Once established, exotics rarely can be eliminated. Most species introductions are the work of humans. Some introductions, such as carp and purple loosestrife, are intentional and do unexpected damage. But many exotic introductions are accidental. The species are carried in on animals, vehicles, ships, commercial goods, produce, and even clothing.
Some exotic introductions are ecologically harmless and some are beneficial.
But other exotic introductions are harmful to recreation and ecosystems. They have even caused the extinction of native species - especially those of confined habitats such as islands and aquatic ecosystems. The recent development of fast ocean freighters has greatly increased the risk of new exotics in the Great Lakes region. Ships take on ballast water in Europe for stability during the ocean crossing. This water is pumped out when the ships pick up their loads in Great Lakes ports.