Allan McDevitt (Twitter: @ShrewGod)
Allan is a lecturer in Ecology and Conservation at the University of Salford. He specializes in the application of molecular techniques for monitoring and understanding ecological and evolutionary processes in multiple mammalian species. But he prefers shrews!
Samuel Browett (Twitter: @ShrewlockHolmes)
Sam recently completed his PhD in the University of Salford. His research focused on understanding dietary and microbial interactions between greater white-toothed and pygmy shrews in Ireland. Sam is currently a Biology Technician in Waterford Institute of Technology in Ireland.
Those of us who reside in Britain and Ireland are at least somewhat familiar with a few high-profile cases of invasive mammals and their impacts on native fauna. These include the North American grey squirrel displacing the red squirrel, or the American mink predating on water voles. These are examples where the impact of an invasive species can be more readily observed through declining abundance of an existing species or noticeable changes to the ecosystem at large. Not all cases are as high profile or obvious however. Away from prying eyes in the Irish countryside, a recently introduced shrew is causing a significant disturbance in the undergrowth.
The greater white-toothed shrew (Crocidura russula) was discovered in Ireland from pellets of barn owls and kestrels collected in 2007. The initial indications were that the species was confined to the southern part of the island. At the time of its discovery it was perhaps reasonably assumed that the species could be a beneficial addition to the Irish fauna, and provide another food source for birds of prey and mammalian predators. However, in an early warning sign during the winter of 2010/2011, it was found that native pygmy shrews (Sorex minutus) were absent in a number of sites where the greater white-toothed shrew was present. As previously Ireland’s only shrew species and the island being one of the strongholds for the species in Europe, the potential disappearance of the pygmy shrew was seen as a worrying development. A much larger survey was undertaken in the summers of 2012 and 2013, and confirmed that the pygmy was indeed being seemingly displaced in areas where the greater white-toothed shrew was established. Recent surveys using live-trapping, presence in bird of prey pellets and public sightings have revealed that the invasive shrew is now present over a large proportion of the island and appears to be spreading rapidly. This disjointed distribution seems to be additionally facilitated by accidental transport with people also and is perhaps to be expected given this shrew’s tendency to live in close proximity to humans (it is often commonly referred to as the ‘house shrew’ in France for example).
This rapid spread is obviously a significant worry for the pygmy shrew in Ireland. What is unclear at present is exactly how the greater white-toothed shrew is causing the displacement of the pygmy shrew in Ireland. Competition from overlap in their diet could be a leading factor. Both species are insectivores and in the presence of competing shrew species, the pygmy shrew normally preys on smaller insects in other parts of its European range. However, the pygmy shrew has been Ireland’s sole shrew species for several thousand years at least, so it may have evolved a dependency on a broader range of prey. With the arrival of the much larger greater white-toothed shrew may impact this available resource. We are currently investigating the dietary overlap between the species in Ireland and France using molecular techniques to tease apart their specific dietary niches. This will give us insights into changes and overlap occuring in real time as the greater white-toothed shrew continues to invade.
On current evidence, the local extinction of the pygmy shrew may continue in Ireland as the greater white-toothed shrew carries on spreading rapidly. Given that eradication of the greater white-toothed shrew is unfeasible at this point because of the large area that it occupies, this may mean that Ireland’s small offshore islands (of which pygmy shrews inhabits many) will become an important long-term refuge.
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