This article refers to the biology of Earth, for extraterrestrial biology, see Extraterrestrial life

In biology the adjective alien, as in alien species, means that the normal locale is somewhere else, that a species or specimen exceptionally has been established in the area in question. Words with similar meaning in biology include exotic, adventive, introduced, and naturalized. These terms contrast with terms as indigenous and endemic. Alien species can sometimes wreak havoc on their new ecosystem, since in many cases, the local predators do not hunt them and so they will have no natural enemy. In some cases, people deliberately also introduced the matching predator, with equally disastrous effects. Introducing of alien species must therefore be prevented as much as possible. This can be difficult. Many marine species travel thousands of kilometers in the ballast water of ocean vessels, which are emptied at the destination port, other lifeforms with their cargo. People also often buy tropical animals and then discover that it is difficult to keep them and release them into the wild. In recent years, tropical species have been moving to the north and south of their original habitat which is attributed to global warming.

See also

• Introduced species
• Invasive species