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Ireland’s Pygmy Shrew, one of the world’s smallest mammals, under threat from white-toothed invader

8th July 2014

A new study published in the scientific journal PLOS ONE has highlighted the significant impacts of an invading species of shrew on the Pygmy Shrew, which is leading to the displacement and disappearance of the latter species.  The Pygmy Shrew is one of the world’s smallest mammals and has been the only shrew species present on this island for thousands of years.  However in 2008 the Greater White-toothed Shrew was discovered in the pellets of Barn Owls through research by BirdWatch Ireland, UCC and Queens University Belfast.  A recent study led by UCD has now shown that the Greater White-toothed Shrew is spreading across the Irish landscape at a rate of more than five kilometres a year and is capable of colonizing the entire island by 2050, something which may have serious consequences for the Pygmy Shrew population.

Pygmy Shrew, Ireland's only native shrew species
Pygmy Shrew, Ireland's only native shrew species

BirdWatch Ireland collaborated on this study, an integral and innovative component of which used analysis of bird of prey pellets to determine the range and expansion rate of the Greater White-toothed Shrew.

"The invading population of the Greater White-toothed Shrew currently covers an area of 7,600 km² and is found in Counties Tipperary, Limerick, Cork, Waterford, Kilkenny, Offaly and Laois", said Dr. Allan McDevitt, UCD School of Biology and Environmental Science, University College Dublin.  "Small satellite populations have also been found in Cork city and more recently in Mullingar, but according to our data they have not yet crossed the Shannon", he added.

Greater White-toothed Shrew: a recently-arrived invasive species that is pushing the native Pygmy Shrew out of former haunts (Photo: John Murphy)
Greater White-toothed Shrew: a recently-arrived invasive species that is
pushing the native Pygmy Shrew out of former haunts
(Photo: John Murphy)

John Lusby, Raptor Conservation Officer with BirdWatch Ireland, commented, "In recent years we have recorded the Greater White-toothed Shrew as the dominant species in the diet of certain predatory birds, such as the Barn Owl.  We also noticed an unusual trend where Pygmy Shrews were completely absent within the diet of raptors in areas where the new shrew was dominant.  The live trapping studies confirm that the Pygmy Shrew is indeed now no longer present in areas where the Greater White-toothed Shrew has successfully colonised and become established."

Prey cache found in a Barn Owl nest in Co. Tipperary: (L - R) 4 Greater White-toothed Shrews, 1 Bank Vole and 2 Wood Mice (Photo: John Lusby)
Prey cache found in a Barn Owl nest in Co. Tipperary:
(L - R) 4 Greater White-toothed Shrews, 1 Bank Vole and 2 Wood Mice
(Photo: John Lusby)

"The sheer speed of the invasion of the Greater White-toothed Shrew and its competitive superiority in eating large insect prey could have severe negative impacts on the Irish population of Pygmy Shrews, and even lead to its local extinction", added Dr. McDevitt.

Barn Owls in a nest cavity inside a building (Photo: John Lusby)
Barn Owls in a nest cavity inside a building
(Photo: John Lusby)

BirdWatch Ireland is also trying to determine the possible implications that this dramatic shift within the small mammal communities may have on sensitive raptor populations, and to this extent has been particularly focusing on studying the effects on Barn Owls.  John Lusby noted, "Through extensive diet work we know that Barn Owls feed almost exclusively on Greater White-toothed Shrews in certain parts of their range, and this species comprises over 85% of the owls' prey intake at some nest sites.  With the continuing expansion of the Greater White-toothed Shrew it is essential to determine how the Barn Owl population will respond.  We have recorded varying breeding success rates within the Greater White-toothed Shrew range in recent years, and although we have observed high mortality rates of young birds in certain years, due to the low density of Barn Owl nest sites, we will need to continue further monitoring before we have a conclusive understanding of the impacts of the Greater White-toothed Shrew's arrival for the Barn Owls and other avian predators."

Barn Owl chick from a monitored nest site: how the recent arrival of the Greater White-toothed Shrew will impact this species in the long term remains unknown (Photo: John Lusby)
Barn Owl chick from a monitored nest site: how the recent arrival of the
Greater White-toothed Shrew will impact this species in the long term remains unknown
(Photo: John Lusby)

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