Introduced species have a major impact on biodiversity.
When non native species become invasive they can transform ecosystems, and threaten native and endangered species.
The impacts of invasive species on Irish biodiversity are widely demonstrated by competition (e.g. grey and red squirrels), herbivory (sika deer) predation (Gammarus pulex on freshwater invertebrates), alteration of habitat (Spartina anglica), introduction of parasites (eel swimbladder nematode) and pathogens (squirrel poxvirus) or dilution of native gene pools (Spanish bluebell).
Alien species that become invasive are one of the main drivers of biodiversity loss across the globe. In addition, alien species have been estimated to cost economies across the world hundreds of billions of dollars each year. This has been recognised in international agreements such as the Convention on Biological Diversity and European and national legislation.