‘Invasional meltdown’ a threat to Irish mammals

bank vole

The bank vole was introduced to Ireland 80 years ago and now occupies a third of the island

Some of Ireland’s oldest inhabitants are facing serious threat and possible extinction because of alien invasive species. The red squirrel (Sciurus vulgaris), Irish hare (Lepus timidus hibernicus) and red deer (Cervus elaphus) are just some of Ireland’s indigenous species which are under threat.

A new study which took place over the last two years looked at the impact of two non-native small mammals – the bank vole (Myodes glareolus) and greater white toothed shrew (Crocidura russula) – on two native small mammals: the wood mouse (Apodemus sylvaticus) and the pygmy shrew (Sorex minutus).

greater white toothed shrew

The greater white-toothed shrew arrived in Ireland more recently than the bank vole

Empirical support for the hypothesis of ‘invasional meltdown’, where the presence of one invading species facilitates another and compounds negative impacts on indigenous species, is equivocal with few convincing studies. In Ireland, the bank vole was introduced 80 years ago and now occupies a third of the island. The greater white-toothed shrew arrived more recently within the invasive range of the bank vole.

Species replacement

Our study demonstrated that the negative effects of invasive on indigenous species were strong and cumulative bringing about species replacement. The greater white-toothed shrew, the second invader, had a positive and synergistic effect on the abundance of the bank vole, the first invader, but a negative and compounding effect on the abundance of the wood mouse and occurrence of the pygmy shrew.

If the current rate of invasion continues Ireland’s native small mammals will die out in at least 80 per cent of their available habitat

The gradual replacement of the wood mouse by the bank vole decreased with distance from the point of the bank vole’s introduction whilst no pygmy shrews were captured where both invasive species were present.

wood mouse

Native small mammals such as the wood mouse and pygmy shrew are threatened by the introduction of non-native species

Such interactions may not be unique to invasions but characteristic of all multispecies communities. Small mammals are central in terrestrial food webs and compositional changes to this community in Ireland are likely to reverberate throughout the ecosystem. Vegetation composition and structure, invertebrate communities and the productivity of avian and mammalian predators are likely to be affected. Control of these invasive species may only be effected through landscape and habitat management.

If the rate of invasion continues as at present throughout the island of Ireland, its native small mammals will die out in at least 80 per cent of their available habitat.

Invasive mammals

The introduction of alien mammals to Ireland over the last 100 years has had major detrimental effects, threatening our indigenous habitats and species. The American grey squirrel, for example, passes a deadly squirrelpox virus to native red squirrels, whilst European hares threaten the ecological and genetic integrity of the native Irish hare through competition and interbreeding.

pygmy shrew

Pygmy shrew

Queen’s University Belfast has called on the Governments in both the Republic of Ireland and Northern Ireland to work together to address the overall problem of invasive mammals throughout the island to ensure that the mechanisms of invasion and the impacts of these aliens are monitored and quantified.

A realistic plan is required to identify the mammal species that are key to maintaining the unique biodiversity and ecology of Ireland and those that should be controlled or eliminated.

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