Red squirrels are native to Europe and abundant throughout most of its habitat. But in Great Britain, Ireland and Italy, numbers have fallen dramatically due to the introduction of gray squirrels from North America. There are thought to be fewer than 250,000 individuals left in the United Kingdom, and most of those are in Scotland. The gray squirrel out-competes the red squirrel in feeding, since it can digest acorns but red squirrels cannot, and also carries a disease called squirrel parapoxvirus which kills red squirrels but doesn't seem to affect gray squirrels. In many areas, the only solution to help bring back red squirrels is for humans to eradicate gray squirrels. But research has shown that perhaps the solution lies in protecting a long persecuted predator.
After years of being poisoned by gamekeepers, trapping and habitat fragmentation, the pine marten disappeared from much of England and Ireland, and it was only still common in northwestern Scotland. However, through protections and the reforesting of some areas, pine martens have begun to spread back into some of their historical habitat. And alongside this redistribution, researchers noticed something else interesting. Where the range of pine martens overlaps with the range of gray squirrels, gray squirrel numbers drop and leave room for native red squirrels to rebound.
For a March 2018 study researchers placed more than 200 feeders in the woods around Scotland with a selection of nuts and seeds that are attractive to squirrels and martens. They were able to extract DNA from sticky strips on the feeders and found that in locations where there were more pine martens, red squirrels were much less likely to visit feeders than gray squirrels.
“It suggests grays are totally naive to the risks of pine martens as a predator,” lead researcher Emma Sheehy from the University of Aberdeen told The Guardian. “In their native range, they don’t have similar predators and that leaves them much more susceptible here.”
The study, published in the Proceedings of the Royal Society B, begins, "The enemy of my enemy is my friend."
In an earlier study released in 2014 on the pine marten's influence on gray squirrels in Ireland, researchers reported, "A distribution survey of the midlands was carried out which confirmed the gray squirrel population has crashed in approximately 9,000 km2 of its former range and the red squirrel is common after an absence of up to 30 years."
It is thought that because gray squirrels spend more time foraging on the ground, they're easier for a pine marten to catch than the more arboreal red squirrel, though there are other factors including how gray squirrels simply react to a predator they didn't evolve with. These include a stress-induced reduction in breeding, changing foraging habits and simply retreating from the areas where pine martens are found. Whatever the reason for the reduction in gray squirrels, it gives the red squirrel a window to recover, and they do.
"It would seem from the evidence gathered to date that the presence of an abundant tree-climbing predator may be the cause of the gray squirrels’ retraction from the midlands, and the native red squirrel’s subsequent recovery," writes researcher Emma Sheehy.
The story of pine martens and red squirrels is a great example of how the balance of predator and prey plays a key role in the health and balance of species in an entire ecosystem. It is possible that by helping to bring back the pine marten, conservationists could make big strides in battling back invasive gray squirrels and bringing back native red squirrels.
Editor's note: This story has been updated since it was originally published in August 2014.
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