Container traffic. The use of containers, the huge metal boxes that are stacked up on ships and off-loaded directly on to trains or trucks, has provided a "quantum leap" in the efficiency of transportation, both for trade goods and for exotic animals and plants. Previously, seaports were the route of entry for many exotics, but with container transport the biological invaders are picked up and delivered directly to inland destinations all over the world. Containers provide a sheltered environment; they sit for weeks waiting to be loaded or unloaded, giving stowaways plenty of time to embark or disembark; and they are difficult for customs inspectors to search thoroughly. Container shipments of used tires from Japan brought the Asian Tiger mosquito to the U.S., South Africa, New Zealand, Australia and Southern Europe.
Ballast water. Many cargo ships are stabilized by pumping seawater or fresh water into huge ballast tanks. They then transport the water, containing an entire animal and plant community, to their destination where the ballast may be pumped out. This is obviously a major source of aquatic exotics. About one third of the exotic species in the Great Lakes have probably been introduced this way. A recent study of a bay in Oregon showed 367 types of organisms released from ballast water of ships arriving from Japan over a four-hour period. In 1990 President Bush signed legislation requiring the U.S. Coastguard to develop tougher standards to deal with dumping of ballast water.
Air traffic. Airplanes
provide another efficient new mode of exotic travel. Mosquitos have survived
flights from Africa to Britain in passenger cabins, and snakes have traveled in cargo bays from Guam
crops have escaped from their plantations and become pests. Olive trees
parts of Australia, Avocado trees on Santa Cruz island in the Galapagos group, and many other
examples have been reported. Agricultural practices have caused the spread of many pest species
and pathogens. Over 20 weeds are found nearly everywhere, as are about 40% of the world's major
crop pathogens. Rats and sparrows are associated with farms everywhere.
Forestry. Trees have escaped from tree farms and become exotic pests. Monterey Pine, native to the western U.S., has invaded areas of South africa, Australia and New Zealand. Many forest pests have been spread with timber shipments.
Aquaculture has caused the spread of numerous species of fish, especially Mozambique Tilapia which is now established in nearly every tropical and subtropical country. Shrimp farms are spreading shrimp viruses around the world, and the shrimp fishing industry is concerned that the viruses will eventually infect wild populations.