The introduction of alien species to ecosystems where they have not previously existed is a growing threat to the biological diversity. Through increasing world trade, with short transport times, increased travel and the use of new environments, alien species have to a larger extent, intentionally or unintentionally, been introduced to the ecosystem. Alien species may through competition, predation, parasitism, genetic pollution or through other direct influences drastically change prerequisites for life for indigenous species and/or affect ecosystem functions

When we refer to alien species, we also include populations, provenances or genes, which have not previously existed in an area, but have been moved out of their natural area of distribution with the intentional or unintentional help of humans. Alien species often originate from foreign countries, but also include indigenous species which have been moved to a new biogeographical region within the same country and as such are alien to their new environment.