Invasive species

An invasive species is a plant, animal, fungus or microbe that has been moved, on purpose or by accident, outside its native range and then spreads and causes damage. Two conditions define it: the species is not native to the ecosystem in question, and its arrival harms native wildlife, the economy or human health. Being non-native is not enough on its own. Most crops and farm animals were introduced from elsewhere, but they are not considered invasive.

Most introductions fail because too few individuals arrive or conditions do not suit them. The ones that succeed may grow explosively, partly because the predators, parasites and diseases that held them back at home were left behind. They can then outcompete native species, eat them, spread disease or change the habitat itself, and together with habitat loss and overharvesting they rank among the leading threats to biodiversity. People move species in ship ballast water, in timber, firewood and packing crates, through the pet and garden trades, and as escaped farm or aquarium stock.

Well-known examples include kudzu vine in the southeastern United States, zebra mussels in the Great Lakes, cane toads in Australia and lionfish in the western Atlantic. Stopping new introductions is generally far cheaper than controlling a species once it is established.