Acclimatization, in a biological sense, is a species' ability to take hold in a new environment to which it is not native. Aiding acclimatization was a movement in the 19th century; naturalists often promoted the transplant of useful plants and animals from their native habitats to anywhere else on earth that they might be able to survive. Although non-native organisms had been traveling along with humans for centuries (for example, the horses brought to the Western Hemisphere by the conquistadors), it had been believed earlier that a species taken away from its point of origin would eventually "degenerate" -- that purebred Arabian horses would, over many generations, turn into donkeys in the New World.

Once this belief had been dispelled, and better transportation had made it more common for plants and animals to survive long journeys, European and American (but particularly French) scientists proclaimed that it was a worthwhile goal to take a species and "impress upon its organization those modifications that will enable it to live and perpetuate its species under new conditions of existence." (Isidore Geoffroy Saint-Hilaire, founder of the French Acclimatization Society). Attempts at introducing zebras, silkworms, llamas and bamboo to Europe did not always succeed, but some attempts at acclimatization have succeeded only too well.

Some examples of organisms that did not need any human help after their transportation to a new location are the spread of rabbits in Australia, the accidental release of the gypsy moth in the Northeast U.S. by E. Leopold Trouvelot (who was trying to breed a silkworm for the New World), the interbreeding of African bees with other honeybees to create "killer bees," and the introduction of the Asian plant kudzu into the Southeast U.S. as cattle fodder and soil stabilizer. All of these introduced species are now considered pests in their new location. Sometimes organisms survive better in their new homes than their old ones, because the predators that existed in their native location are not present. This leads to peculiar situations such as that of the German carp crowding out largemouth bass and bluegill in American lakes, while the closely related silver carp is being beaten out in Korean rivers by imported North American bass and bluegill.

Source: Tenner, Edward. Why Things Bite Back: Technology and the Revenge of Unintended Consequences. New York: Alfred A. Knopf, 1996.