Analysis: The arrival of such invasive mammals as the Bank Vole and Greater White-toothed Shrew are having a major impact on Ireland's ecosystem
The Field Mouse and Pygmy Shrews play ecologically significant roles. They are numerous (there are millions of them) and they've a short lifespan: all pygmy shrews, for example, die in a year, with the youngsters over wintering. Ecosystems cycle materials for sustaining biodiversity and small mammals, such as mice and shrews, are the initial consumers and the foundations of what makes the ecosystem function.
The first new small mammal was the blunt nosed rodent, the Bank Vole, which arrived in Ireland in the 1920s, possibly amongst the large machines that came from Germany during the Shannon scheme. The Bank Vole is a primary herbivore which eats plants and prefers dense cover. As the ground floor of these ecosystems are plants, the Bank Voles changed native Irish plants. As they spread, they changed Ireland’s plant ecology. Around 2005, the Greater White-toothed Shrews arrived, clearly introduced from Europe. They eat insects, mites and sometimes other smaller shrews.
In a harvested field in Wexford in 2021, tillage farmer Michael Boaden saw and filmed an apparent 'snake' of jumbling tiny brown small animals in a 'conga’ line. He had never seen anything like it, and the caravanning shrew’s quickly vanished.
There were five animals in this case, four young shrews (but almost adults), following in their mother, as she led them away to safety. During this caravanning by shrews they use their teeth to attach to the next shrews’ tail and so on, and off they go, rushing to cover behind mother. There is a resemblance to a wiggling larger beast such as a snake. It is not hard to imagine that this would give the shrews protection against an opportunistic, yet easily scared, predator.
Analysis using the four-inch planting pattern of the crop in the background shows these are the size of Greater White-toothed Shrews not Pygmy Shrews. As we know, these are an invasive species, new to Ireland, and a signal of a change in Irelands’ ecology.
The Greater White-toothed Shrews occur in large numbers and are becoming a critical part of Ireland’s ecology. For example, they are now a main prey for most owls, especially Barn Owls. The additional prey they provide are actually likely responsible for a resurgence in this owls numbers. The insects they eat also perform key functions in ecosystems, such as eating fungi, seeds, fruits and plants, as well as other insects, helping decaying materials and pollination.
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From RTÉ lyric fm's Naturefile, Pygmy shrews are tiny creatures with a very long nose, living among the hedges, woods and boglands where they can find plenty of beetles, spiders, bugs, and woodlice to eat.
It has been reported that Pygmy Shrews and larger beetles vanish where the new Greater White-toothed Shrews occur. It is also clear that the spread of the Bank Vole, and Greater White-toothed Shrew will cause changes in other small mammals. This is ‘all change and utterly changed’ with the arrival of both the new Bank Vole and the Greater White-tooted Shrew as they spread.
Both the Bank Vole and Greater White-tooted Shrew cause an 'invasional meltdown' making the Pygmy Shrew vanish, and native Field Mouse scarce. The Bank Vole and Greater White-toothed shrew help each other, in an almost wholesale replacement of other, critical, small mammals, the very foundations of the ecosystem.
Both Greater White-toothed Shrew and Bank Vole have passed through a breeding bottleneck: very few were introduced, which means they are inbred. It's possible and even likely that they will succumb to a rapid infection and perhaps vanish overnight. Where will that leave their predators like the Barn Owls?
As the base of the ecosystem changes in both Ireland and Britain, food chains, parasites and diseases change
And what about the wriggle-snake effect that prompted the Wexford video? While it may work for a while on an island without snakes, will it go on scaring away predators? This is possibly the first time this has happened.
The Greater White-toothed Shrew has now turned up across the water in the North of England. In summer 2020, Field Voles, a species that prefers open habitats unlike the Bank Vole, turned up in Co Monaghan, brought to attention by a veterinarian’s cat. It is likely that this species and the Bank Vole will eventually occupy the entire island with the Greater White-toothed shrew and produce an unrecognisable ‘Irish’ ecosystem.
As the base of the ecosystem changes in both Ireland and Britain, food chains, parasites and diseases change. These will in turn effect the populations of small mammals, which will in turn affect predators such as owls, cats, stoats, foxes and pine martens. It will also eventually impact insects, plants and energy cycles.
The views expressed here are those of the author and do not represent or reflect the views of RTÉ
