From southern India to South America, from Guam to the Great Lakes, a quiet but significant threat to ecological health and biodiversity is outrunning all efforts to control it. Invasive species -- foreign organisms taking up residence in ecosystems where they didn't evolve -- are decimating crops and displacing native populations, even loosing new human diseases, and costing the world economy billions of dollars a year.

The invasions haven't gotten wide attention until recently, and attempts to beat back the intruders have not met with great success. Finally, though, governments are starting to take notice and mount new efforts to combat the bioinvasion that has been spread by the surge in global commerce.

It's estimated that since Columbus, some 30,000 species of imported plants, animals, and microbes have taken hold in North America. In some cases, invasive species have been purposely introduced, and in many others they have stowed away in ballast water of ships, in crevices of airplanes, or aboard unsuspecting travelers.

Most of the time the foreign transplant doesn't gain a foothold in the new environment, but sometimes the organism has neither opponents nor predators and thrives in its new habitat. Examples include the zebra mussels that invaded the Great Lakes, fouling shipping and water systems; the pesticide-resistant whitefly in South America, capable of transmitting 60 kinds of plant viruses; and a comb jelly from the Americas that was accidentally released into the Black Sea, causing $350 million damage to fish stocks.

The best weapon against bioinvasion is to prevent it in the first place, which means identifying species that are likely to survive in foreign environments and keeping them from leaving their native areas. When, despite such measures, bioinvaders make their way to new locales, experts say they should immediately be targeted for control. This will be more feasible, they add, with a global database of critical information on invasive species, which should replace today's fragmented bioinvasion literature.

Preventing further bioinvasion may even require strict but unpopular limits on the global economy, say specialists, such as decreeing that certain goods can never be shipped to certain places, like raw Siberian wood to North America. In the spring of 2001, representatives of 180 nations met in Montreal and agreed to 17 guiding principles that call for such actions as border controls, precautions in international trade, and financial assistance to poor countries in their efforts to fight back against invasive species.