Alien v predator: red squirrels rising

The alien grey squirrel is being routed from midland counties, with their numbers plummeting. The retreat of this American squirrel is proving a boon for our smaller native red squirrel, which is making a comeback in these areas, experts say.

Scientists in NUI Galway tracking the grey squirrel’s decline believe that a resurgent pine marten may be causing the dramatic reversal in squirrel fortunes. Pine martens are cat-sized, tree-climbing predators that can catch and kill grey squirrels.

“We confirmed that the grey squirrel population in the midland counties of Laois and Offaly has undergone a substantial decline,” says squirrel expert Dr Colin Lawton of NUI Galway. “The red squirrel, despite being considered absent from much of this area for a period of about 30 years, is once again widespread and common.”

A gloomy 2007 survey had shown greys expanding into Wexford and Waterford, into north Cork, and right up to Derry and Donegal. Wherever greys go, reds eventually disappear. This outcome is blamed on competition and a pox virus carried by greys but lethal only to reds. But there was also a glimmer of hope, too: a gap in grey distribution.

Woodland owners and gun-club members in the midlands had noticed something was up with grey squirrels as far back as the mid-1990s. Anecdotal reports of disappearing squirrels coinciding with a pine marten comeback are now strengthened by studies by Emma Sheehy, a PhD candidate in Lawton’s group in NUIG. Sheehy, whose research is funded by the Irish Research Council, predicts greys will continue to decline.

Sheehy and Lawton were surprised by a lack of greys squirrels. “We expected them to have disappeared from a few woodlands,” says Lawton. “But it seems to be quite a large area.”

Where you find pine martens you tend not to find greys, so it has been difficult for wildlife biologists to study the grey demise. Greys are rare in Offaly and Laois, says Sheehy. And surveys beyond the Laois/Offaly borders revealed few greys and resurgent red-squirrel populations in north Tipp, south Westmeath, west and northwest Kildare and north Kilkenny.

Pine martens appear to prey more often on grey squirrels than reds, which seem unaffected by the forest carnivore, says Lawton. “Pine martens would find it harder to hunt and capture a red. The time they spend in the canopy and their light size means they go out on to finer branches.” Greys tend to spend more time squirreling for food on the ground.

Pine martens could be gorging their way through our grey squirrels. But Lawton believes the explanation is more complicated: “I would be leaning toward a more stress-induced factor, where [pine martens] are causing the greys to breed less or causing them not to settle in woodlands when they detect a pine marten population.”

Red and grey squirrels feed on tree seeds, but when stores run low, greys will happily eat mushrooms, birds’ eggs, small shoots and so on. “I have seen them eat crisps out of bins in the Botanic Gardens in Dublin,” says Lawton.

Working in the field

Reds had held out in Wicklow, but greys recently began moving in. As part of her research, Sheehy has cage trapped [see panel] up to 20 grey squirrels a day in an oak wood in Wicklow – Tomnafinnoge.

Last year, stopping off on her way down to Waterford Institute of Technology [where she carries out DNA analysis], Sheehy came across scat (droppings) on a log in Tomnafinnoge and decided to collect it for study. “I thought it must be fox,” she says, “but it turned out to be pine marten scat.” And the scat contained grey squirrel DNA.