It might seem slightly paradoxical but in the UK, red squirrel number increase when populations of their native predator, the pine marten, also increase.
When gray squirrels were introduced into the UK by the Victorians as a fancy, it was bad news for the native reds. From a population of around 3.5 million red squirrels, the grays outcompeted and spread disease among them, leaving around 250,000 clinging on largely in Scotland.
Pine martens in England have done poorly too, hunted for their fur and as retribution by farmers, until they too were pushed back into the forests north of the border. But under their own steam pine martens have been slowly increasing in numbers and spreading across the UK, and weirdly this might be benefitting the beleaguered red squirrels.
In a bid to reduce the number of grays and help the native squirrel, eradication programs are common, usually involving trapping and killing the invasive species. This is costly though, and now researchers think that the long-maligned pine martens could actually do the service for free, publishing their results in Proceedings of the Royal Society B.
The team set about trying to assess the relationships between pine martens, red squirrels, and gray squirrels by placing bait out at 200 sites across Scotland. The feeders contained hazelnuts for the squirrels and peanuts for the pine martens, but crucially had sticky pads on them to collect hair samples from any hungry visitors.
Using these hair samples, the scientists were not only able to determine exactly which species were living in the area and visiting the feeders, but also by using genetics they could identify individual animals and track where their core territories were.
This provided the first unequivocal evidence that as the number of pine martens increase, the number of gray squirrels is suppressed, giving a boost to red squirrel populations. So it seems even among woodland creatures: your enemy’s enemy is your friend.
Why the pine martens seem to be preferentially picking off the grays is not fully understood, but could be down to a number of factors.
One that has been floated before is that it might be to do with the size difference between the red and gray squirrels. On average reds are slightly smaller than grays, which means that when arboreal martens hunt them through the branches the red squirrels are able to climb out onto the thinner twigs that won’t support the weight of the marten. The heavier grays, however, have no such luck and so are more likely to make marten lunch.
The data from this study suggests another possible reason. Curiously, in regions where the pine marten numbers were found to be increasing, while the number of red squirrels visiting feeders decreased, no such change happened with the grays. The researchers suspect that as a non-native invader, the grays might be naive to the native predator, as no such analog exists in North America.
The finding, which backs up an earlier study looking into this effect in Ireland, adds yet more weight to the argument that we should be rewilding our countryside, reintroducing the species that we drive to extinction hundreds of years ago to restore the balance, and make the environment not only better for nature, but better for ourselves too.