My Enemy’s Enemy: Scotland’s pine martens bolster squirrel competition

Here in the Northeast United States, we’ve seen quite an uptick in grey squirrel activity over the last couple years. With last year’s “boom” in squirrel breeding, and this year’s “bust” in natural food abundance, its been pretty hard not to notice the influx in road-side “casualties”. Here the phenomenon of “suicidal squirrels” on state highways has been dubbed the “squirrelpocolypse”, or “squirrelgeddon”, depending on which pop-culture media source you draw from.

However, across the pond in the United Kingdom, the rise of the North American grey squirrel (sciurus carolinensis) is anything but welcomed - and the tree-based rodents we’ve come to accept (whether we want to or not) here in the United States are heavily regarded as an invasive pest in the U.K.

The presence of grey squirrels in the United Kingdom is so dire, that it has effectively impacted the region’s native red squirrels (sciurus vulgaris) - commonly characterized by their distinctive ear tufts. As with most threatened wild species, the loss of habitat, motor vehicle impact, and wild predation play an integral role. However, the invasive presence of America’s grey squirrels have also had a detriment on native red squirrel populations.

For starters, grey squirrels carry a disease (a Parapoxvirus), which doesn’t seem to bother the greys, but often kills red squirrels. Grey squirrels are also more likely to consume “green” food sources earlier than red squirrels will, removing viable food supplies. It also appears that red squirrels don’t breed as readily when pressured by other squirrel populations - thus affecting reproduction. If the UK’s countryside were a high school, it would appear the native little red squirrel is destined to eat by itself in the school cafeteria - unless of course it can befriend the school-yard bully. Enter the Pine Marten.

Release the hounds, er, Martens!

According to a recent study released by the University of Aberdeen - Scotland, new research suggests recovering populations of native pine martens, which were also in a population decline across Britain, are suppressing invasive grey squirrel numbers.

Pine martens are a medium-sized member of the weasel family, closely related to the Fisher. In North America, martens are found primarily in the boreal regions of northern America amid habitat densely laced with pine and evergreen timber. The marten is highly regarded in North America as a natural resource for its dense fur.

The UK has been carrying out pine marten research in England, Wales and Scotland since the early 1980s. Mostly focused on investigating distribution and status of pine marten in England and Wales. A pine marten survey in the early 1980s confirmed that while the population in Scotland was recovering and expanding, only very small populations persisted in restricted areas of England and Wales. The primary focus of the marten’s impact on grey squirrels is in various regions of Scotland.

Researchers installed feeding stations on the landscape, which included sticky-traps (material on tabs) to collect hair samples of both squirrels and pine martens. Using DNA extracted from those caught hair samples, they were able to identify individual pine martens in each study area and map where they were spending most of their time. What they found was that pine martens were now returning to areas where they used to be, and are having an impact on invasive gray squirrel populations. With the invasive grey squirrels in decline, researchers believe they are now seeing recolonization of the native red squirrel.

Researcher Emma Sheehy confirms that “exposure to pine martens has a strong negative effect on grey squirrel populations, whereas the opposite effect was observed in red squirrel populations who actually benefitted from exposure to martens."

The study, titled “The enemy of my enemy is my friend: native pine marten recovery reverses the decline of the red squirrel by suppressing grey squirrel populations”, took place between 2014 and 2017. It included evidence from a 2014 investigation by Sheehy which suggested that European martens may be responsible for a decline of gray squirrels in Ireland.

When the well runs dry

While this news appears to demonstrate that natural predators can be utilized to maintain (and even manage) invasive species, it leaves several open doors for the future. Here in North America, a growing movement seeks to place predatory species on a pedestal of God-like greatness; where the likes of wolves and grizzly bears are “above” the confines of wildlife management as “self ecological regulators”. If Europe’s pine marten populations do fully recover, and are able to effectively eliminate a non-native invasive species, what happens then? When the abundance of martens outweighs the presence of grey squirrels, will the native red squirrel inevitably be next on the menu? Martens are a weasel-like species specifically built for running down squirrels. Will a full, unmanaged rebound of martens wipe out the very native squirrel populations European conservationists are seeking to protect? Will Europe’s recovered pine marten wind up with the same fate as grey squirrels here in the Northeast United States - subject to boom & bust years with sprawling highways mitigating the surplus cull?

Look, I’m not trying to be the Debbie-Downer pessimist, but as a sustainable-use conservationist I have a tendency to see the writing on the wall ahead of time, especially in recent years.

As it stands right now, the news of the European pine marten’s recovery, and its subsequent mediation of non-native prey species is the primary example of a conservation success story. I just hope that IF the time ever does come, when Scotland’s martens have expanded their carrying capacity, the powers that be in the UK seize the opportunity for long-term sustainable management.

The study suggests shared enemies within a wild ecosystem may propagate competitive interactions between varying species. Can a predator of a threatened species actually assist with the recovery of that species?

For now, we continue to follow the great work taking place with furbearer study in Europe. Time will certainly tell if the marten’s predatory instinct can realign historic squirrel presence.

The study was funded by the Irish Research Council, Forestry Commission Scotland and the European Commission's FP7 program. References from The Vincent Wildlife Trust, PubMed Central® and Sheehy E, Sutherland C, O'Reilly C, Lambin X. 2018 contributed to this article. (Main header photo: www.britishwildlifecentre.co.uk)