Maine environmental groups voiced concern last week that the Bath Iron Works’ new dry dock, recently arrived from China, might harbor species of snails and algae that could invade Maine’s waters. To do so, they would have had to survive a nearly 16,000-mile trip through waters with salinities and temperatures quite different from those of their native habitat.

However, past experience with exotic species, invasive plants and animals from other lands that often overwhelm and displace native species, show that the threat is a very real one. As Martin Enserink writes in the Sept. 17, 1999, issue of Science, “As the world shrinks and travel and trade boom, plant and animal species have become globetrotters.” The result may well be increasing instances of environmental and economic disaster.

Perhaps there is no better example of the successful transplanting of a foreign species than that of the common English sparrow. Eight pairs were brought to Brooklyn, N.Y., in the fall of 1850 and released the following spring. Today they inhabit almost the entirety of North America.

The danger posed by exotic species was brought home in 1986 when the larvae of zebra mussels were pumped, along with ballast water, from a freighter into Lake St. Clair near Detroit. Since then, according to a report from the Maryland Sea Grant Program, they have spread throughout the Great Lakes, where the cost of scraping them from the intake and outflow pipes of power plants and water treatment facilities is estimated to be between $50 million and $100 million annually. Closer to Maine, a Sept. 24, 1997, report from the Environmental News Network says that they have invaded Lake Champlain on the New York-Vermont border.

Vermont is also plagued with another aquatic invader known as Eurasian water milfoil where, according to Aaron Miller writing online for MaineToday.com, it infests 50 out of 285 lakes in the Granite State. Milfoil likely came to the United States as an aquarium plant and began to spread when people emptied their aquarium tanks into lakes. It was first spotted in the Great Lakes in the 1970s and has since spread to lakes throughout North America hitching rides on the bottoms of pleasure boats. Maine is reportedly the only state in which milfoil has not been found.

Maine is considering stringent laws requiring boat owners to make sure the bottoms of their boats are free of any plant fragments before entering the water, but there will be almost no way to enforce any regulations. And milfoil will likely end up in Maine waters even if every boater complied with the rules as it takes only a tiny piece to grow a new plant.

Don Schmitz and Daniel Simberloff, writing in the Summer 1997 issues of Science and Technology, warn that global trading is rapidly accelerating the introduction of exotic plant and animal species into the United States. They write that, of 47 harmful species introduced into the country between 1980 and 1993, a total of 38 were directly related to trade. Once established, they have contributed to the decline of 42 endangered native species and the likely extinction of three others. The authors say that nearly 25 percent of the nation’s agricultural gross national product is lost to foreign pest invaders and the costs of controlling them.

Just how quickly an invader can displace a native species is illustrated by Enserink in his article. In just one year, all of the major native species in Utah’s Canyonlands National Park were displaced by Eurasian cheatgrass, changing the area’s ecosystem forever. Almost 121/2 million acres in Idaho and Utah are now nearly overgrown with cheatgrass, say Peter Vitousek et al in the September-October 1996 issue of American Scientist. While New England does not face the same threat, 877 out of 2,872 identified species, or more than 30 percent, are non-native exotic species, say the same authors.

Schmitz and Simberloff say the Department of Agriculture spends more than $100 million annually for agricultural quarantine and port inspection and Enserink estimates that about 3,000 potential pests are stopped at the border each year. The effort is much like keeping back the tide with a leaky bucket and the end result a seeming foregone conclusion.

Enserink writes that another century of global trade will lead to “ecological homogenization” of the planet with a few highly successful species, such as the zebra mussel, cheatgrass and English sparrow, dominating ecosystems everywhere, a condition he dubs a “global McEcosystem.”

Clair Wood taught chemistry and physics for more than 10 years at Eastern Maine Technical College.