Wetland. I INTRODUCTION Wetland, geographic area with characteristics of both dry land and bodies of water. Wetlands typically occur in low-lying areas that receive fresh water at the edges of lakes, ponds, streams, and rivers, or salt water from tides in coastal areas protected from waves. In wetlands, the surface of the water, called the water table, is usually at, above, or just below the land surface for enough time to restrict the growth of plants to those that are adapted to wet conditions and promote the development of soils characteristic of a wet environment. Wetlands provide habitat for a wide variety of plants, invertebrates, fish, and larger animals, including many rare, threatened, or endangered species. The plants and animals found in wetlands include both those that are able to live on dry land or in the water and those that can live only in a wet environment. Wetlands are found on every continent except Antarctica and in climates ranging from the tropics to the tundra. They occupy about 6 percent of the land surface of the world, or approximately 890 million hectares (approximately 2.2 billion acres). The United States contains about 111 million hectares (about 274 million acres) or approximately 12 percent of the world's wetlands. II TYPES OF WETLANDS Wetlands can be classified into three general categories: marshes, swamps, and peatlands. Within each of these categories, wetlands can vary widely. Because wetlands depend on water sources, their boundaries can change. The characteristics that describe each category include vegetation, soil type, water supply, and water chemistry. A Marshes Marshes are periodically or continually flooded wetlands characterized by nonwoody emergent plants, plants that are adapted to living in shallow water or in moisturesaturated soils. Different species of emergent plants often occur in zones within a marsh; zones are determined by the elevation of the soil surface relative to the water level. Water chemistry in marshes depends on the water sources and varies from salt water (from incoming and outgoing ocean tides) to mineralized fresh water (from groundwater, streams, and surface runoff) to poorly mineralized fresh water (mostly from precipitation). Marshes often have mineral soils. Coarser soils such as sand are found in areas subject to waves or flowing water; in more protected areas, silts and clays accumulate with dead plant matter to form organic soils. Examples of marshes include the Everglades in south Florida; the prairie potholes of central North America; numerous coastal areas along the Great Lakes; and salt marshes along middle- and high-latitude ocean coasts around the world. B Swamps Swamps are dominated by trees or shrubs and occur in a variety of flooding conditions. Standing water can be present in swamps during all or just a small part of the year. Water chemistry in swamps can vary greatly, depending on the water source. Swamp soils can be rich or poor in nutrients and vary in mineral or organic content. Swamps often occur along river floodplains, in shallow, quiet waters of lakes, and along subtropical to tropical coasts. Examples of swamps include bottomland hardwood forests in the lower Mississippi River Valley; the Okefenokee Swamp on the Georgia-Florida border; and mangrove forests in Florida Bay, tropical Africa, South America, and Southeast Asia. C Peatlands In peatlands, plants are produced more quickly than they can decay, and partially decomposed plant material, called peat, accumulates. Peat provides an organic soil that influences plant growth. Water chemistry often determines which plant species grow in a peatland. Peatlands with groundwater sources (known as fens) often have more mineralized water and are dominated by sedges, grasslike flowering plants. Peatlands that receive water only from precipitation (called bogs) have poorly mineralized water and are usually quite acidic, especially if sphagnum mosses are abundant. Peatlands are more common in northern regions. Examples of peatlands include the Red Lake Peatlands in Minnesota; those of the Hudson Bay Lowlands in Canada; and extensive areas of Scandinavia, eastern Europe, and western Siberia. III HOW WETLANDS WORK The existence of all types of wetlands, as well as many of their biological characteristics, is determined by water: the amount that flows into and out of wetlands and the amount that is stored there. For example, in freshwater marshes, large emergent plants such as cattails can form dense stands that shade out other plant species. If periodic high-water years occur, the large, dominant plants are killed, providing openings where seeds from other plants can germinate and grow during low-water years. When water levels remain low, the dominant plants grow also, eventually shading out the smaller plants again. This cycle maintains diversity, both in the species of the plant community and in the habitat provided by mixed stands of large and small plants. Diverse habitats benefit fish that require different conditions for spawning, feeding, or seeking shelter from predators. These habitats provide the combinations of open water and protective emergent plants that are preferred by waterfowl, and they provide a variety of food and cover for the smaller organisms that are used as food by many birds and larger animals. Swamps along river floodplains are affected by the distribution of water in a different way. Rivers often meander across the floodplain. Sediments flowing with the river water are deposited on the inside of the river bends. During floods, a river may form a new channel that takes a shortcut between bends and leaves the original bend abandoned. Both of these processes change the shape of the floodplain and create new, diverse wetland habitat. Rivers may also build up banks that isolate the river from the floodplain. Periodic flooding then causes water to overflow the banks and deposit nutrient-rich sediments onto the floodplain, where they nourish the trees of the floodplain swamp. The timing of floods also influences the use of the swamps by fish and wildlife, especially for spawning and waterfowl migrations. Water influences peatlands in still other ways. The moist conditions created by precipitation or groundwater that flows into a peatland promote plant growth and also slow the decay of dead plant matter. If groundwater rich in minerals flows into a peatland, the chemistry of that water can affect which plant species grow there. If precipitation supplies much of the water to a peatland, sphagnum mosses often grow. Sphagnum makes the water more acidic, limiting the species of plants and animals able to grow there. A Plant and Animal Adaptation Other plants and animals have special adaptations suited for living in a wet environment. Most emergent plants have air spaces in their stems that enable oxygen to be transported to roots that grow in sediments with no oxygen. Some of the trees that grow in swamps form a set of roots above the soil surface or above the water that allows them to get oxygen to the lower roots. In saltwater wetlands, specialized cells can limit the amount of salt that enters a plant, or specialized organs can excrete salt from the plant. Microorganisms often have adaptations that allow them to live in water or wet soil without oxygen. Some larger aquatic animals, such as certain species of crab, have specialized gills or other organs to increase the uptake of oxygen from water or to use it more efficiently; other animals may reduce activity to minimize the need for oxygen when concentrations are low. In saltwater wetlands, animal cells either can be adapted to survival in salt water or they can regulate the amount of salt that passes in and out of them. Wetland animals may also have specialized ways of reproducing and feeding. Some animals affect the water distribution of a wetland in special ways: beavers build dams that may flood large areas, and alligators dig holes that retain water during the dry season. IV ROLE OF WETLANDS Because they have both land and aquatic characteristics, wetlands are some of the most diverse ecosystems on earth. The different plant species of a wetland provide habitat for varied animal communities. In addition to microorganisms and invertebrates, reptiles, such as turtles, snakes, and alligators, are common in wetlands. Many amphibians--frogs, salamanders, and toads--live in wetlands during at least part of their life cycle. A large number of fish species require wetland habitat for spawning, feeding, or protection from predation. Birds are attracted to wetlands by abundant food resources and sites for nesting, resting, and feeding. Many breeding and migratory birds, especially waterfowl, are associated with wetlands, as are mammals such as muskrats, nutria, mink, raccoons, and beavers. About one-fourth of the plants, one-half of the fishes, two-thirds of the birds, and three-fourths of the amphibians listed as threatened or endangered in the United States are associated with wetlands. Inland wetlands may help control floods by storing water and slowly releasing it to downstream areas after the flood peak. Wetlands can reduce wave action and slow down the flow of water, lessening erosion and causing sediments to settle out of the water. This improves water quality, as does the removal of nutrients and contaminants from the water by growing wetland plants and by chemical processes in wetland sediments. Wetlands may also serve as sites where surface water can seep into the ground and replenish the groundwater. Wetlands provide many opportunities for recreational activities, such as bird-watching, hunting, fishing, trapping, and hiking, and they provide educational opportunities for nature studies and scientific research. Some North American wetlands are of archaeological interest because Native American settlements were located near them. Peatlands in Denmark and England have yielded human fossil remains from about 2000 years ago, well preserved by acidic and anaerobic (low-oxygen) conditions. Wetlands are also valuable for the food and timber harvested from them. A Wetland Conservation Issues Since the late 1700s, over half of the wetlands of the United States, excluding Alaska, have been lost. About 35 percent were gone by the 1950s; wetland destruction during the next two decades resulted in an additional loss equal to the combined area of Massachusetts, Connecticut, and Rhode Island. Although still substantial, the rate of loss has since decreased, with inland marshes receiving better protection but forested wetlands sustaining more damage. Wetland losses have resulted in greater flooding and erosion, reduced water quality, and reduced populations of many plants and animals. Beginning in the early 1980s, wetland science emerged as a separate field of study, and better information concerning the importance of wetlands was made available to the public. As a result of the heightened awareness of wetland values, attitudes began to change; laws such as Section 404 of the Clean Water Act of 1972 (revised 1975), which regulates the dumping of solids into wetlands and waterways, and the 1985 Swampbuster provisions of the Flood Securities Act were passed to protect and preserve wetlands; public and private programs were developed to restore wetlands; and wetland losses began to decrease. In 1988 recommendations were made by the National Wetlands Policy Forum for a program of "no net loss of wetlands," with stronger wetland protection policies but also recognition that some wetlands will inevitably be destroyed by development. Under this program, lost acreage and function may be recovered by the creation of new wetlands and the restoration of degraded wetlands. By 1991 over 60 countries had joined the Convention on Wetlands of International Importance Especially as Waterfowl Habitat, adopted in Ramsar, Iran, in 1971 (enforced since 1975) and known as the Ramsar Convention. Member countries are required to designate at least one wetland as a conservation project to add to the List of Wetlands of International Importance. The Ramsar List includes more than 30 million hectares (74 million acres) of wetlands in more than 500 locations--still only 3 percent of the total wetland area of the world. Twenty of these sites are considered to be seriously at risk, and many have no management program. Conflicts over wetland policies remain, since the general public benefits the most and individuals the least from restrictions on developing wetlands. Much of the controversy centers on the legal definition of a wetland. Many wetland advocates believe that a greater area of a wetland should be protected than the area suggested by some landowners or developers. Contributed By: Douglas A. Wilcox Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.
