Alien

Definition

In ecology and biogeography, a species introduced by human activity—intentionally or accidentally—into a region or ecosystem outside its native range, where it establishes and may spread. Alien species become 'invasive' when they cause ecological or economic harm. The term is taxon-neutral but is frequently applied to insects, arachnids, and other arthropods transported via trade, horticulture, or travel.

Etymology

From Latin alius ('other'), via Old French alien and Latin alienus ('belonging to another'); ecological usage emerged in the 20th century alongside 'exotic' and 'non-native'.

Example

The brown marmorated stink bug (Halyomorpha halys) is an alien species in North America and Europe, native to East Asia, that has become a major agricultural pest since its accidental introduction.

Synonyms

• non-native
• Exotic
• introduced
• Non-indigenous

Related Terms

• Invasive species
• native range
• biological invasion
• establishment
• naturalized
• cryptogenic species
• Vector
• propagule pressure

Usage Notes

Specialists distinguish 'alien' (origin outside the region) from 'invasive' (causing harm). 'Alien' is absolute—a species is either native or alien to a given region—whereas 'invasive' is relative and impact-based. Some style guides prefer 'non-native' or 'introduced' to avoid science-fiction connotations. Contrast with 'indigenous', 'endemic', 'autochthonous'.