That makes invasive species a major threat to the diversity of life on earth. Nearly 20% of the world's endangered vertebrate species are threatened by exotics, and almost half of all species in danger of extinction in the United States are imperiled at least in part by non-native species.

The culprits arrive by sea

Ballast water from international shipping is a major culprit in the spread of aquatic species. On any given day, some three thousand to ten thousand species are moving around the world in ship ballasts. When the ballast water is discharged, so are the organisms, after which they often cause incalculable damage. For example, a ballast water-induced invasion of the Black Sea by the Atlantic jellyfish in the early 1980s was instrumental in the collapse of the fisheries there by the end of that decade.

Did you know that on any given day, as many as 10,000 different species are moving around the world in ship ballasts?

The U.S. Great Lakes have also been hard hit by bioinvasions over the last several decades. A recent villain is the zebra mussel, which probably originated in the Caspian Sea and was likely first released into the Great Lakes from a ship's ballast water tank in the mid-1980s. Zebra mussels have now spread widely throughout the lakes and other waterways of eastern North America, where they have wreaked havoc with delicate ecological systems by ingesting large quantities of algae — a fundamental component of aquatic food webs.

Zebra mussels also multiply rapidly, clogging water intake pipes and encrusting aquatic infrastructure and boats. The associated economic losses for this problem alone are enormous — with losses expected to add up to at least $3.1 billion within the next few years.

Bugs in water and on land

Terrestrial ecosystems are no less at risk. The damage wrought by the pesticide-resistant whitefly is a warning of the high stakes involved. The whitefly caused tens of billions of dollars of agricultural damage in California in the early 1990s. It then moved on to South America, where it has helped spread crop viruses that led to the abandonment of more than one million hectares of cropland.

The rapid growth of global transportation has created convenient ways for thousands of exotic species to travel the globe.

In the United States, the aggressive purple loosestrife plant has become a widely known symbol of the broader threat. It is thought to have first been accidentally introduced into North America in the late 18th century in wool imports and solid ship ballast.

Later, it was more deliberately imported for ornamental and likely for medicinal purposes during the 19th century. It has already taken over more than 600,000 hectares of temperate and boreal wetland, crowding out native vegetation that is used by wildlife for food and shelter.

A call to arms

The bioinvasion problem cries out for an international response. Among the steps that could be taken are inspections, limits on ballast water discharges and the adoption of a precautionary approach that prohibits the knowing introduction of exotic species — unless they have been shown to be benign.

At present, some 23 different international treaties make at least some mention of exotic species. These include the 1951 International Plant Protection Convention, the 1982 Law of the Sea and the 1992 Convention on Biological Diversity. Although many of these agreements are quite weak, some of them include important commitments.

The 1959 Antarctic Treaty, for example, banishes all exotics from the region unless they are specifically listed on an annex of exceptions or the bearer is granted an import permit. Besides legally binding treaties, a range of "soft law" instruments such as codes of conduct and action plans also address the bioinvasion threat.

Trade-offs with trade agreements

Still, tougher international agreements are clearly needed to address this problem adequately. Yet any accord stringent enough to alter today's rising tide of biotic mixing could run into conflicts with world trade rules.

Stepped up international cooperation is needed to ensure that economic globalization does not pave the way to ecological decline.

In what may be a foreshadowing of controversies to come, China has complained that a ban imposed by the United States in late 1998 on the import of goods in untreated wooden packing crates amounted to an unfair trade barrier.

The U.S. government imposed the ban after determining that Chinese packing crates were a primary culprit in the recent introduction of the voracious Asian long-horned beetle. That particular beetle is an invasive insect that poses a major threat to the health of U.S. hardwood forests. The European Union subsequently placed similar restrictions on Chinese packaging, while China in turn limited the use of U.S. and Japanese crates made from coniferous trees after discovering wood-eating worms in some of them.

As world trade and travel continue to surge, the bioinvasion threat can only intensify. Stepped up international cooperation is needed to ensure that economic globalization does not pave the way to ecological decline.