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.
In northern England and Scotland, there is a big community effort to control grey squirrels and help the reds survive but last year saw a dramatic expansion in the grey squirrels’ northward range, with animals moving north from Dunkeld to Pitlochry for the first time and spotted west in Aberfeldy and north of the River North Esk in the east.
There is a grey squirrel population around Aberdeen, which has been restricted by culling in recent years, but the fear is that northward-scampering greys from Scotland’s central belt will meet the Aberdeen population and rapidly expand, endangering reds across north-east Scotland.
“There seems to have been some movement of greys last year linked to a bumper beech mast year and possibly because trappers weren’t out because of Covid,” said Gareth Ventress, an environment forester at FLS. “We need to stop the grey squirrels in their tracks.”
The dens are roofed wooden boxes fixed to trees five metres off the ground and filled with wood shavings, providing a secure and cosy nest in which martens can breed safely in spring and shelter in winter, reducing mortality rates for juveniles.
If martens nest on the ground, their kits are vulnerable to predation from foxes, and while they can rear young in old buzzard nests, these are often exposed.
It is hoped that the dens will increase the population density of pine martens and ensure they are living in areas where they can predate the greys.
Pine martens have already returned to the region north of Perth, but dens will encourage them to frequent woods close to the A9 and the railway line north to Inverness, which provides a corridor of scrub and trees along which the greys can expand.
“In many ways the A9 corridor is defendable because it’s such a narrow corridor of land, but if you travel along it you notice there is tree cover most of the way up,” said Ventress.
Dens will also be installed farther east, around the A90 in Angus, after greys last year crossed the River North Esk into Aberdeenshire.
Research from Ireland, Wales and Scotland was once thought to show that pine martens created “a landscape of fear”, driving away grey squirrels. But ecologists have since found that grey squirrels are in fact naive and “fearless”, and that’s what makes them susceptible to predation. The science is supported by real-world examples of resurgent pine martens alongside prospering red squirrels, which have returned to parts of Aberdeenshire where they haven’t been seen for many years.
But recent studies have shown that pine martens won’t colonise urban areas, which remain a stronghold for grey squirrels. If the countryside is constantly topped up with new greys from their urban stronghold, the species is likely to rapidly adapt to survive alongside the marten.
Another recent study in Ireland found that while martens increase red squirrel populations in native, broadleaf woodlands, the effect is reversed in non-native conifer plantations, where the marten reduces red squirrels. This could be due to the lack of alternative prey and the lack of hiding places for red squirrels in highly simplified landscapes.
According to Ventress, the threat to reds from pine martens in conifer plantations may only apply in Ireland, where there are no field voles, the marten’s main prey in Scotland. Irish plantations also tend to be solely sitka spruce, unlike in Scotland where many plantations include native Scots pine and other trees.
Ventress added: “Pine martens are not the answer to the grey squirrel threat by any stretch, but they are part of the answer. The key to this pine marten-grey squirrel relationship seems to be pine marten density. With the boxes we can potentially localise some of the pine marten population.”
Ensuring grey squirrels don’t move farther north will also help foresters restore and establish native woodland in the Highlands. Grey squirrels strip bark from young trees, causing disease, damaging timber and sometimes killing trees.
“Keeping the grey squirrel populations back will help us diversify, and nature and woodland restoration has a better chance,” said Ventress.
• This article was amended on 21 February 2022 to note the theory that pine martens created a “landscape of fear” for grey squirrels has been disproven.
