Only around 140,000 of the UK’s red squirrels remain in various strongholds, with Scotland being home to around 75% of the population.
The survival of many of these strongholds relies on active management by organisations, volunteers and land managers to remove invasive grey squirrels - introduced from North America in the 1870’s. Although some have natural barriers that prevent grey squirrel invasion, including islands like the Isle of Wight and Brownsea Island.
Maps showing grey squirrel spread and red squirrel decline 1945, 2000 & 2010 by Craig Shuttleworth & Red Squirrel Survival Trust
Red squirrel declines
Grey squirrels have replaced red squirrels in large areas of the UK, as they outcompete them for food and habitat, and transmit squirrel pox virus. Competition stresses and weakens the red squirrels, and reduces their ability to fight off disease and reproduce. Once grey squirrels move into a new area it is only a matter of time before the red squirrels die out.
In the presence of squirrel pox, red squirrels die off 17-25 times faster. Grey squirrels are carriers of the virus, which they rarely contract but can be reservoirs for high concentrations of the disease. Squirrel pox outbreaks in red squirrel areas are generally linked to grey squirrel encroachment and disease transmission.
Grey squirrel spread
Current estimates put the grey squirrel population in the UK at over 2.5 million individuals. They are now widely spread in large areas of England, Wales, the Scottish Lowlands, Northern Ireland and Ireland, where they tend to live at higher densities than the native red squirrel.
Grey squirrels have a greater resistance to tannin, a chemical found in seeds such as unripened acorns. Whereas red squirrels can suffer from tannin poisoning, so have to wait until acorns have ripened and the tannin has reduced to eat them. Grey squirrels can decimate this important food source early in the year, so outcompete red squirrels in broadleaved woods.
Red squirrels were pushed into areas with more conifers, as they can exploit the smaller seeds more easily than grey squirrels. However, red squirrels can live in a variety of different habitats, as long as grey squirrels are not present. Active conservation efforts to manage grey squirrel populations are important for maintaining and expanding red squirrel strongholds.