Michael Viney: White-toothed invader puts Irish pygmy shrew at risk

New species arriving through human activity turning Ireland into ‘ecological junkyard’

While the human world deliberates on the pace of its own disintegration, this column distracts with affairs of Ireland’s smallest mammals ...

“There is something indescribably sad, almost poignant, about a dead shrew, flat on its back with its little legs in the air. As animal corpses go, only that of an elephant is more pathetic.”

Prof James Fairley, a veteran zoologist, was moved to write this of the pygmy shrew in An Irish Beast Book, his engaging review, half a century ago, of the island’s furry wildlife. In 2001 his Basket of Weasels found Ireland’s only shrew still “common throughout rural Ireland” as the prey of stoats, owls and pine martens. It was often found dead in autumn at the end of a brief and hectic life or brought home by the cat as a doorstep trophy.

Today the Irish pygmy shrew is fast becoming an endangered species, displaced by an alien invader, the greater white-toothed shrew. And Ireland’s experience is now taken as a warning by ecologists in Britain. They have been alarmed at the discovery of the first white-toothed shrews, transported from Europe, in a garden in Sunderland, on England’s North Sea coast. In Ireland, UCC zoologist Dr Paddy Sleeman agrees: “This is very, very bad news for small British mammals.”


The teeth of the pygmy shrew have reddish tips, being coated with iron to chomp through the chitinous armour of some 125 beetles and other insects in 24 hours. This ceaseless, sleepless consumption comes to more than the shrew’s own body weight (a minuscule 5 or 6g, or a third of the weight of a mouse). Two hours without food can be fatal.

The white-toothed shrew (with white hairs on its tail) is up to three times bigger than the pygmy. Its females are territorial and aggressive (as are the pygmy shrews). It can hunt bigger insects. And it can carry a bacterial disease.

The pygmy’s exceptional metabolic rate complicates the reckoning of how it first arrived in Ireland. Geneticist Dr Allan McDevitt of Atlantic Technological University and the University of Salford in the UK suggests the shrew may have arrived as a stowaway in livestock fodder in a boat from Britain about 5,000 years ago.

He credits the animal with eager climbing and burrowing after food on the voyage. His Twitter address, ShrewGod, speaks for his devotion to the study of Sorex minutus, the pygmy shrew, and Crocidura russula, the invasive greater white-toothed shrew.

The latter’s arrival in Ireland was discovered in 2007 from skeletal remains in the pellets of barn owls and kestrels collected in Tipperary and Limerick. Seven live shrews were then trapped in four places in Tipperary – proof the species had, indeed, arrived.

New species entering Ireland could damage the island’s store of resilient, long-established biodiversity

Genetic comparison of jawbones showed that France was the most probable source of the new animals. This echoes research into the arrival of the hazel dormouse found established in Co Kildare in 2010.

DNA analysis suggested origins in France, from which Kildare racehorse owners regularly imported hay. Tipperary, too, is known for its racing stables.

Dozily delightful dormice, once established in woodland, tend to stay put with limited wandering. Greater white-toothed shrews, however, spread themselves at a remarkable rate. By 2013 they had colonised some 7,600sq km of Ireland and were advancing at more than 5km a year.

A current map of sightings of the shrew at Biodiversity Ireland shows a solid spread across central southern counties, with a hot spot of numbers advancing through Co Westmeath by 2021. Isolated pockets of the shrew, with one in central Cork city, suggest help from human activity.

The sinister side of this colonisation emerged when live-trapping surveys by a 10-strong “shrew crew” of scientists, led by Dr McDevitt, found that the pygmy shrew was completely absent at sites where the new shrew was established and only present at the edge of and beyond its advance.

The species coexist in some overlapping habitats in France, so why this change in Ireland? “The speed of the invasion,” says the McDevitt report, “and the homogenous nature of the Irish landscape may mean that Sorex minutus has not had sufficient time to adapt to the sudden appearance of Crocidura russula. This may mean the continued decline/disappearance of Sorex minutus as Crocidura russula spreads throughout the island.” The pygmy’s last refuges, he says, may be on small, offshore islands.

Does it really matter if one shrew replaces another? When the bigger one arrived, it found some welcome as additional prey for owls, kestrels and hen harriers. That was before its impact on the pygmy began to emerge.

Prof Ian Montgomery of Queen’s University Belfast was the first to show the replacement happening. For him it was the latest warning of “an invasional meltdown”. New species arriving through human activity were turning Ireland, he said, into an “ecological junkyard”. This damages the island’s store of resilient, long-established biodiversity that, as he sees it, could help the Irish ecosystem through the throes of climate change.