Expansion of global trade, and increases in human mobility have resulted in unprecedented invasion by nonnative species. These invasive species can produce severe, often irreversible impacts on agriculture, recreation, and our natural resources. Invasive species threaten biodiversity, habitat quality, and ecosystem function. They are the second-most important threat to native species, behind habitat destruction, having contributed to the decline of 42% of U.S. endangered and threatened species. Introduced species also present an ever-increasing threat to food and fiber production. In the United States, the economic costs of non-native species invasions reach billions of dollars each year.
Whether they are called invasive, non-native, alien, exotic, or non-indigenous, introduced species are those that evolved elsewhere and have been purposely or accidentally relocated . While some species have invaded habitats on their own (e.g., migrating wildlife, plants and animals rafting on floating debris), human exploration and colonization have dramatically increased the diversity and scale of invasions by exotic species. Introduced species often find no natural enemies in their new habitat and therefore spread easily and quickly. Invasive species is a problem on land and in the oceans, in deserts, islands, forests, rivers, lakes, farms, almost everywhere.
When ships unload their cargo, they often fill their ballast tanks with water to provide balance for their return journey. In addition to water, many aquatic organisms are sucked into these tanks and given transport. A ship will then empty its ballast tank (and various aquatic stowaways) at the next port it takes on cargo. Many invasive species have become introduced into new areas this way. A relatively simple control mechanism is to exchange ballast water on the high seas between ports to remove invasive species before they reach the destination port. Other methods being explored are using filters to trap organisms as the tanks are filled or heating the water to kill them.
Invasive species may be associated with harmful algal blooms. Many species of phytoplankton are also transported around the world in ships’ ballast water and discharged in areas where they did not previously occur. Others are distributed accidentally through the transfer of shellfish for aquaculture. The rapid changes and deterioration of many coastal environments are being accompanied by a series of harmful-and often unexpected-events involving marine phytoplankton. The recent emergence of Pfiesteria, the tiny “ambush predator”; this species has caused massive fish kills and serious human illness in North Carolina, and has recently become a problem in Maryland.
The yellow crazy ant, responsible for the Christmas Island frigatebirds and Abbot’s boobies, is a diminutive insect (4-5 mm long) that has been introduced across the tropics as a by-product of international trade. It is highly predatory and has been used as a biological control agent. In the process, it invades urban, agricultural and native natural ecosystems with catastrophic effect. The ant has been nominated as one of the 100 of the “world’s worst” invaders by the IUCN Species Survival Commission Invasive Species Specialist Group.
The brown tree snake (Boiga irregularis), native to eastern Indonesia, the Solomon Islands, New Guinea, and the northern and eastern coasts of Australia has caused ecological, economic, and human health problems on Guam since it arrived shortly after World War II. It is an aggressive predator and has caused local extinctions of native bird, bat, and lizard species.
The zebra mussel is a voracious filter feeder which removes suspended particles from the water, changing the physical characteristics of the invaded habitat and causing native clams to starve to death. Colonies of zebra mussels attach to boats, pipes, and the shells of other mollusks.
