Ireland’s smallest mammal, the protected pygmy shrew, has “completely vanished” in some areas due to the introduction of new species.
The serious threat and possible extinction which foreign species the black vole and the greater white toothed shrew cause to the native wood mouse and pygmy shrew, were the subject of a two year study by researchers at Queen's University Belfast.
If the rate of invasion continues as at present throughout the island its native small mammals will die out in at least 80 per cent of their available habitat, researchers said
There was "local extinction" and "rapid and complete" replacement of the pygmy shrew in areas where both non-native species were present, according to the study Invasional Meltdown which is published today in journal Biological Invasions.
The researchers looked at thesynergistic and combined effect of the two invasive species and showed that the one can compound the effects of another can be very strong even if no impact was expected.
The greater white toothed shrew helped to increase the abundance of the black vole but to decrease the population of wood mouse and pygmy shrew, the study found.
In areas where the black vole was longest established the wood mouse numbers have halved, the researchers found.
The black vole rodent was introduced from Germany some 80 years ago and now occupies about a third of the island to the south-west. The white toothed shrew was introduced about 15 years ago.
The pygmy shrew and the wood mouse arrived on the island some 8,000 years ago. The pygmy shrew is a protected species but the wood mouse is not, the study said.
The gradual replacement of the indigenous species with foreign species could have major consequences due to the central role of small mammals in the ecosystem for birds, mammal predators and seeds, researchers found.
The black vole feeds more on green plant material than the wood mouse while the white-toothed shrew is three times larger than the pygmy shrew.
Lead researcher Professor Ian Montgomery said the introduction of alien mammals over the past 100 years had "major detrimental effects". He urged governments north and south to address the overall problem of invasive mammals.
"It is no longer tenable to treat each invasive species as an isolated case," he said. !We should establish a realistic plan identifying the mammal species that are key to maintaining our unique biodiversity and ecology and those that we should eliminate or control."
Small mammals occupy central positions in food webs, so major changes in species composition which are already occurring, will have both top-down and bottom-up effects in the ecosystem affecting bird and mammal predators as well as the invertebrates, seeds and seedling that small rodents and insectivores feed on.
The study was the first of its kind to systematically analyse the cumulative effects of invasive mammal species on indigenous animals in a process known as "invasional meltdown".
