Invasive Species Invasive, alien species, those which colonise, expand and out compete native species (Smith & Smith, 2009), are a major threat to our habitats, terrestrial and aquatic species, and biodiversity. Agricultural and leisure industries are affected as well as conservation welfare and the continued wellbeing of man, flora and fauna. Whole ecosystems can be distorted and the economic cost of awareness, prevention and eradication systems is substantial. Most invasive species have been introduced by mandeliberately or otherwise. As an island, Ireland has been subjected to less invasive species than larger nearby land masses e.g. Europe, but islands have a less diverse species population and are therefore more vulnerable when invaded. As international movements increase, risks spread. Some non-native species are not obviously detrimental on introduction, but become so in the future and intensive monitoring is required. Negative invasive species can compete and overcome native species in 6 ways. 1. Competition: Food sources and habitat reduced for native species. 2. Herbivory: Non-native herbivores trample or overgraze vegetation. They may not kill the species they consume, but any removal of vegetation tissue affects its survival capacity (Smith & Smith, 2009). 3. Predation: Killing of weaker native species, possibly leading to extinction. 4. Habitat alteration: Removal and reduction of natural habitat by flooding, soil erosion and water