Species whose introduction and/or spread by human action outside their natural distribution threatens biological diversity, food security, and human health and well-being. âAlienâ refers to the speciesâ having been introduced outside its natural distribution (âexoticâ, ânon-nativeâ and ânon-indigenousâ are synonyms for âalienâ). âInvasiveâ means âtending to expand into and modify ecosystems to which it has been introducedâ. Thus, a species may be alien without being invasive, or, in the case of a species native to a region, it may increase and become invasive, without actually being an alien species.