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.

Joshua P Twining , Author provided

For conservationists, it almost sounds too good to be true. But why do pine martens seem to benefit native red squirrels at the expense of invasive greys?

Striking fear into red squirrels

Researchers sifting through masses of pine marten faeces demonstrated that pine martens eat both squirrel species, but they tend to eat more grey than red squirrels. We know pine martens are opportunistic omnivores, switching food throughout the year and consuming whatever is available. So it’s probably not the preferences of pine martens that lead them to eat more grey squirrels, but the availability of the squirrels themselves.

Read more: Why the pine marten is not every red squirrel's best friend

One way that prey species avoid predators is with chemical signals. Predators use scent cues to mark territories and communicate with one another, but prey animals have evolved behavioural responses to these scent cues. By eavesdropping on these chemical signals, prey species can alter their behaviour and avoid predators, increasing their chances of survival. Could the grey squirrel, an animal not native to Britain or Ireland, be naive to the risk posed by the pine marten?

Armed with camera traps, squirrel feeders and a solution of pine marten poo, we set out to find out if this was the case. We repeated the experiment at 20 different locations across Northern Ireland, amassing more than 8,000 minutes of squirrel footage. The results are published in Royal Society Open Science.

Emerging red-eyed from our lab, we realised that red squirrels showed a clear fear response to pine marten scent while greys didn’t. Reds visited feeders less, fed for shorter periods of time and were more vigilant – standing on their hind legs with their head upright and tail twitching from side to side. Meanwhile, the greys continued as if nothing had changed. In some cases, grey squirrel visits to feeding stations actually increased while their vigilance decreased around pine marten scent.

Failing to recognise the scent of a predator as a threat leaves the grey squirrels vulnerable. In hindsight though, their behaviour isn’t surprising. Red squirrels and pine martens have shared the same evolutionary landscape for millennia – we would expect them to be aware of each other. But grey squirrels and pine martens have co-existed in the same place for a mere blink of the eye. Although the pine marten predates red and grey squirrels, they have more success in catching greys because they appear oblivious to the threat.

Joshua P Twining , Author provided

Healthy native predator populations have wide ranging benefits to the environment they inhabit. Conservation efforts are helping predators to recover in parts of Europe, ensuring they can restore and regulate ecosystems. If the recovery of a small weasel can have such benefits, there is immense potential for larger predators like lynx and wolves to restore balance to fragmented and degraded ecosystems.

Pine martens aren’t a panacea for red squirrels though. Human-led efforts will still be important in red squirrel conservation, especially because prey species can develop responses to new predators in just a few generations. Who knows whether grey squirrels will develop anti-predator behaviours.

But if, like the red squirrel, greys learn to recognise and avoid the pine marten, they could one day learn to survive alongside it. Useful though they may be, the pine marten – or any native predator – is more than a solution to a human-made problem. They are an essential and iconic part of the natural world. The conservation and recovery of predators may not always be easy, but in a human-dominated world, we must learn to live alongside what precious biodiversity we have left.