Martens limit the damage caused by humans

For well over a century, the red squirrel as we know it has been fighting a losing battle in Great Britain, losing ground year after year. At the end of the 19th century, gray squirrels were first introduced to Great Britain from America. Since then, they have been on an unstoppable march, establishing themselves on the British Isles and pushing their native relatives, the red squirrel, to the brink of endangerment.

This North American rodent is somewhat more robustly built than its European cousin (Sciurus vulgaris) and also carries a virus: immune to it themselves, gray squirrels infect their relatives, who succumb to the disease in large numbers.

Poison and culling have failed

In an effort to get the situation under control, conservationists in Great Britain are trying everything to prevent the gray squirrel from spreading further and to help the European red squirrel recover. Gray squirrels may be shot, poison is being considered, yet all government strategies to eradicate the introduced species have failed. The government has admitted defeat against the gray squirrel.

The pine marten as savior

What humans have failed to achieve may finally be accomplished by another species. The pine marten (Martes martes), which has suffered greatly at human hands in the past, is currently making a comeback. And wherever this small predator spreads, gray squirrel populations shrink while red squirrel populations begin to grow again.

This is the finding of a US-American-Scottish research team in the latest issue of the journal «Proceedings of the Royal Society B». The biologists documented the return of the pine marten to regions in Scotland that had long been abandoned — including clear effects on squirrel populations. The study ran from 2014 to 2017 and built on a 2014 investigation that suggested the same connection for the shrinking of the gray squirrel population in Ireland.

Coevolution protects the red squirrel

The connection is not immediately obvious: after all, martens hunt both red and gray squirrels. However, the factor of coevolution likely comes into play here, as biologist Emma Sheehy explained to the BBC. Red squirrels have evolved alongside the pine marten over a long period of time and are accustomed to it and its hunting methods — which protects them to a certain degree.

Gray squirrels, on the other hand, had no such neighbor in their original homeland, making them less cautious around martens and thus easier for them to catch. “The enemy of my enemy is my friend,” the researchers summarize the relationship in their paper. At the same time, they describe it as a win-win situation, since both the pine marten and the red squirrel have protected status.

Gray squirrels are larger and bulkier than European red squirrels. This advantage over their relative could become a disadvantage when it comes to predators. Their size ultimately makes the animals more sluggish and therefore easier prey. Moreover, in the course of their evolution in America, they never had to learn to flee from pine martens, while red squirrels have this escape mode genetically “programmed in.”

The scientists assume that the pine marten is indirectly responsible for the rise in the red squirrel population. And they give the government a strong hint that nature sometimes knows how to help itself and can still find a solution to a problem even after traps and poison have already failed. More on wildlife and on biodiversity.