Red squirrels have been classified as an endangered species because their numbers have declined due to the spread of the North American grey squirrel that brings the virus parapox, which is deadly to native red squirrels, according to the Young People's Trust for the Environment. Moreover, larger grey squirrels directly compete with them for food and nesting sites.
The red squirrel has disappeared from large regions of Britain, as the grey squirrel has taken its place. The larger grey squirrel first arrived in the country in the mid-19th century. Research reveals that grey squirrels have higher chances of surviving than red squirrels because they put on much more body fat. In terms of competition for food and space, larger grey squirrels win over red squirrels. More importantly, grey squirrels carry the squirrelpox virus, which does not affect them but harms red squirrels.
Both red and grey squirrels are tree-dwelling mammals that depend on similar food sources and live in a similar ecological niche. However, they are different in their ability to digest bigger seeds from broadleaved trees, specifically acorns. Thus, grey squirrels have a competitive advantage in mixed and broadleaved woodland. They eat large seeds before these seeds are fully ripe, allowing them to obtain more nutritional benefits. This leads to weight loss and lower breeding success among red squirrels.