Organisms can be described as being either a native species or an introduced species. A native species is found naturally in an ecosystem. Native plant and animal species have evolved in a specific ecosystem or have arrived naturally, carried by wind, water, or other animals. Native animals may have migrated into an area on their own, possibly for food or reproduction, or to escape a natural catastrophe. The native plant species of the Arctic tundra, for example, include Arctic moss, Arctic willow, and the masque flower. Shrubs and trees grow low to the ground and have other adaptations that help them survive bitter cold winters and short reproductive periods. Animals native to the tundra include Arctic foxes, musk oxen, caribou, snowy owls, lemmings, and Arctic hares.
An introduced species is an organism from one ecosystem that is brought to live in a different ecosystem. Introduced species compete against endemic species (species that live in one geographical area) for food and space. Accidental introduction may occur by a nonnative species stowing away in ships or airplanes and inhabiting the new ecosystem in which it arrives. Zebra mussels entered the Great Lakes ecosystem by adhering to the hulls of cargo ships arriving from other geographical areas. In their native habitat of the Caspian Sea, the zebra mussel population is controlled by predation; however, zebra mussels have no natural predators in the Great Lakes, and their presence has seriously damaged the lakes' ecosystem.An introduced species can become an invasive species if it reproduces rapidly and spreads widely. An invasive species is an organism that does not occur naturally in an ecosystem but thrives and often damages its new ecosystem.Two intentionally introduced invasive species have been nearly impossible to control: kudzu (a type of perennial vine) and cane toads. In the southern United States, kudzu was introduced from Japan to be used as cattle fodder, except that cattle would not eat kudzu. No organism native to the southern United States feeds on kudzu, so over time, the invasive plant has out-competed native grasses, shrubs, vines, and trees in that ecosystem.