The physical roughness of the Earth -- its structural variety -- has tended to hold its living communities in place. The barriers that surround any particular ecosystem help set the terms of life within it. They tie a particular assemblage of plants and animals together, and they tend to exclude predators, competitors and diseases that evolved elsewhere.

Islands provide the extreme case. In their isolation, many island creatures have evolved into forms found nowhere else -- the giant tortoises of the Galapagos, for example, or the colorful "picture-winged" fruit flies of Hawaii.

But the planet is scored by thousands of more subtle barriers, too. A "rain shadow" downwind from a mountain ridge may be too dry for forest; an ocean current may isolate two distinctive coral reefs.

Even the lives of highly mobile creatures are likely to be governed by barriers of one sort or another. The salmon that hatch in the rivers of western North America may swim together in the ocean, yet each strain returns to its own river to breed, thereby preserving its distinctive genetic identity.

And so, from one horizon to the next, a subtle matrix of barriers has allowed the communities of life to work out evolutionary answers to a particular spot of land, a stream or a set of ocean currents. Natural barriers are the instruments of evolution.

Jellyfish and mosquitoes

Today these barriers are losing their ecological reality, as more and more organisms are moved around them. A western Atlantic jellyfish, for example, is pumped out of a ship's ballast tank and into the Black Sea, where it wrecks the fisheries. Escaped garden plants strangle North American wetlands and rare island forests.

Plantations of Australian eucalyptus trees displace native forests throughout the developing world -- and sometimes native forest peoples as well. The farming of commercial shrimp species obliterates coastal fisheries and the local economies that depend on them. Range-devouring weeds sprout from contaminated crop seed; virus-laden mosquitoes emerge from shipping containers.

Water hyacinth, an aquatic weed that is suffocating East Africa's Lake Victoria, comes from South America; the disease that is killing off crawfish in European streams comes from the crawfish that live in North American streams; melaleuca, a tree that is invading the Florida Everglades, is from northern Australia; and the infamous kudzu that grows throughout the South is from Asia.

As a global threat of extinction, bioinvasion may already rank just behind "habitat loss" -- a much more general category that can be taken to include almost any kind of physical disruption. For certain types of organisms, exotics are clearly the principal threat. During the past century in the United States, for example, exotics have been a factor in 68 percent of fish extinctions.

The cultural effects of exotics can be as profound as the biological ones. Human pathogens, for instance, travel as readily as crop pests or weeds, and entire branches of humanity have fallen away as a result.

Unchecked diseases

The diseases brought to the Americas by European colonists precipitated one of the greatest cultural crises in history. In the century following the conquistadors' arrival, as many as two-thirds of the Western Hemisphere's native inhabitants -- perhaps 30 million people -- may have succumbed to smallpox, malaria and other Old World diseases to which they had almost no resistance. To a considerable degree, the Europeans

inadvertently "created" the wilderness they then went on to explore.

Today, miners and settlers continue to spread pathogens to the native peoples of the Amazon basin, with disastrous effect. Since the mid-1980s, about a quarter of the Yanomami people have succumbed to exotic diseases.

The rapidly expanding volume of migration and travel is pulling ,, most of humanity into a single microbial system, and no society may really be prepared for the results. Epidemic cholera has recently returned to the Americas, yellow fever may be poised to invade Asia, and we have barely begun to identify the malign synergies produced by overlapping epidemics.

Nor are the social effects of bioinvasion limited to disease. Exotics ruin our crops and suppress our fisheries. They cause declines in forest and rangeland productivity. Aggressive exotic water weeds and shellfish are fouling dams, power-plant intake pipes and irrigation canals. Some exotic plants are increasing the rate and intensity of brush fires; others are reducing water tables. In these and many other ways, exotics are costing societies all over the world billions of dollars every year.

Biological diversity

Conceptually, the problem of invasion comes down to this: Why should adding a species to an area end up reducing that area's biological diversity? The exotic dandelion on your lawn, for example, is presumably just one more species in the local plant community -- its only known victims are people who value uniform grass.

The paradox disappears once you look from the individual exotic to the process as a whole. The proportion of exotics that cause serious trouble is difficult to estimate, but a very rough rule of thumb, sometimes called the "tens rule," is that 10 percent of exotics introduced into an area will succeed in establishing breeding populations, and 10 percent of those will go on to launch a major invasion.

When that happens, the exotic has escaped the predators, diseases and other factors that kept it in check in its native range, and has found nothing comparable in its new range. It is facing organisms that did not evolve in its presence and that may not be adapted to compete with it or escape from it.

There is little consolation in the fact that 90 percent of these impacts are "duds" and only 1 percent of them really detonate. The bombardment is continual, and so are the detonations.

Weedy invaders

Many invaders seem to owe their explosive ecological power to a complex of traits known as "weediness." Weedy invaders mature quickly, multiply prolifically, spread easily and often do especially well in disturbed conditions.

Animal "weeds" tend to be highly adaptable in their diets. The ubiquitous rats and house sparrows, the zebra mussels invading North American waterways, the water hyacinth and the melaleuca -- all these organisms are weeds.

As the weeds spread, displacing more and more local diversity, the world becomes a steadily more homogenized place. The same weeds begin to crowd every rangeland; water hyacinth smothers warm-climate waters the world over. Goats gnaw the shrubbery to stubble on island after island.

In all these places, as the local creatures disappear, the ecosystem they were part of tends to weaken. An artificially simplified community, like a machine that is missing a lot of its parts, is more likely to break down. A fire, say, or an outbreak of disease that might have had little effect in a healthy, complex community may seriously disturb a simplified, sick one. And that disturbance will lay the area open to yet more invasion.

This is the cycle of degradation that is coming to characterize our era.

This article is excerpted from "Life out of Bounds," published by the Worldwatch Institute.

Pub Date: 11/02/98