Red squirrels (Sciurus vulgare) are hard to find — they are almost extinct in England. They survive on Anglesey, North Wales and in parts of the Highlands and islands of Scotland, where isolation has saved them from the scourge of squirrel pox, carried by the successful grey squirrel (Sciurus carolinensis) that raids our bird feeders but it’s not as simple as that. Last week we visited some in…

Pine martens are to be deployed as wildlife bouncers along the east coast of Scotland and the A9 corridor to halt the northward march of grey squirrels.

More than 35 artificial pine marten dens are being installed by Forestry and Land Scotland (FLS) at strategic locations on the grey squirrels’ path of northward migration in an attempt to save the Highland red squirrel populations.

Red squirrels have been driven out of most of the British countryside since the introduction of greys in Victorian times, mainly because greys carry the squirrelpox disease, which leaves them unharmed but is fatal to the reds.

But recent research has found that the return of the predatory pine marten has reduced grey squirrel populations. Because the native red squirrels and martens evolved alongside each other over thousands of years, the reds appear less affected by martens.

For as long as modern humans have been moving around the planet, we’ve been bringing animals, plants and microorganisms with us. But by introducing invasive species to ecosystems in which they did not evolve, we’ve unwittingly created problems that we now spend a great deal of effort and resources trying to rectify.

The North American grey squirrel is one such invasive species that was brought to Britain and Ireland during the 19th and 20th centuries. Since their introduction, grey squirrels have replaced the native red squirrel across much of its former range, mainly by transmitting squirrel pox to reds, a deadly virus to which greys are immune.

Almost all attempts to counter historic blunders with grey squirrels have been met with limited success, but it would seem that nature already has a solution.

Until recently, the European pine marten was a little-known member of the weasel family that lives in trees and is restricted within the British Isles to the northern reaches of Scotland and the western coast of Ireland. But in recent years, it has started to reclaim some of its former range. In parts of Ireland and Scotland where this native predator has recovered, there have been subsequent declines in grey squirrel populations, allowing reds to recover.