Coastal Heritage Magazine
Triumph of the Weed
The biological invaders are coming! The Earth, conservationists say, could become increasingly dominated by hardy, prolific, adaptable exotic species such as the zebra mussel and the fire ant.
Closed Ecosystem. Andrew Lohrer, a marine ecologist with the University of South Carolina, adjusts one of 48 tubs used to run experiments on native and invasive crabs, their prey, and their predators at the Baruch Marine Laboratory in Georgetown County. Photo by Wade Spees.
Triumph of the Weed
For ages, a carnivorous jellyfish (Mnemiopsis leidyi) lived in the western edge of the Atlantic Ocean from Cape Cod to South America. But nowhere else. Then one day in 1982 a Soviet research vessel discovered the jellyfish in the Black Sea in southeastern Europe. The creature found ideal conditions there: an abundant food supply and few competitors or predators. By 1988, the jellyfish flourished spectacularly, devouring so many Black Sea anchovy eggs and larvae that commercial fish catches dropped by 90 percent. How did the jellyfish travel thousands of miles across the Atlantic Ocean? International trade carried it abroad, probably hitchhiking in a ship’s ballast water tanks.
Several years later, a freshwater mussel also caught a ride across the Atlantic in ballast water, but it traveled west, from Europe to America. In 1986, the zebra mussel (Dreissina polymorpha), a native of the Black Sea, suddenly appeared in the Great Lakes, where it reproduced at an extraordinary rate, clogging industrial and utility pipes. Barges then transported the nuisance mussel throughout the Great Lakes and into rivers from New York to Minnesota and as far south as Louisiana and Tennessee. Recreational craft carried the mussel into smaller lakes.
International trade and travel similarly send untold numbers of non-native species around the world each year. Non-native species are plants, animals, insects, or microorganisms carried far from their historic homes. They are also called “exotic” or “alien” or “non-indigenous” species.
On the Cover. Water hyacinth (Eichhornia crassipes), an invasive aquatic plant, fills up reservoirs and clogs waterways. South Carolina bans its importation and sale. Photo by Wade Spees.
The British ecologist Charles S. Elton first popularized the threat from exotic creatures in a 1956 book The Ecology of Invasions by Animals and Plants. Elton argued that “we are living in a period of the world’s history when the mingling of thousands of kinds of organisms from different parts of the world is setting up terrific dislocations in nature.” Elton warned of an unprecedented biological diaspora—creatures scattered from their native habitats to foreign lands.
Exotic species are particularly numerous in North America and Europe, plus Australia and New Zealand. These places have sophisticated economies dependent on global trade, according to Stanford University ecologist Peter Vitousek. About 24 percent of Canada’s plant species and 46 percent of New Zealand’s plant species are exotic. By contrast, about three percent of Egypt’s plant species and one percent of Tanzania’s are non-natives.
Species from abroad slip into the United States through every imaginable pathway. Plant seeds arrive here hidden on farming equipment, packing crates, and other materials. Exotic insects hitchhike on timber, in agricultural produce, and in nursery products. Travelers smuggle in non-native fruits and vegetables that can harbor plant pathogens. Mosquitoes that spread malaria and other diseases travel on cars, planes, railway cars, and trucks. The pet and nursery industries import many alien species for aquarium tanks and gardens.
Restless Americans also carry exotics from one part of the country to another. A species native to the eastern United States can be a nuisance west of the Rocky Mountains, or vice versa. The marsh grass Spartina alterniflora is a crucial element in the East Coast estuarine food chain and provides nursery habitat for marine fisheries. Decades ago, to improve habitats, resource managers introduced this plant to the West Coast. Wrong move, some biologists say. The plant has taken over mud flats, displacing valuable feeding areas for native wading birds and marine life. “We worship Spartina alterniflora on the East Coast,” says James T. Morris, a marine biologist at the University of South Carolina. “But on the West Coast it’s considered an extreme pest,” and resource managers fight it.
In the aquatic realm, ports bear the greatest brunt of biological invasions. While docked in port, a ship will draw water from the estuary into special ballast containers. Ballast provides extra weight, so the ship rides lower in the water and has greater stability in the open ocean.
Slippery Characters. At a port facility, an inspector with the U.S. Department of Agriculture examines imported bananas for potentially harmful pests. Photo courtesy of USDA/APHIS.
But ballast water isn’t sterile. It’s rife with fish, jellyfish, clams, mussels, sea slugs, algae, bacteria, and viruses, which can survive a transoceanic trip lasting weeks. When a ship enters its destination and releases ballast water, a Noah’s ark of small marine creatures is flushed into the estuary. An exotic species can become established if the new environment is similar enough to the old one.
Still, very few exotics survive long in a new environment. Predators eat them. Or exotics starve, unable to compete successfully against natives for food. Many shrivel in inhospitable cold or heat. An introduced species has to find suitable conditions—the combination of a satisfactory climate, food supply, soil or water type, and a lack of natural diseases, parasites, and predators.
But at least 6,600 species of foreign origin have became established in the United States and Canada since Europeans began exploring and colonizing North America five centuries ago. Many exotic introductions are economic blessings. Nearly every plant crop and all livestock animals cultivated in North America are descendants of introduced species. The only native plants raised by American farmers are corn (maize), potatoes, some beans, squash, sweet potatoes, and some berries. The turkey, the Muscovy duck, and perhaps one kind of chicken are the only native domesticated animals raised in the United States, according to Clemson University historian William F. Steirer, Jr. About 25 percent of fish species, including many sport fish, in the United States are introduced non-natives. American gardens and lawns are covered with non-indigenous plants, and most of our pets are exotics.
In rare instances, an exotic species finds an environment where natural enemies are few and it can reproduce and spread aggressively. Ecologists call these creatures “biological invaders,” which cause damage to forests, rangelands, crops, commercial fisheries, recreational and historic sites, water supplies, and human health.
Water hyacinth is among the most troublesome biological invaders in South Carolina. Water hyacinth (Eichhornia crassipes), a fast-growing plant, fills up reservoirs and clogs water supplies. “Some water gardeners see it as pretty, and it’s a coollooking plant, but it’s so problematic that the state has banned its importation and sale,” says Steve de Kozlowski, manager of the S.C. Dept of Natural Resources (DNR) Aquatic Nuisance Species Program.
Weed Patrol. Jim Levesque, a contract herbicide applicator for the S.C. Dept. of Natural Resources, sprays Roundup on patches of water hyacinth (Eichhornia crassipes) in the Waccamaw River. Water hyacinth, native to Brazil and introduced to the United States in the mid-nineteenth century, reproduces rapidly and forms huge mats that can obstruct waterways. Photo by Wade Spees.
People have introduced many invaders by accident. The fire ant (Solenopsis invicta) was inadvertently imported from Argentina into Mobile, Ala., around 1940 in dry ship ballast. Now the imported ant is a major pest in the South and a growing problem in the Southwest.
Other invaders have been purposely introduced with best intentions. Between 1935 and 1942, the U.S. Soil Conservation Service grew 85 million kudzu seedlings, promoted these vines for erosion control, and paid southern farmers to plant them. Since then, the plant has proliferated throughout the South, covering millions of acres of forests.
Some invaders are difficult to label as nuisance or benign. People in one geographic region might loathe a particular invader, but in another place they ignore or prize it. Chinese tallow (Sapium sebiferum)—known as the “popcorn tree”—was introduced to America in the 1770s. The tree is found throughout the South Carolina lowcountry, where many regard it as a valuable shade tree. The state of Florida, though, bans Chinese tallow because it soaks up moisture from the water table.
Some scientists argue that we shouldn’t worry about most alien species. Exotics and their human handlers have always displaced or diminished populations of native plants and animals, notes Peter Del Tredici, a botanist and director of living collections at the Harvard University Arnold Arboretum. As climate has changed and changed again, species have adapted, relocated, or died out. “Evolution is about survival,” he says. “The bottom line of evolution is, who survives?” West Coast birds and marine life, for example, would eventually adapt to Spartina alterniflora and other introduced species.
Pop Art. An invasive species can be prized in one region and despised in another. Holiday wreaths decorated with “popcorn” from a Chinese tallow tree hang on a fence in downtown Charleston. The Chinese tallow (Sapium sebiferum) was introduced to the Americas in 1770s. In South Carolina, the tree is admired for its popcorn-like fruit. Florida, though, bans Chinese tallow as an invasive pest. Photo by Wade Spees.
Today’s biological invasions, however, are quite different from the natural ebb and flow of species over millions of years, says James T. Carlton, director of the Maritime Studies Program of Williams College-Mystic Seaport in Connecticut. There are two kinds of biological invasions: range expansions (natural movements) and human-mediated introductions. Range expansions flow along predictable corridors. By contrast, modern trade and travel have allowed people to “dissolve all barriers of time and space, moving species around the world,” says Carlton. “Brazilian estuaries do not naturally exchange species with Japanese estuaries. But humans cause species to flow in a matter of hours or days between such areas.”
We have legitimate reasons to fight invaders that cause genuine nuisances such as economic losses, says Mark Sagoff, a philosopher at the University of Maryland. But our dislike of some exotics is based on historical attitudes toward certain ecosystems. We want to keep out exotic species that disrupt landscapes and seascapes we have treasured for aesthetic or cultural reasons. Sagoff argues that “there is no biological property that distinguishes native from exotic species—it is wholly a matter of history and our preference.”
Many Americans believe, inaccurately, that ecological stability is more natural than flux, says Sagoff. And this flawed notion underpins a misguided preservationist ethic in regard to exotic species. The problem is that “we want ecosystems to stay just as they are, to freeze nature in time.”
Numerous interest groups lobby fiercely for and against exotic species. Hunters, birders, boaters, and fishermen shout when invasive species interfere with their hobbies. But sometimes hobbyists fight each other over how to cope with a biological invasion. Boaters on some South Carolina lakes despise Hydrilla verticillata, an invasive aquatic plant, but many fishermen and hunters want to keep it because the plant provides food and habitat for sport fish and waterfowl.
Profiling Invaders
Today’s harmless exotic can turn into tomorrow’s nuisance. Some non-natives lie low for decades or centuries in their new environments before causing problems. “They arrive and establish small populations,” says Daniel Simberloff, an ecologist at the University of Tennessee. “There is a lag time, and then suddenly they explode. There’s no one answer why. There may be a genetic change or a change in the environment that makes a species more suitable” in a particular place.
Someday scientists might have the tools to predict which species are likeliest to wreak havoc. Researchers have already outlined a number of typical invader characteristics, designing a profile of potential troublemakers. Some plant nuisances, for example, have small seeds that can spread far and wide. Even so, invaders rarely fall into such neat categories. “We’re still in the infancy stage in the science of predicting invasiveness,” says Don C. Schmitz, a biologist with the Florida Department of Environmental Protection.
Marine invasions are among the most mysterious because we know far less about marine species’ life histories than we do about those of terrestrial creatures. Researchers draw core samples from sediments and identify pollen records to learn which land plants existed in North America before Europeans arrived here. With this information, scientists can judge whether a plant lived in a particular place centuries ago.
Such pollen records do not exist in ocean sediments. “Our terrestrial colleagues are much better at finding and understanding invasions than (marine scientists) are,” says Carlton. “I don’t have my hands around a good record” before 1800 when the first biologists began identifying marine species in East Coast estuaries. As a result, “we tend to underestimate the real scale of the invasions and the staggering numbers of species moved around before biologists came on the scene.” Without a long-term historical record, scientists can not determine whether large numbers of marine species are natives or newcomers.
International Gateway. Port facilities are major pathways for accidental introductions of exotic species, particularly via ballast water. Nations with sophisticated economies based on world trade are likeliest to continue facing threats from invasive species. Photo by Wade Spees.
That’s just the beginning of the mystery. Gregory M. Ruiz, a biologist with the Smithsonian Environmental Research Center, says that once a marine organism arrives from abroad, scientists “don’t know whether (it) is going to reproduce; if it does reproduce, if it will persist; how abundant it will be and what impact it will have. We can make good guesses, but we can’t really predict how a species is going to behave in a novel environment.”
A dizzying variety of invaders arrive in American estuaries, bays, and shorelines every year, says Andrew Lohrer, a marine ecologist at the University of South Carolina. “People have tried to generalize about the broader patterns of invasions in the marine environment. But there are many different ways that species, native or exotic, can make their livings in a habitat.”
Some exotics are successful invaders because they grow quickly to sexual maturity, Lohrer points out. “They are extremely opportunistic, colonizing habitat rapidly and reproducing like mad before they are out-competed by native species.” Other species use a contrasting strategy. Although growing very slowly to maturity, they are fierce competitors, dominating food supplies and other resources.
Many invasive species apparently prefer disturbed habitats. Disturbed sites are places where earth-moving machinery has dredged up sediments and ripped out vegetation, or places where people have established and maintained monocultures such as crops and grasses.
“It is not any old disturbance” that enables exotics to become invaders, says Simberloff. “It’s places that are heavily disturbed (by humans) on a regular basis that seem to be least resistant to invasions.” Landscapes adjacent to highway projects, such as ditches, “are seas of exotic plants.”
Still, invaders also overtake habitats that are relatively undisturbed by humans. “Disturbance is not a prerequisite for an invasion,” says Carlton, but disturbance does offer many exotics a foothold in a new habitat.
The Unique and Commonplace
By mixing species around the world, people are interfering with evolution, disrupting a natural order, some ecologists say. But skeptics argue that we’ve affected evolutionary processes since man first began tilling the soil and raising livestock. Over thousands of years, human beings have transported species to new places, and people have bred plants and domestic animals to encourage specific traits.
In recent decades, however, people have dramatically accelerated their interference in evolution. With applications of antibiotics and pesticides, intense commercial fishing, and species introductions, “humans may be the world’s dominant evolutionary force,” argues biologist Stephen R. Palumbi of Harvard University in a recent study in the journal Science. “Accelerated evolutionary changes are easy to understand—they derive from strong natural selection exerted by human technology.” Introduced species have caused unusually swift evolutionary changes in some native animals. Introduced predatory fish, for example, have caused rapid evolution of color patterns and life history traits in some native fish.
Another question is whether exotics diminish biological diversity in invaded habitats over the long term. Biological invasions are causing an international extinction crisis, some ecologists say. Almost half of the species on the United States’ list of threatened or endangered species are at risk partly due to exotic invaders. “There is a near consensus among scientists that exotics are the second largest contributor to species extinction (after habitat loss and degradation),” says Phyllis Windle, a senior scientist with the Union of Concerned Scientists, a nonprofit organization in Washington, D.C. “And it’s a growing problem.”
The planet, some conservationists claim, could become increasingly dominated by hardy, prolific, adaptable “weedy” plants and animals, such as water hyacinth, Chinese tallow, the zebra mussel, and the imported fire ant.
Michael L. Rosenzweig, an evolutionary ecologist at the University of Arizona, says that habitat loss, not biological invasions, will continue to be the primary culprit for extinctions. Evidence strongly suggests, he adds, that evolution, given enough time, would repair the losses of species caused by exotics around the world. But evolution cannot repair the damage when diversity is lost to habitat degradation.
Moreover, invaders usually increase local biological diversity, says Rosenzweig. “A new species comes in, and it often doesn’t push anybody around. The invader restricts the native’s realized (ecological) niche. Competition and predation don’t always result in extinctions; it’s that simple.”
Yes, it’s true that invaders frequently increase local biodiversity, but we shouldn’t measure an ecosystem simply by how many species live there, argues Carlton. San Francisco Bay is probably the most invaded marine ecosystem in the world, with 250 more species today than it had in 1850, yet virtually no native species in the bay have been driven extinct since then. “Exotics have removed virtually none of the original species as far as we can tell and it’s much richer,” Carlton says. “But it isn’t simply the raw numbers of species that’s important. Native species are in far, far fewer numbers now, and their role in the ecosystem is vastly reduced. What we have done is disrupt the way that the system once operated. That’s what we lament.”
Against the Wall. Steve de Kozlowski, manager of the S.C. Dept. of Natural Resources Aquatic Nuisance Species Program, worries about Phragmites australis stands like this one on Sandy Island in Georgetown County. Bird hunters dislike Phragmites because it drives out aquatic plants that provide food for waterfowl. Photo by Wade Spees.
Disruption, though, is a natural process, other scientists say. Natural events such as climate changes have always disrupted how ecosystems function. A warmer or colder climate, for example, forces species to take different roles in an ecosystem. “Any ecosystem at any time has lots of common species and a few rare species,” says Morris. “When climate changes, common species often become rare, and rare species become common.”
Modern biological invasions, however, are far different from natural disruptions, Carlton says. Through ballast water and other pathways, exotics are pouring into marine systems on an unprecedented scale causing rapid alterations, according to Carlton. And it is not only the famous invasions that are troubling; the small ones change systems as well. “You can’t put a species into a community and not have an impact. It has to eat something, and it has to take up space if it’s a plant or an animal.”
Equally troubling, says Carlton, is the idea that invasions seem to create ecological conditions that allow additional aliens to flow in. “Invasions beget invasions,” he points out. Eventually, an ecosystem can encounter what some biologists call “invasional meltdowns.” The theory is that “invasions themselves are a disturbance that disrupts the system, and then you have an opportunity for more invaders.”
Ecologists have found the most damning evidence against biological invaders on oceanic islands such as Hawaii and other isolated places. For tens of thousands of years, isolated islands were cut off from continental evolutionary processes. Once in a great while, a storm or freak sea current would carry a species—a bird or plant or an insect—from one of the continents across the vast Pacific Ocean to the islands. Most larger mammals and reptiles couldn’t survive long oceanic trips. So remote islands remained species-poor, lacking many predators that had shaped continental ecology.
In the nineteenth century, Europeans began introducing species such as rats, snakes, and other predators into islands. These introduced species consumed native wildlife, especially flightless birds, which had not evolved defensive mechanisms against predators.
Exotic invaders pose “an extremely serious problem in areas that have been isolated” by seas or by climate, acknowledges Del Tredici. “There you have plants and animals that have evolved in isolation, and when they are exposed to invaders, you can have a huge extinction crisis.” Remote lakes, surrounded by high mountains, are also biotic “islands” vulnerable to extinctions. Another “island” habitat is the southern part of Florida, a subtropical ecosystem cut off by saltwater and climate from similar habitats.
After invasions, “islands” often have more species than before, but the earth overall has lost biodiversity, ecologists say. The commonplace species has replaced the unique one.
In North America and other continents, species evolved amongst diverse populations of predators and pathogens. Thus when biological invaders arrive in South Carolina or Massachusetts or Illinois, they are far less likely to cause extinctions than invaders in Hawaii or South Florida, scientists say. “But,” says Lohrer, “there can be exceptions to this rule.”
Stopping Invasives
Twenty federal resource agencies are responsible for preventing many biological nuisances from entering the country, but major loopholes still exist. The U.S. Fish and Wildlife Service requires a special permit to import any foreign wild animals and some nuisance plants, though the agency doesn’t address accidental importation of species. The U.S. Department of Agriculture’s Animal and Plant Health Inspection Service (APHIS) primarily regulates species that pose a threat to agriculture and forestry. Yet APHIS usually does not address species that cause problems outside those specialties. Moreover, the federal government has lacked an overall plan with adequate funding to stop exotic introductions.
Holding Back the Tide. The invasive water hyacinth sends out underwater roots and takes over lakes, reservoirs, bays, and canals. Jim Levesque, a contract herbicide applicator for the S.C. Dept. of Natural Resources (DNR), sprays on Sandy Island in Georgetown County. Photo by Wade Spees.
In 1990, Congress passed legislation to plug a major loophole—introductions of aquatic nuisances pouring into the Great Lakes via ballast water. Before ships can enter the Great Lakes, they must now dump ballast from the previous port at sea and then exchange it with offshore ocean water. Organisms from the open ocean are unlikely to survive when discharged into freshwater lakes or brackish estuaries.
In 1996, Congress passed the National Invasive Species Act, which reauthorizes the 1990 act. The 1996 law stipulates voluntary guidelines on ballast water for ships that enter all U.S. waters. But these measures aren’t working. Ships coming into the United States from outside the Exclusive Economic Zone, excepting the Great Lakes, are supposed to file reports with the U.S. Coast Guard outlining what measures the ships have taken to treat—that is, exchange—ballast water. But only about 28 percent of all ships entering U.S. waters filed these reports during the period of July 1, 2000 to June 30, 2001, says Lt. JoAnne Hanson of the U.S. Coast Guard Environmental Standards Division. Under the current law, “we have no means to enforce it,” says Hanson. “There are no civil or criminal penalties associated with noncompliance.”
The act is up for reauthorization in Congress, and some marine scientists and policymakers want the law changed to make ballast exchanges mandatory.
In 1999, former President Bill Clinton signed an executive order to combat exotics and foster improved cooperation among federal agencies. The order established an interagency National Invasive Species Council, whose members wrote a comprehensive plan to minimize the economic, ecological, and human health impacts of invasive species and determine additional steps to prevent their introduction and spread. The plan includes 57 “action items” addressing coordination, prevention, research, and more.
The Bush administration has not formally adopted the plan. “With the shift in administrations, everyone’s waiting to find out what the council’s activities will be,” says Simberloff.
Once a biological nuisance takes hold in the United States, state agencies have the major responsibility to fight it. Yet neighboring states often regard a particular exotic species in different ways. South Carolina resource managers say that water hyacinth is among their worst aquatic nuisances. North Carolina, however, does not include water hyacinth on its list of banned aquatic plants.
We must vigorously fight dangerous invaders, everyone agrees. But how should we address exotics that do not cause a direct economic or human health concern? Should we become more suspicious of all invaders, a small number of which could eventually prove harmful? Should we establish a coordinated defense of American borders against every kind of alien? Or should we welcome a flood of non-native creatures just as earlier American generations did?
Government agencies, conservationists say, must provide better screening of intentional exotic introductions for use in gardens, aquariums, and aquaculture. Agencies must plug major pathways of accidental introductions such as ballast water. And government must, of course, fight invaders that present a direct threat to human health, crops, and industries.
Before the 1990s, few people noticed or cared about exotic species. Over the past decade, though, many Americans have become increasingly alarmed about strange new creatures appearing in our landscapes.
This new concern over exotics reflects a change in our values, says Carlton. “A hundred years ago, we operated under an environmental ethic that (encouraged) moving species around to improve nature. Now we think the absolute reverse. We exist in a time when we have a desire to restore the environment, to do no more harm. Today, we have a fear of loss, and that pervades everything we do.”
What is “Native”?
It’s hard to find a native species in coastal South Carolina.
European explorers and colonists began reshaping North American biology as early as the sixteenth century. From the beginning, Europeans surrounded themselves with plants from home. In 1629, Captain John Smith reported that most of the woods around Jamestown, Virginia, had been cut down and “all converted into pasture and gardens; wherein doth grow all manner of herbs and roots we have in England in abundance and as good grass as can be.”
Settlers soon encouraged exotic forage plants such as white clover and a Eurasian grass that Americans later called Kentucky bluegrass. These plants spread across the colonies.
The climate has steadily warmed since Europeans arrived here, driving up sea levels and moving salt water farther inland. A brackish marsh that you see near the coast today was likely a freshwater system in 1700. Thus entire plant communities— and the animal species that depend on them—have moved over three centuries.
During the past 75 years, South Carolinians have accelerated the scale and intensity of ecological alterations. Major rivers have been dammed to create lakes for water supplies, electricity, and recreation. The water flow into many coastal rivers has been continuously manipulated for various human uses. Urban development and modern farming have sent excess nutrients into waterways, which have been dredged and altered.
Today few plant assemblages in coastal South Carolina could accurately be called “native.” In places like South Carolina that have a “long history of massive human disturbance at all levels, the concept that there is a plant community that is native to that area is a biological fallacy,” says Peter Del Tredici, a botanist at Harvard University.
With a few exceptions, resource managers are not really protecting wild, untouched ecosystems from exotic threats. Instead, Del Tredici says, “we are deciding that ‘native’ species are more valuable than exotics. And we want to do something that helps encourage ‘natives’ to grow as opposed to (organisms that) we don’t like. That is called gardening.” There’s nothing wrong with gardening, he adds, “but let’s not say that we’re returning things to their natural state.”
Phragmites: The Kudzu of Wetlands?
Some say Phragmites, an invasive plant, is disastrous for the wetlands of South Carolina. But is it really so bad?
A sturdy weed, growing as high as an elephant’s eye, has invaded Georgetown County’s freshwater wetlands over the past two decades. Phragmites australis elbows out native plants such as wild rice that provide foodfor migratory birds.
“Phragmites grows so thick that it pushes out aquatic plants that provide food for waterfowl,” says Jack Whetstone, aquatic plant specialist with the S.C. Sea Grant Extension Program.
“Native animals and migratory waterfowl aren’t adapted to Phragmites,” agrees Steven de Kozlowski, manager of the S.C. Dept. of Natural Resources (DNR) Aquatic Nuisance Species Program. “The plant doesn’t provide the food or habitat that native plants offer. It comes in here and changes the whole food chain.”
Phragmites has lived along the northeastern U.S. coast for 3,000 years. No one knows the historical extent of its range, but for most of the twentieth century it was a minor part of brackish tideland vegetation. Then in the 1960s and ‘70s, it spread rapidly through the New England coast and south into the Mid-Atlantic states. In the early 1980s, it invaded a Georgetown County dredge disposal site, according to Bob Joyner, manager of the Yawkee Wildlife Center. The weed, which probably hitchhiked south on dredging equipment, has infiltrated historic hunting plantations in Georgetown County. “Phragmites is one of the most serious threats we have to our wetlands,” Joyner says.
One morning last August, de Kozlowski pointed out a Phragmites stand on the southern end of Sandy Island in Georgetown County. The stand was so thick that it established a vertical wall of vegetation 15 feet high. “The weed forms a dense monoculture that spreads and soon you have a huge field of this stuff.”
The weed often overtakes small tidal pools where migratory birds search for food in the former rice fields along coastal rivers, says Sandra Upchurch, a biologist for the ACE Basin National Estuarine Research Reserve. As a result, ducks have to look elsewhere for nourishment. “The big scare is that Phragmites can diminish the ecological value of these freshwater areas.”
But sometimes we attack an exotic before we understand how it fits into a particular niche in an ecosystem, skeptics say. Phragmites could be performing an important job in nature that we don’t yet recognize, says Peter Del Tredici, a botanist and director of living collections at the Harvard University Arnold Arboretum. “A lot of exotic species have been condemned before we’ve understood how these plants function in an ecosystem.”
Now Phragmites is disappearing in Europe. Ironically, Europeans, who mourn its decline, prize the plant’s capacity to take up excess nutrients. In the United States, Phragmites grows rapidly where excess nutrients from urbanized areas and pollution sources have flooded into coastal areas, says James T. Morris, a marine biologist at the University of South Carolina. “I think it’s nutrients,” driving the plant’s success. “Phragmites is responding to a change in the environment, either high nutrient loads or something else.”
So what would happen if resource managers knocked back the plant to its earlier East Coast range? “If you eliminate that plant,” says Morris, “something else is going to replace it. And the replacement will be less adapted” to current environmental conditions.
South Carolina resource managers, however, worry that if they don’t attack Phragmites stands every year, then it could spread and destroy migratory waterfowl habitat.
To Joyner, Phragmites is an implacable enemy. And he is outraged that anyone could see the invader as anything but a menace. “It’s the greatest threat to our marshes, next to sea level rise,” he says. “Anybody who thinks Phragmites is an advantage ought to lie down and have kudzu crawl over him.”
Further Reading
Carlton, James T. Introduced Species in U.S. Coastal Waters: Environmental Impacts and Management Priorities. Arlington, Va.: Pew Oceans Commission, 2001.
Cox, George W. Alien Species in North America and Hawaii: Impacts on Natural Ecosystems. Washington, D.C.: Island Press, 1999.
Crosby, Alfred E. Biological Imperialism: The Biological Expansion of Europe, 900-1900. Cambridge: Cambridge University Press, 1986.
DeVoe, M. Richard (ed). Nonindigenous Estuarine & Marine Organisms. Proceedings of the Conference and Workshop. Charleston, S.C.: South Carolina Sea Grant Consortium, 1992.
Elton, Charles S. The Ecology of Invasions by Animals and Plants. Chicago: University of Chicago Press, 2000.
Flannery, Tim. The Eternal Frontier: An Ecological History of North America and Its Peoples. New York: Atlantic Monthly Press, 2001.
Mooney, Harold A. and Richard J. Hobbs (eds). Invasive Species in a Changing World. Washington, D.C.: Island Press, 2000.
Palumbi, Stephen R. “Humans as the World’s Greatest Evolutionary Force.” Science, Sept. 7, 2001.
Pederson, Judith (ed). Marine Bioinvasions: Proceedings of the First National Conference, Jan. 24-27, 1999. Cambridge, Mass.: Massachusetts Institute of Technology.
Vitousek, Peter M. et al. “Biological Invasions as Global Environmental Change.” American Scientist, Sept.-Oct., 1996.