Once exotics take hold in an environment, they can be extremely difficult to eliminate.
Biological invaders are especially a problem in regions where extensive international trade occurs.
Human pathogens hitchhike across borders and oceans, stowing away on research animals, tropical fruits and vegetables, creatures imported as pets, and mosquitoes. Ballast water is another frequent conveyance for exotic pests. When a ship enters port, its ballast water is dumped, spilling out exotic species that may include human pathogens and toxic algae. Biological invasions are exacerbated when exotics spread from freshwater ports into rivers and inland lakes. In the United States, the zebra mussel, introduced to the Great Lakes in 1986, has already spread as far away as Louisiana and Tennessee.