Last year, President Clinton signed an executive order that committed the federal government to what Agriculture Secretary Dan Glickman called “a unified, all-out battle” against the spread of alien species in the United States. Praising the order, Interior Secretary Bruce Babbitt said, “There are a lot of global bioinvasive hitchhikers, and now is the time to take action. The costs to habitat and the economy are racing out of control.”
There are good reasons to try to keep out of the United States pathogens and other organisms that are known to be dangerous. On the whole, however, exotic species -- which include nearly all crops -- confer benefits that far outweigh their costs. An “all-out battle” against exotic species in general cannot be justified.
Those eager to wage an expensive war against exotic species argue that we should treat any alien organism as guilty until proved innocent. They justify their position by citing invaders like the zebra mussel, which clogs intake pipes, and kudzu, which can grow over and smother native vegetation. But examples are not arguments. By analogy, opponents of welfare programs may publicize the deceptions of a few welfare cheats to create the impression that welfare recipients, as a class, are dishonest. In any large group of species -- native or exotic -- selected at random, some will cause damage. However, no one has shown that exotics are more likely than natives to be harmful. Although alien organisms alter ecosystems -- for example, by increasing the variety of species in them -- we have no evidence that the changes are generally for the worse.
Actually, even the exotic species most condemned as nuisances may be beneficial. The “poster” alien species, the zebra mussel, traveled in the 1980’s from the Caspian Sea through canals to the Danube River, and by ship to Lake St. Clair near Detroit. It then spread throughout lakes and rivers in the eastern United States. It clogs intake and distribution pipes and cooling systems, which in America -- unlike in Europe -- were not originally designed to keep it out.
The mussel also does a lot of good. It devours the excess nutrients and associated algae that result from agricultural runoff and municipalities’ waste discharge, and through that process it helps to clean lakes and rivers. Scientists from Ohio State University have reported that because of the mussel, at least 14 species of native aquatic plants have reappeared in Lake Erie after an absence of more than 30 years. The mussel has made the water clearer, so the plants once again have enough sunlight to flourish. Scientists credit the mussel, along with antipollution efforts, with returning Lake Erie to a condition similar to that of a century ago.
In addition, the Oklahoma University biologist Douglas Hunter has observed that the fecal pellets that the zebra mussel excretes, by enriching the bottom of lakes and rivers, provide an excellent habitat for insect larvae, leeches, snails, and other invertebrates that yellow perch and other native fish feed on. As a result, the catch of yellow perch in Lake St. Clair increased fivefold from 1990 to 1996. Various freshwater fish, like drum, feed on zebra mussels, as do diving ducks, which have returned to Lake Erie in record numbers.
Kudzu, the other poster invasive species, often grows where it is not wanted, but otherwise it acts like a model citizen. A nitrogen-fixing legume, kudzu restores depleted, marginal areas, nourishing the soil while cleaning the air. In fact, during the 1930’s, the Civilian Conservation Corps grew 85 million kudzu seedlings and paid farmers to plant them in fallow areas to prevent erosion and improve soil conditions. Today, the plant makes a high-quality fodder for cows and other livestock -- it sells for $2 a bale -- and cooks use its nutritious leaves and roots in many excellent dishes, from snacks to quiche.
It is easy to select from among the estimated 50,000 exotic species found in the United States several that cause economic damage, or to which high costs might be attributed -- if only by figuring out how expensive it would be to get rid of them. One could as easily attribute the same kinds of costs to similarly selected native species -- like the tick that delivers Lyme disease, the mosquito that carries malaria, and the stinging jellyfish that often makes it unbearable to swim in the Chesapeake Bay during the hot summer months.
In a much-cited report in BioScience in January, David Pimentel and three of his graduate students at Cornell University concluded that “nonindigenous species in the United States cause major environmental damage and losses totaling approximately $137 billion a year.” The authors called attention, for example, to the damage that exotic insects inflict on crops. Nearly all U.S. crops are themselves exotic, however, while most of the insects that prey on them are native.
The exotic species that Pimentel and his coauthors deplored as harmful often have native cousins that are as bad or worse. The authors found that "[t]he single most serious pest bird in the United States is the exotic common pigeon,” to which they attribute a cost greater than $1-billion, because it takes $9 to eradicate each bird. Pigeons “foul buildings, statues, cars, and sometimes people, and they feed on grain.” Actually, any pigeon foolhardy enough to leave the city to look for grain would quickly be killed and eaten by a hawk. Passenger pigeons, now extinct, did dine in their billions at farmers’ expense -- but when we look back at that native species, we often forget the damage it did.
Not all critics of exotic species generalize from invidious examples. Some worry about the extent to which aliens have colonized our aquatic and terrestrial environments. The biologists Andrew N. Cohen and James T. Carlton reported in Science in 1998 that of roughly 400 species found in San Francisco Bay, 234 were known to be exotic and an additional 125 were of unknown origin. Other researchers have identified more than 139 alien aquatic species in the Great Lakes, and 116 in the Chesapeake Bay. With at least 4,598 non-native species in the wild, Hawaii ranks as the most invaded state; Florida is a close second.
Why is the extent of colonization a problem? In a 1994 report, the U.S. Aquatic Nuisance Task Force argued that exotics “cause significant ecological problems because they have been introduced into a habitat in which there are no co-evolved controls.” If the species had no controls, however, we would be buried under them. Besides, once an exotic species, like a crop, has become established, its controls or predators often find a way to immigrate as well. Round gobies now chow down on the zebra mussel in the Great Lakes as they do in the Caspian, although in America the gobies are themselves eaten by native predators like smallmouth bass, walleye, and yellow perch.
Biologists sometimes attribute to immigrant species some of the same characteristics that nativists and xenophobes have ascribed to immigrant humans: sexual robustness, excessive breeding, low parental involvement with the young, a preference for degraded conditions, and so on. In fact, no quality that anyone can observe identifies a species as exotic. Such species fit so well into ecosystems that the only way to tell them apart from native species is to learn the history of the place. Species for which no historical record exists are called cryptogenic -- that is, of unknown provenance -- because no one can tell if they are native or not.
Decades ago, ecologists proposed that ecosystems exhibit a structure or equilibrium to which they return after being disturbed. One could argue that exotic species upset that natural balance. Peter L. Chesson, a theoretical ecologist at Ohio State University, has observed that the balance-of-nature idea is now “dead for most people in the scientific community.” Ecosystems lack order, purpose, and design; they have no balance to disrupt. The San Francisco estuary, for example, is no more or less balanced now than it was 500 years ago, even though most of its flora and fauna are exotic.
If exotic species typically damaged ecosystems, ecologists could readily distinguish between the ecosystems that had been invaded and harmed, and those that remained in a prelapsarian state. But only the historical record can distinguish an intensely colonized from a pristine environment. In fact, scientists have not identified any ecological property -- except, perhaps, for the presence of greater biodiversity -- that distinguishes a more from a less invaded or colonized ecosystem.
Some commentators worry that alien species can drive native ones to extinction. However, in the San Francisco Bay, a highly invaded ecosystem, exotics have not caused the extinction of a single native species. More generally, the National Heritage Network’s Central Databases list 84 native animals and plants of all kinds as extinct, and 173 as missing, in the continental United States. However, few -- if any -- of those losses can be attributed to exotics. Native species -- not just exotics -- compete with and deplete the populations of other native species; indeed, endangered species, native or not, sometimes prey on each other.
Many Americans appreciate native species and support private programs -- like the wildflower-restoration efforts that Lady Bird Johnson has started -- to conserve them. Those efforts are laudable. But although the distinction between native and non-native has aesthetic and historical significance, it is otherwise irrelevant. It does not predict a species’ economic or ecological effect.
Good reasons exist for controlling known pests, whether they are native or exotic. Good reasons exist for taking pride in local flora and fauna. But no good reason can be given for waging an expensive battle against all exotic species.
Mark Sagoff is a senior research scholar at the Institute for Philosophy and Public Policy in the School of Public Affairs at the University of Maryland at College Park.
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