Red squirrels find unlikely ally in an old predator

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The return of the formerly ostracized pine marten to Scotland is helping native red squirrels regain ground from their invasive grey cousins.

In the late 1800s, wealthy Victorians shipped North American grey squirrels (Sciurus carolinensis) to Britain to adorn their estates. Unfortunately, the exotic ornaments carried squirrel pox, which is fatal to native red squirrels (Sciurus vulgaris). Grey squirrels also damage woodlands by stripping bark from trees, causing millions of pounds in damage to forestry. But controlling invasive species can be costly and controversial if culling is involved.

Pine martens (Martes martes) — cat-sized carnivores in the weasel family — have been slowly returning to Irish and Scottish woodlands since the 1980s, when it became illegal to hunt them. A study in Ireland found that areas with rising numbers of pine martens have fewer grey squirrels, yet more red squirrels, despite being a predator of both.

To explore this relationship further, four researchers, including two from the Waterford Institute of Technology in Ireland, surveyed these three woodland critters in three parts of Scotland. Since the three areas had been recolonized by pine martens at different times, their squirrel populations have been exposed to predation for different periods.

The researchers fixed boxes of nuts and seeds to more than 200 trees across the three regions. Sticky tape on the lid of the boxes trapped fur from any animal that came to feed from them, while motion-triggered cameras snapped the animals in action. “This wouldn’t have been possible without all the volunteers that went out periodically to collect the fur,” notes lead author Emma Sheehy.

In the lab, the researchers analysed the DNA from the fur to determine precisely which animals stopped for a snack and where. “In this way, we collected huge amounts of data without coming into contact with any of the animals,” says Sheehy.

As predicted, areas with greater pine marten activity had fewer grey squirrels, suggesting that the martens had been eating more than just nuts and seeds. This gave the competitive advantage back to red squirrels, which popped up more in areas with more martens.

Red squirrels had been living alongside pine martens long before greys arrived, and so may have adapted to escape nimbly through the trees, whereas the naïve greys still blithely forage around on the ground, exposed.

By suppressing grey squirrels, pine martens are helping both red squirrel conservation efforts and the UK timber industry.

Next, Sheehy hopes to determine exactly how pine martens suppress grey squirrel populations as opportunities to study them increase. “Pine martens have been reported in North England,” she says. “The natural spread is slow, but various wildlife trusts are giving them a helping hand elsewhere.”

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  1. Proceedings of the Royal Society B 285, 20172603 (2018). doi: 10.1098/rspb.2017.2603