Wetlands - Invasive Species

People are not the only ones who can dramatically alter wetlands. Nonnative—also called exotic—species FIGURE 7.5
Percentage of wetland acreage lost, by state, 1780s–1980s
SOURCE: Adapted from Thomas E. Dahl, "Table 1. Wetland Losses in the United States 1780s to 1980s," in Wetland Losses in the United States 1780s to 1980s, U.S. Department of the Interior, Fish and Wildlife Service, 1990 can be as devastating to wetlands as humans by changing the nature of the ecosystem, thereby interfering with its function and the survival of native plants and animals. Plants and animals introduced either accidentally or deliberately can cause unexpected harm by displacing native species from their habitat or by placing stress, such as disease or predation, on a native species.

In 1899 the nutria or coypu (Myocastor coypus) was introduced into California for the fur-farming trade. This introduction was originally viewed as a way to provide economic benefit. Subsequently, state and federal agencies as well as private interests were responsible for introducing nutria into the wild in fifteen states to provide a new fur resource. In coastal states such as Maryland and Louisiana, the results have been disastrous.

Nutria are large (about fifteen pounds), semi-aquatic rodents that live in fresh, intermediate, and brackish marshes and wetlands and feed on the vegetation. They eat all vegetation in an area, changing a marsh to a barren mudflat. Nutria feed on the base of plant stems and dig for roots and rhizomes in the winter. Their grazing strips large patches of marsh and their digging turns over the upper peat layer. This conversion of marsh to open water destroys valuable habitat for muskrat, wading birds, amphibians, reptiles, ducks, fish, crabs, and a host of other species, as well as causing erosion and siltation. According to the U.S. Geological Survey, nutria currently affect 100,000 acres of coastal wetland in Louisiana alone. To date, the best control method is trapping and harvest for meat and fur.

FIGURE 7.6
Sources of recent wetland losses, 2000
SOURCE: "Figure 5-4. Sources of Recent Wetland Losses," in 2000 National Water Quality Inventory, U.S. Environmental Protection Agency, August 2002

FIGURE 7.7
Causes degrading wetland integrity, 2000
SOURCE: "Figure 5-5. Causes Degrading Wetland Integrity," in 2000 National Water Quality Inventory, U.S. Environmental Protection Agency, August 2002

Plant species can be as harmful as animals. Eurasian watermilfoil, phragmites (common reed grass), hydrilla, and purple loosestrife are introduced species that have disrupted wetland systems. Purple loosestrife (Lythrum salicaria) is a good example. It is a perennial herb with reddish-purple flowers that may reach six feet in height under the right conditions. It was an important medicinal herb and ornamental as early as 200 years ago on the East Coast and was probably introduced for this reason. It has no known North American predators and has high reproductive capacity—up to 300,000 seeds per stalk. Because it can out-compete most native wetland plants, it can change the character and ecological function of a marsh. FIGURE 7.8
Sources degrading wetland integrity, 2000
SOURCE: "Figure 5-6. Sources Degrading Wetland Integrity," in 2000 National Water Quality Inventory, U.S. Environmental Protection Agency, August 2002 This is a serious threat since many wetland and other wildlife species are adapted to and depend on specific plants.