On a global basis, alien species are listed among the greatest threats against biodiversity, but what is an alien species?

Alien species are species that occur outside their ​natural range and dispersal potential. Alien species are spread by human activity, intended or unintended, to new areas.

Alien species do also include taxa below species level such as subspecies, varieties, cultivars and hybrids. The term alien species includes all life stages that might survive and subsequently reproduce, i.e. seeds, eggs, spores or other biological material.

Regionally alien species

The risk assessment covers species that are alien to Norway. In addition, the possibility exists to risk assess regionally alien species.

Regionally alien species are species native to Norway, that have been introduced to novel areas within Norway by humans after year 1800. Occurrences in the species’ natural range (extent of occurrence, past or present) are referred to as regionally native; occurrences outside these species’ natural range and dispersal potential are referred to as regionally alien. For example, the fish species Rutilus rutilus naturally belong in South-Eastern Norway, but have been introduced into fishing waters in Central Norway. Rutilus rutilus is therefore considered a regionally alien species in Central Norway.

General definition of alien species 

The general definition of alien species applied in Norway follows IUCN (2000):

 “Alien species” (non-native, non-indigenous, foreign, exotic) means a species, subspecies, or lower taxon occurring outside of its natural range (past or present) and dispersal potential (i.e. outside the range it occupies naturally or could not occupy without direct or indirect introduction or care by humans) and includes any part, gametes or propagule of such species that might survive and subsequently reproduce.

IUCN (2000). IUCN guidelines for the prevention of biodiversity loss caused by alien invasive species. Gland: IUCN.