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.