Species whose introduction and/or spread by human action outside their natural distribution threatens biological diversity, food security, and human health and well-being. Alien refers to the species' having been introduced outside its natural distribution (exotic, non-native and non-indigenous are synonyms for alien). Invasive means tending to expand into and modify ecosystems to which it has been introduced. 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.