Pine marten populations re-established in the Highlands more than 45 years ago, in Central Scotland between 8–14 years ago, and have recolonized the Scottish Borders in recent years. Red squirrels are present in all three areas, whereas grey squirrels have not yet reached the Highlands, but are present in Central Scotland and the Borders. The research team used these three areas as their study sites.
To understand more about the number of pine marten, red squirrel and grey squirrel in each area, multi-species feeders (wooden boxes with a liftable lid containing nuts and seeds) were attached to trees. To access the bait, the martens and squirrels have to climb the tree and lift the feeder lid with their head. On the underside of the lid, sticky strips capture hair samples as the animals feed. The DNA extracted from the hair samples was used to identify each species, and Bushnell camera traps were rotated between the sites to capture images of the feed box activity. By using this method the researchers are able to identify individual martens via their DNA and use this information to understand more about the range that individual martens occupy.
The research monitored a total of 725 detections at the feeders (115 red squirrel, 101 grey squirrel and 509 pine marten) and found more red squirrels and less grey squirrels where pine marten were present. This suggests pine marten are having a suppressing effect on grey squirrel populations, giving the red squirrels a chance to rebound in number, as negative factors such as the disease spread by grey squirrels is less present. One reason for this could be that red squirrels have coevolved alongside pine marten in the UK, whereas grey squirrels have not and are therefore not adapted to respond to the predator.
These findings are of great importance for pine marten conservation, as it is clear evidence of an ecosystem service that pine marten can provide to control grey squirrel populations. In the UK we spend considerable amounts of money on grey squirrel control each year. Pine marten could help in managing our grey squirrel problem, which would in turn play a valuable role in supporting red squirrel populations to thrive again.
This may sound like a case for removing grey squirrels from the UK altogether, which perhaps isn’t the most ethical solution (let alone a particularly viable one at present), but it is clear that we need much better control over the size of grey squirrel populations. This will in turn support the return of red squirrels, while having wider positive impacts on biodiversity by protecting susceptible tree species to bark stripping, such as sycamore, beech, oak and pine.
By conserving and improving pine marten populations, this new research provides a compelling case for how we can better manage grey squirrels while boosting red squirrel numbers in the UK.