Are wild horses in North America native or invaders? Scientists argued
Horses evolved in North America, spread into Eurasia, then died out in their homeland. When Spanish conquistadors brought them back, they returned to the continent of their ancestors, and in 1943 researchers debated whether feral herds counted as native or exotic. The episode shows how slippery the word 'invasive' can be.
An invasive species is an introduced one that damages its new surroundings, ecologically, economically or both. Organisms have always colonised new ground, but people have multiplied the speed and reach, first through early migrations, then during the Age of Discovery, and again with global trade. Well-known offenders include the kudzu vine, water hyacinth, European rabbits, domestic cats and red swamp crayfish.
The definitions are contested. Some use the label only for harmful non-natives; others stretch it to native species spreading into farms or wild areas, and a few have even applied it to humans, though our adaptability arguably defies a fixed category. The US government, under Executive Order 13112, uses a narrow version: an alien species whose arrival causes or is likely to cause economic, environmental or health harm. To cut through loaded vocabulary, Colautti and MacIsaac proposed judging individual populations by their success in a place, treating natives and newcomers alike and not assuming every successful arrival is harmful.
Modern study owes much to Charles Elton's 1958 book on invasions by animals and plants, though research stayed sparse until the 1990s and has focused heavily on land plants. The field, usually called invasion ecology or invasion biology, still lacks standard terms because it borrows from agriculture, zoology and pathology.
Invasion usually starts slowly. Newcomers often linger at low numbers, struggling to breed, and may need several introductions before they take hold. Repeated traffic, such as ships moving between ports or vehicles on major highways, supplies a steady stream of arrivals, a pattern called high propagule pressure; shifting excavated soil can do the same. Once established, a fast-spreading species can monopolise limited nitrogen and phosphorus after disturbances like fire, outpacing native grasses. Cleared land creates edge habitats where unfamiliar species can gain a foothold.
Source: Invasive species
