Eastern grey squirrels are a common sight in both natural and human modified landscapes throughout Eastern North America. However, thanks to human introduction, they are often just as common in areas far outside their native range, including: western/northern North America, the United Kingdom, Ireland, Italy, South Africa, and Australia (although the Australian population has since been extirpated via control program intervention; Koprowski 1994). The characteristics that enable eastern grey squirrels to colonize urban environments may help to explain why they are such proficient invaders of novel ecosystems around the globe.
Unfortunately, these invasions are seldom benign. In some cases eastern grey squirrels can negatively impact native tree species, or facilitate the decline of native vertebrates, either through competitive exclusion, or disease mediated processes. Perhaps, the best known example is the decline of eurasian red squirrel populations in the British Isles, however eastern grey squirrel introductions have also been linked to declines in western grey squirrels and Mearn’s squirrels in North America.
It is due to this invasive capacity of the eastern grey squirrel that has led the International Union for Conservation of Nature (IUCN) to list them among the world’s 100 worst invasive species.
Koprowski, J.L. (1994). Sciurus carolinensis. Mammalian Species, 480:1-9