It's not enough simply to worry about direct human assaults on wetlands such as habitat destruction and fragmentation. Some wetlands are under attack by nature itself - although humans have had a hand in this process as well. The problem here is the invasion by nonindigenous, alien species that can overrun ecosystems with only limited natural checks on their advance. The Giant Salvinia plant - an exotic Brazilian fern that quickly blankets freshwater wetlands in a three-foot thick mass of leaves that kills fish and other plants - now threatens the bayous of Louisiana. Another invader brought to this country from Australia as an ornamental tree, Malaleuca quinquenervia, is rapidly spreading in the Florida Everglades, siphoning off already scarce freshwater supplies.
But many ecologists believe that purple loosestrife (L. Salicaria), a flowering plant imported from Europe about 200 years ago, poses the biggest menace to North American wetlands. The invasive plant has spread to wetlands over much of the United States, creating what one writer has called "a magnificent sea of purple." Nevertheless, the beautiful flowers are considered a plague by some, as they can choke out cattails, sedges and other native wetland plants upon which hundreds of species of birds, mammals, amphibians and reptiles depend. Purple loosestrife can completely take over a marsh ecosystem, with hundreds of seedlings taking root in a single square foot area, leaving no room for other species.
One plant releases an estimated 2.7 million seeds per year, which adds up to 24 billion seeds in a one-acre patch. Once purple loosestrife moves in, it's hard to get rid of it. The most effective aproach is to uproot the plants by hand and burn them, but that strategy is obviously not practical for large areas. As a result, scientists are turning to biological control techniques. Four species of purple loosestrife - eating beetles have been approved by the Department of Agriculture for release in the United States, and researchers in the control program predict the beetles can reduce the plant populations by 90 percent. Time will tell whether the insects can catch up with the plants, which have about a two-century headstart.
In any case, many experts believe that other approaches will be needed to curb the spread of purple loosestrife. These alternatives must be examined carefully to make sure the cure is not worse than the problem. The widespread use of toxic herbicides, for example, would pose clear environmental hazards. The last thing we want to do - to paraphrase a famous Vietnam War era quote - is to destroy the wetland in order to save it.
More than half of the wetlands in the United States have been lost to human activities and as much as 90 percent of the wetlands in states such as California and Ohio have been wiped out.