The eastern gray squirrel ( Sciurus carolinensis ) outlasted many of the magnificent extinct species of Pleistocene megafauna because they are well adapted to survive in environments modified by man. They are just as much at home in suburbs, city parks, and 2nd growth forest of the countryside as they are in the middle of a pristine wilderness. Unlike western gray squirrels ( S. griseus ), they are not shy around man and will nest in backyards or even attics. They are nimble squirrels, able to jump from tree top to tree top in the young dense forests that replace abandoned agricultural lands. And they have a unique way of spreading their populations. Every September, juvenile eastern gray squirrels begin to expand their range and forage for acorns and nuts. After they have spent enough time burying acorns in a certain area, they establish an home range there. This process is known as the “September shuffle.” During colonial times when there were still vast tracks of timber, this September shuffle could seem like a massive migration, especially during years of poor mast production following a year of heavy mast production that increased squirrel numbers. (See: https://markgelbart.wordpress.com/2012/08/24/squirrel-migrations/ )
Eastern gray squirrels thrive everywhere they’ve been introduced–England, Ireland, Italy, South Africa, western Canada, and Australia (where they were eventually eradicated by man). Much to the consternation of English naturalists, they have almost completely displaced native European red squirrels ( S. vulgaris ) on the British Isles. There are several possible reasons for this displacement. Eastern grays are carriers of a virus that may be fatal to European red squirrels. They may also disrupt red squirrel mating and outcompete them for food, and they are simply better adapted to living adjacent to people. But the most compelling ecological explanation involves comparing European and American Ice Ages.
The American eastern gray squirrel in Brandon Hill Park, Bristol, England. Gray squirrels are better adapted to living in deciduous woodlands than native red squirrels and they are displacing them.
The Eurasian red squirrel is being displaced by introduced American gray squirrels in Great Britain and Italy.
In Europe glaciers covered a greater percentage of territory than they did in North America. Most of the unglaciated region consisted of grassy or shrubby mammoth steppe with pockets of spruce and pine growing in moist protected areas. Some southerly lowlands supported more extensive conifer forests. The deciduous oak forests that dominate most of Europe today were restricted to narrow strips along the Mediterranean coast. Because glacial stages were 5-10 times longer than interglacials, European red squirrels became better adapted to live in conifer forests. However, in North America, even during the severest stadials, there were always extensive oak and oak/pine forests that supported large populations of gray squirrels. Eastern grays evolved the ability to digest acorns better than red squirrels can. Although eastern grays are not native to Europe, they are a better fit for the interglacial oak forests that exist there today. Ecological displacement of 1 species by another has occurred thousands of times during earth’s history. People may object to the displacement of European reds by eastern grays because man played a role in the introduction of the latter, but it is not unnatural or detrimental to the overall ecosystem in this case.
Eastern grays along with California ground squirrels, introduced fox squirrels ( S. niger ), and turkeys are displacing and outcompeting western gray squirrels on the Pacific coast of North America. All seem to be better adapted to anthropogenic environments. Western grays are now restricted to deep wilderness preserves. Introduced eastern gray squirrels are also displacing American red squirrels ( Tamiasciurus hudsonicus ) in British Columbia.
The American red squirrel is being displaced by gray squirrels in some parts of its range as well.
Taxidermic mounts of eastern and western gray squirrels. The western is larger with a bushier tail. Introduced Eastern gray squirrels and fox squirrels, as well as native California ground squirrels and turkeys are outcompeting western grays in suburban areas of California heavily modified by man.
Eastern gray squirrels have co-existed with fox squirrels for hundreds of thousands of years but are more common in many areas, including Richmond County, Georgia where I live. Eastern grays quickly recolonize agricultural land replaced by dense 2nd growth forest, while fox squirrels prefer mature forests with widely spaced trees. Because most of southeastern North America was clear cut between 1865-1945, gray squirrels have been quicker to return and spread throughout their range. Without human introduction fox squirrels may never return to formerly clear cut land.
Eastern gray squirrels boldly live next to people and their unique September shuffle makes them a super squirrel, able to expand their populations and survive where other squirrels can’t.
Bruemmer, Corrie; Peter Lurz, Karl Larsen, and John Gurnell
“Impact and Management of Alien Eastern Gray Squirrel in Great Britain and Italy: Lessons from British Columbia”
Proceedings of the Conference on the Biology and Management of Species and Habitats at Risk 1999