Bio-Invaders

Are we under attack by "non-native" species? Should we care?

"That kind of information is dangerous," scolded Jodi Cassell. Cassell, who works with the California Sea Grant Extension program, was speaking at a symposium on "Alien Species in Coastal Waters: What Are the Real Ecological and Social Costs?" at the February American Association for the Advancement of Science meeting in Washington, D.C. She wasn't alone in her alarm. "We have members of the press here," warned a member of the audience. "I am very concerned that they might think that his view is the dominant view."

The target of this shushing was Mark Sagoff, a philosopher from the University of Maryland who has worked with Maryland's Sea Grant program to determine how the Chesapeake Bay's unique ecology defines a sense of place. Sagoff's sin? He'd had the temerity to point out the benefits that the much-loathed zebra mussels had brought to the Great Lakes.

Introduced via discharged ballast water from European freighters in the mid-1980s, zebra mussel populations have been exploding in the Great Lakes. Tens of thousands of the tiny, striped shellfish can occupy a square meter of any hard surface--like rocks, docks, and boat hulls. Observers initially feared that zebra mussels would clog water-intake pipes for municipalities and power plants and perhaps out-compete native shellfish for food. However, it turns out that the things are voracious "filter feeders." They strain algae and nutrients like fertilizer runoff from the lakes' waters. As a result, zebra mussels have played a significant role in improving water quality by clearing the lakes of polluting organic matter.

"There has been a striking difference in water clarity improving dramatically in Lake Erie, sometimes six to four times what it was before the arrival of the zebra mussels," according to the U.S. Geological Survey's Nonindigenous Aquatic Species Database. "With this increase in water clarity, more light is able to penetrate deeper allowing for an increase in macrophytes (aquatic plants). Some of these macrophyte beds have not been seen for many decades due to changing conditions of the lake mostly due to pollution. The macrophyte beds that have returned are providing cover and acting as nurseries for some species of fish." What's more, zebra mussels provide food and habitat for all sorts of native fish and ducks.

Having Sagoff point out such positive developments was more than his colleagues on the AAAS panel could bear. To them--and to most professional ecologists--zebra mussels are simply "bad." So too, say ecologists, are all other "non-native" or "invader" species that set up shop in ecosytems different from the ones in which they originated.

Why ecologists feel this way is no small matter. It is one of the hottest questions in contemporary ecology, and one which has tremendous policy implications: Should massive regulatory steps be taken to make sure "non-native species" are kept out of any given ecosystem? This is the same issue that the signatories to the Convention on Biological Diversity are hashing out in Nairobi, Kenya, as this issue goes to print. The convention, an international agreement negotiated during the 1992 Earth Summit, is the first comprehensive global treaty to address all aspects of biological diversity, including genetic resources, species, and ecosystems. The results from Nairobi could well be the start of a global system for controlling non-native species. Delegates from 168 countries, including the U.S. (which has signed but not ratified the convention), are considering the "Guidelines for the Prevention of Biodiversity Loss Caused by Alien Invasive Species" devised by the World Conservation Union earlier this year.

Among other things, these guidelines want to apply the very problematic "precautionary principle" to the introduction of alien species. (See "Precautionary Tale," April 1999.) The WCU provisions call for sanctions against people or companies that intentionally introduce species without the prior authorization of national "biosecurity" agencies. They further recommend establishing "appropriate fines, penalties or other sanctions to apply to those responsible for unintentional introductions through negligence and bad practice." The activities of transport companies would "be subjected to appropriate levels of monitoring and control" by the biosecurity bureaucracies. In other words, a decision to regulate non-native species will likely end up regulating international trade, too.

The two basic positions regarding the debate over non-native species were laid out in clear relief at the AAAS meeting. So were the essentially aesthetic underpinnings of those who would devote huge resources to keeping "invaders" out of a given ecosystem. Panelist David Pimentel, an ecologist at Cornell, estimated that efforts to clear zebra mussels from municipal and city water-intake pipes, boat hulls, and docks cost about $200 million a year. Pimentel noted that he and his colleagues have "conservatively" estimated that the 50,000 non-native species introduced into this continent were costing the American economy $137 billion per year. Jodi Cassell and like-minded audience members were clearly worried that if the Sagoffs of the world go around talking about the benefits as well as the costs of non-native species, they might undermine efforts to extirpate invader species from our shores.

Sagoff countered by pointing out that even Pimentel admits that the vast majority of introduced species do not have adverse costs. Pimentel's "50,000" is just a big scare number, noted Sagoff. "Besides, more than 60 percent of insect pests are native. So why single out non-natives in toting up the costs?"

There's another important point worth making on behalf of the invaders: We have reaped enormous benefits from non-native species. Ninety-nine percent of crop plants in the United States are non-native, as are all our livestock except the turkey. "There is no basis in either economic or ecological theory for preferring native species over non-native species," said Sagoff. He further challenged his fellow panelists to name any specifically ecological criterion by which scientists can objectively determine whether an ecosystem whose history they don't know has been invaded or not. Are invaded ecosystems less productive? No. Are they less species-rich? No. And so on. Tellingly, the panelists had to agree that there is no objective criterion for distinguishing between "disturbed" ecosystems and allegedly pristine ones.

Despite that inability, the Convention on Biological Diversity's Subsidiary Body on Scientific, Technical, and Technological Advice stipulated last year that "it is important to differentiate between natural invasions and human introductions of species."

Why? From a strictly ecological point of view, should we care whether a species arrives on a piece of driftwood or on a cargo boat? Why not just regard the introduction of non-native species as fascinating experiments? Science magazine estimated last year that 99 percent of all the biomass--that is, the total of all living matter--in some parts of the San Francisco Bay belongs to non-native species. Yet native species continue to live in the Bay. University of California at Davis evolutionary biologist Geerat Vermeij concluded in a 1991 Science article: "Invasion usually results in the enrichment of biotas [the total flora and fauna] of continents and oceans." In layman's terms, introducing species tends to raise the total number of species living in a given ecosystem, not decrease it.

Most recorded extinctions are of species confined to oceanic islands which cannot compete with introduced continental species or humanity's habitat changes. For example, the brown tree snake came to Guam from New Guinea or the Solomon Islands during World War II. (They apparently hitched a ride on either Allied or Japanese ships or planes.) The birds and lizards of Guam were not adapted to snake predators and so were decimated by this alien species. However, continental species are better able to weather invasions. Even the Convention on Biological Diversity's Subsidiary Body concedes, "There are no records of global extinction of a continental species as a result of invasive species."

Of course, that isn't to say that non-native species don't sometimes cause economic harm. Take the case of the American chestnut. There was a time when it was said that an enterprising squirrel could travel from Maine to Georgia on the interlocking branches of chestnut trees. Yet an introduced fungus killed off nearly all of them before 1950.

The loss of American chestnuts was economically damaging, but the ecological costs are much less clear. The disappearance of such a dominant tree species from the Appalachians might have been expected to have had far more major consequences for the survival of other species in the ecosystem than it apparently has had. If the fungus had arrived before European settlers, it is unlikely that the absence of chestnuts would even have been noted.

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