be likely to cause, economic or environmental harm, or harm to human health. The concept goes beyond simply being non-native, as many non-native species exist without causing harm.
The black squirrel, being a color variant of the native Eastern Gray Squirrel, cannot meet the primary criterion of being non-native within North America. Because it is simply a differently colored member of a species already native to the eastern United States and Canada, it is not an alien introduction. Therefore, the black squirrel is categorically not an invasive species in its native North American range.
The Eastern Gray Squirrel has been introduced to other continents, such as the United Kingdom, where the entire species, including the black morph, is considered invasive. However, the black coloration does not change the species’ fundamental native status in North America. Its occasional local dominance over the gray morph is a natural evolutionary dynamic, exhibiting a natural genetic variation.
Geographic Distribution and Range Expansion
The perception that black squirrels are newly arrived or invasive is often due to their current high concentrations in certain locations. Historically, the black morph was more abundant across the Eastern Gray Squirrel’s range, particularly in dense, shaded forests. Today, the highest frequencies of black squirrels are generally found in the northern part of the species’ range, especially around the Great Lakes Basin, including parts of Michigan and Ontario.
Human involvement has created isolated populations of the black squirrel outside its natural high-frequency zones, leading to the impression of a sudden invasion. Intentional translocations occurred in the late 19th and early 20th centuries, with squirrels moved for aesthetic reasons to places like Washington D.C. and parts of the UK. While these populations are technically introduced to those specific local environments, the animals remain a native species variant, not an invasive foreign species.
The Role of Melanism in Their Success
The black coloration, or melanism, provides the black squirrel with several selective advantages that contribute to its success in specific environments. One significant benefit is thermoregulation, as the dark fur absorbs more solar radiation, which helps the squirrel maintain a higher body temperature during colder months. This thermal advantage is particularly noticeable in the northern parts of their range, where the frequency of the black morph is highest.
Melanism may also offer a degree of camouflage, though the advantage shifts depending on the environment. The dark coat provides better concealment in dense, shaded forests and against the dark, often wet, surfaces of urban environments, such as pavement and shadows. This combination of better heat absorption and improved camouflage in certain habitats allows the black morph to sometimes outcompete the lighter gray morphs, leading to the localized population dominance.