Bio-invasions: the spread of exotic species.

• Author: Bright, Chris
• Position: Cover Story

Human mobility has radically increased the rate at which large numbers of living things are moving from one ecosystem to another. As more and more of these mobile "exotic" species invade natural communities that cannot cope with them, more and more native species lose out. Exotics are undermining global biological diversity - and becoming a growing economic burden as well.

Earlier generations of Americans thought the Florida Everglades required a cure. The immense marsh at the tip of the peninsula - a hot, unwholesome expanse of mosquito-infested sawgrass - was an obstacle to the advancing fronts of civilization and industry. So around the turn of the century, officials of the U.S. Department of Agriculture (USDA) began distributing seeds of the melaleuca tree (Melaleuca quinquenervia), a thirsty, fast-growing native of Australia. The tree had already been planted to drink up "fever swamps" elsewhere - and there was the possibility of timber production. Melaleuca took root in the popular imagination; by the 1930s, nurserymen weren't able to grow enough to satisfy demand. One forester even took to broadcasting melaleuca seed over the Everglades from his airplane.

But the melaleuca's only product has been a spreading thicket of trouble. Its impenetrable stands displace virtually all other vegetation. Its dense root mat oozes substances poisonous to other plants. Its airborne secretions are poisonous to people: they can cause severe respiratory and skin irritation. And the melaleuca is "fire adapted" - it spreads by burning. Its inner bark is a wet, insulating sponge, while its outer bark is tinder-dry and its leaves are laced with a flammable oil. So even though it sucks up water four times as fast as the native sawgrass, it burns with explosive force. A few days afterwards, the tree sprouts new growth and rains millions of seeds onto the burnt-over land. Germination begins in three days and a seedling may reach six feet in its first year. The melaleuca may already have invaded as much as 600,000 hectares of Florida wetland and if it is not controlled, says Ronald Myers, an expert on the problem, "the Everglades will be no more."

The melaleuca's rampage fits a pattern typical of exotic species - species introduced into ecosystems in which they are not native. Freed from the diseases, predators, and other factors that keep them in check in their native habitats, exotics can wreak ecological havoc. And as they spread, they displace ever greater numbers of rarer species, whose ranges are more circumscribed. About 30 percent of the creatures on the official U.S. Endangered and Threatened List, for example, are there at least in part because of exotics.

Of course, the migration of species into new habitat has always been a part of nature, but human interference has so greatly amplified the process as to make it, taken globally, a phenomenon without precedent in the history of life. In Hawaii, where exotic invasions have reached epidemic proportions, an average of 18 new insects or other arthropods have established themselves every year over the past half century or so. That's more than a million times the natural rate of invasion for that group of organisms. Hawaii may be an extreme case, but all over the world, exotics are accounting for an ever larger share of the biota - the local assemblage of living things. Yet little is being done to stop this process, largely because it's bound up with so many economic activities - everything from intentional introductions of exotic grasses for grazing cattle, to accidental releases of shrimp viruses from aquaculture shipments.

Like other forms of environmental degradation, exotic invasions exact a price. In 1957, for example, the Nile perch (Lates niloticus) was released into Africa's Lake Victoria to improve the fishing. But the perch, a voracious predator, eliminated nearly half the lake's 400 native fish species. And it proved an inferior food fish. Its oily flesh must be smoked, so nearby forests were logged. Now the perch itself appears to be in decline, due to lack of prey, overfishing, and the deoxygenation of algae-choked waters provoked by the loss of the herbivorous fish. Uganda, Kenya, and Tanzania - the countries surrounding the lake - must endure the loss of both fisheries and forests, even as their populations continue to grow.

The full costs of a disruptive invasion are beyond reckoning, but the immediate costs may be clear. In the United States, the USDA estimated losses and control expenses during the 1981 outbreak of the gypsy moth (Lymantria dispar), a forest pest native to Europe, at $764 million. (About 40 percent of all serious insect pests in the United States are exotic; so are at least half the weeds.) The zebra mussel (Dreissena polymorpha), a shellfish from the Caspian region that is spreading throughout eastern North America, could increase the cost of U.S. power production. Since it breeds prolifically and encrusts almost any available surface, including powerplant water pipes, the mussel could force the power industry to spend $800 million redesigning its plants - plus $60 million annually on maintenance. Exotics can pose serious public health threats as well. Over the last decade, the Asian tiger mosquito (Aedes albopictus), already widespread in Asia, established itself in Brazil, southern Europe, South Africa, and the continental United States. This mosquito is known to carry dengue fever, yellow fever, and encephalitis.

THE DOMINO EFFECT

Not every exotic is a monster, of course - the ginkgo tree on the lawn will probably never take over the countryside. It's likely that only a small percentage of exotics even manage to survive in their new homes. And according to many ecologists, an exotic that does establish itself will not necessarily do measurable harm. But at present, there is simply no way to identify the serious troublemakers - the invasive exotics - until the damage is done.

Part of the reason for this is that the invaders do their damage in so many different ways. Sometimes they cause a kind of ecological domino effect. The zebra mussel, for instance, is stripping the plankton out of more and more North American lakes and rivers, forcing an abrupt shift in the basic community food source - from plankton to bottom sediment. Over the long term, that shift may suppress plankton-feeding fish species, and increase bottom-feeding organisms like aquatic worms and crawfish. Such effects can extend far beyond the immediate ecosystem. In the 1960s and 1970s, for example, the U.S. state of Montana introduced the exotic opposum shrimp (Mysis relicta) into the state's Flathead River. It was hoped that the shrimp would serve as an additional food for a species of salmon, also exotic, that had been introduced as a game fish. But the shrimp proved a formidable predator of the local zooplankton - the main food of juvenile salmon. The salmon population collapsed, taking with it a wide spectrum of terrestrial species that had come to depend on the fish. Among the wildlife the shrimp has displaced are eagles, gulls, otters, coyotes, and bears.

The "food web" - the network of predator-prey relationships - is not the only ecological process that's vulnerable. Fire-adapted exotic grasses are changing the role that fire plays in natural areas of the American west, Hawaii, and Australia. Other effects are more complex. Why, for example, should the arrival of the Argentine ant (Iridomyrmex humilis) threaten the Cape Floral Kingdom, an extremely diverse and unique South African plant community? Because some 1,300 plant species depend on native ants for seed burial, and the native ants are being displaced by the invader.

But the damage can also be direct. Sometimes, for instance, the intruders simply eat the natives. Rats are the most important exotic predators of island birds, which have often evolved in the absence of any predators. This lack of evolutionary defenses explains...