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The effects of alien invasive species on biodiversity have been described as “immense, insidious and usually irreversible” (IUCN 2000). There is no doubt that invasive species can cause severe economic and ecological damage (Mack et al. 2000). They may soon surpass habitat loss as the main cause of ecological disintegration globally (Vitousek et al. 1997, Chapin et al. 2000) and are probably already the main cause of extinctions in island ecosystems. The breaching of biogeographic boundaries by the widespread, recent human transport of species has caused rapid and radical change in biological communities, including multiple extinctions. To minimise further extinctions and other ecological changes, the most important priority is to reduce the risks of new invasions. After prevention, the next priority is to eradicate existing invasive species, where this is possible. These aims are embodied in the United Nations Convention on Biological Diversity, which states that parties to this convention should “ prevent the introduction of, control or eradicate, those alien species which threaten ecosystems, habitats or species”.