I have to admit I had never heard of ‘Invasional Meltdown’ until a few weeks ago. Scientists around the world have found it difficult to show a clear example of it, but it seems that it is happening in Co. Tipperary right now.
‘Invasional meltdown’ is an ecological theory which says that when two invasive species arrive in an area, their effect on each other is positive and their combined effect on native species is negative.
A paper by W. Ian Montgomery, Mathieu G. Lundy and Neil Reid, from Queen’s University Belfast, entitled ‘Invasional meltdown’: evidence for unexpected consequences and cumulative impacts of multispecies invasions, was published in December 2011 in a journal called Biological Invasions. The core area of their study was in west Tipperary and east Limerick.
There are four species involved in this life and death struggle. The Wood Mouse and Pygmy Shrew are native to Ireland and have been here for hundreds if not thousands of years. The Bank Vole was introduced to Ireland, either accidentally or deliberately, about 80 years ago in the Foynes area of Co. Limerick and its range now includes almost all of Munster, south Galway and the south-eastern parts of Leinster.
The Greater white-toothed Shrew was first recorded in Ireland when its bones were found in an owl pellet in west Tipperary in 2007. Its range is still confined to west Tipperary and east Limerick. Given the extent of its range, it is estimated that this shrew was in Ireland for about ten years before it was discovered.
This presented scientists with an ideal situation in which to study ‘Invasional meltdown’. Using standard trapping techniques, they could measure the abundance of each species in three different situations (a) areas where there were only native species (b) areas where the Bank Vole and native species occurs and (c) areas where both the Bank Vole and the Greater white-toothed Shrew along with our native species occur.
They found that where the Bank Vole occurs, the population of Wood Mouse has declined. More dramatic has been the complete extinction of the tiny Pygmy Shrew in areas where the invasive Greater white-toothed shrew occurs. The Greater white-toothed shrew is roughly three times bigger than the Pygmy Shrew. Where the two invasive species occur together, they are doing better than if they were on their own.
All of these studies were carried out on agricultural land with fields and hedgerows. It is possible that the Wood Mouse and the Pygmy Shrew will survive in wooded areas as these may not be attractive to the invasive species. As these small mammals play an important role in the food chain, it will be interesting to see the effect of these changes on the ecology of west Tipperary.