Halting march of grey squirrel

THE American grey squirrel, and its flea, arrived here from England in 1911 when a ‘scurry’ was presented to the bride during wedding celebrations at Castleforbes County Longford.

Released into the countryside, the intruders and their descendants prospered. They have colonised eastern and northern counties, displacing the resident red squirrels everywhere. This alien also strips bark from trees. A report on the Irish Squirrel Survey claims that “commercial broad-leaf crops can be written off and native trees left severely damaged … If left unchecked, the grey squirrel will severely curtail both commercial hardwood production and the conservation and expansion of native hardwood cover in Ireland”.

The survey, carried out in 2007, found that greys had “expanded their range dramatically” during the previous decade and that they had “breached the Shannon in a few places”. The grey is now in 26 Irish counties and, at current rates of expansion, will colonise three more in the next two decades. Trapping and shooting are ineffective in controlling it but, thanks to a discovery made during the squirrel survey, there is a glimmer of hope. Surveyors seem to have found the grey squirrel’s Achilles heel; there is a native animal which may be able to halt the expansion. We might even recover some lost ground. The scientists are tight-lipped, but a remarkable research project is underway at NUI Galway.

The red squirrel’s decline has been relentless wherever greys are found. However, surveyors noticed two exceptions. In Laois-Offaly and Cavan-Leitrim-Fermanagh, the reds seemed to be holding their own. Greys were present but their numbers were low. Significantly, the areas appeared to have lots of pine martens.

The pine marten, long persecuted by game keepers and harvested for its skin, was until recently one of Ireland’s rarest mammals. Then, following legal protection in the 1970s, its range and numbers increased. The ‘cat crainn’ preys on small and medium-sized animals. Grey squirrels, being ground-foragers, are relatively easy for a marten to catch. The red squirrel, much lighter than the grey, is a tree-top feeder which behaves like a bird. Martens, although they are tree dwellers, are too heavy to venture onto thin upper branches, so red squirrels are not easy targets. The pine marten and red squirrel have co-evolved over millennia and equilibrium has been established between their populations. The grey squirrel, not having encountered martens in its native North America, hasn’t evolved a defensive strategy for dealing with them.

If martens can halt the grey squirrel’s spread, could they help us control the invader? Emma Sheehy, of NUI Galway’s Department of Zoology, has embarked on a three-year study to find out. She began by investigating whether there really are more red squirrels than grey ones in areas where pine martens are common. Members of the public were asked to report sightings. Then she carried out a ‘hair-tube’ survey; plastic pipes baited with food were attached to trees. Sticky tape on the inside of a tube traps animal hair which can be subjected to DNA analysis to identify its owner. Next, she collected marten droppings, known as ‘scats’. DNA extracted from these reveals what the animals have been eating. She reckons that she walked over 1,000km to find about 400 scats. These are very difficult to locate in areas with few martens so a trained sniffer-dog was brought from England. It picked up eight scats in an area in which Emma could find only one! Finally, she trapped and tagged red and grey squirrels in Offaly where marten numbers are high.

The crucial finding was that grey squirrel populations which used to be strong had all but disappeared from areas with lots of martens. Moreover, the numbers of red squirrels had increased. When similar studies were carried out in two Co Wicklow woods, where marten numbers are very low, greys were found to be numerous. Emma can’t say, as yet, why martens deter greys. However, she is optimistic that there is some sort of interaction between the species. Nor can she say whether the pine marten can save our red squirrel, but there seems to be light at the end of the tunnel.

* The Irish Squirrel Survey. 2007. Michael Carey, Geoff Hamilton, Alan Poole and Colin Lawton.