Tangled thorny masses of Himalayan blackberries in Bellingham, Washington, vacant lots. Dense thickets of Scotch broom inducing sneezes along miles of Interstate 5 in western Oregon. Invading mussels from the Black Sea displacing native ones in Montana streams. Atlantic salmon in the Pacific and Pacific salmon in the Great Lakes. In an age of hypermobility, species are moving around the planet in directions and at rates unprecedented in global history.

While organisms have evolved ingenious mechanisms to disperse on their own, human activities are enhancing that natural ability, creating large-scale introductions of non-native species in more and more areas. Some introductions, like the succulent South African freeway plant introduced around Los Angeles for erosion control, are deliberately planned by humans. Others are quite inadvertent and capable of doing tremendous damage, such as the zebra mussel (Dreissena polymorpha), which has caused several hundred million dollars in economic and ecological damage in the Great Lakes and is spreading west at an alarming rate.

Non-indigenous species (also known as exotic species, alien species, introduced species, or non-native species) are defined as species that have been transported long distances (almost always because of the activities of humans) from the place where they evolved. A subset of non-indigenous species, known as invasive species, are especially good at establishing viable, reproducing populations in their new homes. These invasive species, by interacting within native ecosystems, can create genuine ecological havoc in the absence of their own predators, parasites, and pathogens back home.

How do species actually get moved around by people? The answer is as variable as human enterprise itself, ranging from hitchhiking on vehicles or in shipments of crops to arriving as parasites on imported pets or on ornamental plants. Ballast water has proven to be an especially efficient mechanism for inadvertently moving critters en masse to far-off locales and new habitats. What is ballast water? It's water taken into holding tanks by large ships, like tankers and freighters, to keep them stable on the high seas. It typically is slurped up in coastal waters where the ship starts its journey, and released in the coastal waters near its destination. Ballast water tanks can contain up to 70 metric tons of water each! Furthermore, up to 30,000 ships are cruising the world's waters on any given day, and the size, speed, number, and routes these ships follow are increasing every day. Every gallon of water in every ship's ballast tanks on every journey has the potential to transport the adults or larvae or eggs of non-indigenous marine or freshwater organisms. The hardy invaders among these organisms are creating substantial new problems in ports, bays, and estuaries along the West Coast of the US.

Zebra mussels are the most famous aquatic non-indigenous species, but many others are currently having dramatic effects on West Coast economies and ecosystems. For example, the vast majority of the benthic (bottom-dwelling) organisms in San Francisco Bay are now from other places, having displaced many native populations and even entire species. One species of Chinese clam there, Potamocorbula amurensis, is such an efficient filter feeder that regular blooms of phytoplankton have completely disappeared in northern parts of the delta. Phytoplankton blooms, of course, generate much of the primary productivity upon which the rest of the San Francisco Bay community depends; in this way a single exotic species may be capable of altering the entire community structure of a region. Even the local congressional delegation has noticed.

Other non-indigenous organisms may be pathogens (disease-causing organisms) to other creatures; a good example of this is whirling disease, a European protozoan parasite affecting the nervous systems of trout species in at least 11 Western states. Indeed, the Madison River in Montana has seen a 90% decline in rainbow trout populations since the disease arrived, probably with introduced fish stocks. Sensitive West Coast estuaries are being invaded at unprecedented rates by the exotic cordgrass Spartina alterniflora; altered substrates and hybridization with native cordgrasses is threatening these most productive of coastal ecological communities from Padilla and Willapa Bays in Washington State all the way down to California.

Finally, there are plenty of terrestrial invaders as well. The Asian Longhorned Beetle, an invasive species from Asia that destroys hardwood, has been causing quite a fuss in several ports across the United States recently. The main concern is that the beetle may arrive on cargo ships arriving from international ports; entomologists across the nation have been spreading the alert lest this invasive menace establish because it poses such a potentially large economic threat.

• Chapter 2 (Ecosystems: What They Are), the section "Global Biomes," pages 42-46, describes some of the aquatic habitats (estuaries, lakes and rivers, and intertidal zones) most vulnerable to ballast water introductions. An overview of general ecology in terms of species interactions, the means by which most exotic species either directly or indirectly do damage, appears in the same chapter, in the section "The Structure of Ecosystems," pages 29-42.
• Chapter 11 (Wild Species: Biodiversity and Protection), the section "Biodiversity," pages 275-284, discusses the loss of biodiversity and the general problem of exotic species, including a specific focus on the brown tree snake in Guam (page 279).
• Finally, Chapter 17 (Pests and Pest Control), the section "Alternative Pest Control Methods," pages 424-431, explains how natural enemies of pest organisms may be used to control populations of pest or nuisance organisms. It also discusses some potential problems associated with using biological control (usually the introduction of a non-indigenous species to control another non-indigenous species) in aquatic ecosystems.

Sea Grant Nonindigenous Species (SGNIS)

This address is for the Sea Grant Nonindigenous Species (SGNIS) website. Supported by this federally funded (but run by state agencies) program, there is a wealth of information on zebra mussels and other aquatic non-native species.

National Ballast Water Information Clearinghouse

This site provides extensive text and a number of links related to ballast-mediated species invasions.

National Academy Press

This site, part of the National Academy Press online reading room, has the text of the comprehensive National Research Council report by some of the nation's top marine scientists. This report is called "STEMMING THE TIDE: Controlling Introductions of Nonindigenous Species by Ships' Ballast Water."

Species Roulette

This fun but scary site has a game of biological roulette to explore how quite a few species have been introduced and what happened next.

Nonindigenous Species in the San Francisco Bay and Delta

This site contains a press release from California Congresswoman Ellen O. Tauscher calling for a hearing on nonindigenous species in the San Francisco Bay and Delta. The congresswoman displays a good grasp of the biology of non-native species in the Bay area and the problems they are causing there.

Translocation from World Resources Institute

This site contains guidelines from the World Resources Institute, a non-governmental organization, on translocations of living organisms. It includes sections on how nonindigenous species impact native species, gene pools, and societies.

Alaska Ballast Hitchhiking

This site contains an article from Alaska Sea Grant on ballast hitchhiking organisms in that state.

Effects of the European Green Crab

This site, provided by the Sustainable Ecosystems Institute, describes the impacts of European green crab invasions along the Oregon and California coast.

USDA Invasive Species Information Center

This terrific website, maintained by the National Agricultural Library of the US Department of Agriculture, contains information about invasive species, their impacts, federal efforts to address them, along with links to other agencies that deal with invasive species.

Asian Longhorned Beetle

This website, maintained by the USDA, contains a wealth of information on the Asian longhorned beetle, including identification and instructions pertaining to reporting sightings.

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