Why can red and grey squirrels not live together?

The answer is a little complex, but in short, grey squirrels consume the same types of foods as the native red squirrel. This means that they compete for resources, the much smaller red squirrel suffers as a result. In addition, grey squirrels typically live at densities of around five or six adults per hecatre (a football pitch sized area) of woodland, whilst only 0.5-1.5 adult red squirrels would be found in a similar sized area.

It is therefore easy to see how the much bigger grey squirrel can take over habitat from red squirrels. Recent scientific studies have shown that young red squirrels find it difficult to survive in woodlands that contain established grey squirrel populations. With relatively few young animals entering the population, the red squirrels gradually disappear.

Occasionally, mixed red and grey squirrel populations are found to persist for several decades, however, although these are interesting, they are wholly unrepresentative of what happens almost everywhere else in the UK.

One explanation for such cohabitation is that the grey squirrels in such areas have never been found with anti-bodies to the 'parapox' virus that can cause heavy mortality in red squirrels. Eleswhere, a high proportion of greys are found with anti-bodies to this virus, and it appears likley that the grey is involved in transmitting the virus into red squirrel populations.

Thus, grey squirrels out compete red squirrels for resources and are also likely to introduce diseases into red squirrel populations.