Invasive species, or invasives, are plants and animals that have found their way into areas outside their normal geographic range. In many cases, humans have transported them to their new homes. Invasive species have been called a type of biological pollution (University of Rhode Island, 2001; Mack et al., 2000). Unlike pesticides or sewage, invasive species do not dissipate over time. With no natural enemies in their new habitat, invasive species often grow, reproduce and spread quickly.
Non-native species are often introduced to estuaries in the ballast water of ships. When ships are empty, they take in water to help keep them balanced. When cargo is loaded onto the ships, they release the ballast water. In addition to water, aquatic organisms are sucked into the ships' ballast tanks. When ships take on water in one part of the world and release it in another, aquatic plants and animals are transported along with the water and introduced into foreign estuaries. The San Francisco Bay estuary is probably the most invaded estuary in the world. Over 230 non-native species now live there; so many, in fact, that they now dominate the ecosystem. Over 160 invasive species are now found in the Chesapeake Bay, and their numbers are growing.
Invasives often cause ecological damage and economic losses where they are introduced. Competing with native species for food, or preying upon native species, invasive species have drastically reduced the populations of native species and have, in some cases, caused their extinction. Purple loosestrife, for example, was introduced to the United States from Europe as an ornamental plant in the early 1800s. Today it has invaded estuaries in 48 states, crowding out 44 species of native plants. Controlling purple loosestrife costs about $45 million a year (Pimentel et al., 1999).