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Exotic Invaders
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10340 |
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Section : |
NATURAL SCIENCE
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| Issue
Date : |
12 / 1993 |
2,200 Words |
| Author
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James T. Carlton
James T. Carlton is the director of the maritime studies program of Williams College and Mystic Seaport in Mystic, Connecticut. |
In the short time that you take to read this article, a million aquatic animals and plants from China will be released in San Francisco Bay. A million more marine organisms from Europe will be released in the Chesapeake Bay. And a million more from Japan into Sydney, Australia. And a million more from Sydney into the Hawaiian Islands.
Carried in vast, silent, and invisible conveyor belts around the world--the ballast tanks and ballasted cargo holds of oceangoing ships--these releases of introduced organisms have grown considerably in the past 10 years to more and more successful invasions, and thanks to a few spectacular successes, have led to a closer examination of what the world's maritime communities can do to reduce this problem.
Introduced species are plants and animals that have been transported to new regions by human activity--unintentionally such as in ships' ballast water, or intentionally such as when fish or shellfish are released, to start new populations. Fish biologists frequently refer to introduced species as exotic species; botanists call introductions alien species; ecologists call them biological invasions, and the U.S. government uses the term nonindigenous species. All mean about the same thing--the novel appearance of an organism whose newly discovered presence is believed to be due to other than natural means of dispersal.
The growing threat
Unintentional invasions as a result of ships releasing ballast water began about 100 years ago, shortly after water replaced rocks, stones, and sand as the preferred medium for providing a ship with extra weight for stability and other purposes. In the early and mid-1980s, however, for reasons that remain unclear, the release of ballast around the world took on a startling new profile as invasions linked to ballast release began to appear with greater frequency. The fresh and marine waters of the world suddenly began to itch with invasions:
In Tasmania and southern Australia, Japanese species of red-tide-causing microorganisms known as dinoflagellates appeared along the coast, causing the shutdown of both natural shellfish harvests and aquaculture facilities. Clams and mussels ingest these planktonic organisms while feeding and become poisonous to humans.
In San Francisco Bay, a Chinese species of small brackish-water clam suddenly appeared and soon was measured in densities of
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