Ireland has been subject to invasion by a number of alien species down through the years. Some, such as the beech tree, date from prehistoric times. However other, more recent invaders are having a detrimental effect on the country's native species. Montbretia (Crocosmia x crocosmiiflora) is displacing species like Irish Spurge (Euphorbia hyberna) in hedgebanks. Zebra mussels (Dreissena polymorpha) not only clog waterways, but also decimate microalgae populations. Rhododendron ponticum threatens the biodiversity of native oak woodlands of Glenveagh, Glengarriff and Killarney by a combination of shading over young trees and shrubs and exudation of toxins into the soil. Indeed, according to Invasive Species Ireland, there are 250 hectares infested with R. ponticum in Killarney National Park alone.
Another unwelcome intruder is the Australian Flatworm, Australoplana sanguinea alba. This was first reported in Belfast and Dublin in 1983 and in the interim has spread to the entire island. A. s. alba feeds on earthworms, however not to the same extent as the ravenous New Zealand Flatworm, Arthurdendyus triangulatus. Its impact on native species has not yet been fully understood. Research carried out at the University of Manchester by Giulio Santoro and Hugh Jones suggests that A. s. alba, while not having a significant effect on overall earthworm population, selectively fed on anaeic earthworms. These are species that burrow deep in the soil and their decrease in an area leads to a reduction in soil drainage. This is especially serious in heavy soils that are prone to water-logging. Since much of the grazing land in Ireland is of this type, the Australian Flatworm may pose serious problems not only to the island's biodiversity, but also to its agriculture in the future.