a species introduced either accidentally or deliberately by human actions into places beyond its natural geographical range. Familiar examples include the house sparrow, domestic pigeon, and starling, which were all deliberately introduced into North America and other places from their native ranges in Europe. rats of several species were unintended stowaways on oceanic voyages, probably those of Columbus and certainly those of the Polynesians as they colonized Pacific islands. Infected humans took the human immunodeficiency virus (HIV), which causes AIDS, from Africa to the rest of the world. In much the same way, early European colonists of the New World and the Pacific took other sexually transmitted microbes (see sexually transmitted disease), common cold viruses, smallpox viruses, and a variety of other disease-causing organisms to populations with no resistance to them. For some species, the introduction was deliberate but the spread of the species and its consequences were unexpected. For example, kudzu (kudzu vine), a vine introduced from Japan to the southeastern United States to stabilize steep banks of soil created by road construction, has spread up and over large parts of the native forest, choking it.

Introduced species sometimes cause massive economic harm. For example, introduced cacti (cactus) in many parts of the world reduce productive rangeland to impenetrable, thorny bushland. In addition, by crowding out a region's competitively inferior indigenous species, introduced species are a major factor in the extinction of species and consequent loss of biological diversity (see biodiversity).