WETLANDS ARE WATER-BASED ecosystems that cover approximately 6 percent of the Earth's land area. an estimated 3.3 million square mi (8.6 million square km). Wetlands occur on every continent except ANTARCTICA and in every climatic zone from the humid tropics to DESERT to tundra. Three climatic zones of the world (tropical, subtropical, and boreal) support 70 percent of the world's wetlands. Together, tropical and subtropical zones have an estimated wetland area of 1.3 million square mi (3.4 million square km); the boreal zone has an estimated 1 million square mi (2.6 million square km) of wetlands.
Wetlands include many familiar ecosystems, such as swamps, bogs, fens, mires, and moors, which have different names in various regions of the world. As wetlands often occur at the interface between dry terrestrial ecosystems and permanently saturated aquatic ecosystems (rivers, lakes, estuaries, and oceans) they are sometimes considered to be ecotones, transition areas or interfaces between ecosystems.
Wetlands are not always easily defined or identified. Moreover, wetland definitions are numerous and sometimes contradictory: over 50 different definitions of wetlands are used worldwide. In 1979, wetland scientists in the UNITED STATES developed a comprehensive definition of wetlands based on three criteria: vegetation, soils, and hydrology—the delivery and retention of water in the ecosystem. Wetland hydrology is typified by the presence of water at or near the ground surface for at least part of the year. The amount, timing, and duration of water supply are important in defining wetlands. For example, a wetland may be permanent—having standing water or saturated soil horizons across the year or seasonal with water present for a portion of the year. Plants typical of wetlands are hydrophytes (plants adapted to wet conditions) that grow wholly or partially in water.
Oxygen is often limited in the saturated soils of wetlands and hydrophytes cope with root anoxia by having structural and physiological adaptations to enhance oxygen delivery to tissues. Dominance of vegetation by hydrophytes is a good wetland indicator even if standing water is absent, as it may be in seasonal wetlands. Hydric soils, those that develop under saturated, low-oxygen or anaerobic conditions, dominate wetlands. Redoximorphic features (physical attributes that develop under anaerobic conditions) are typical of hydric soils and include gray layers, mottles and oxidized rhizospheres (rooting zone of the soil horizon). Gray layers and mottles are formed when iron compounds are reduced by bacteria under anaerobic conditions; oxidized rhizospheres are thin traces of oxidized iron formed from excess oxygen diffusing from plant roots.
There are many types of wetlands worldwide and numerous efforts have been made to categorize and classify them in a useful manner. The U.S. Fish and Wildlife Service has developed a hierarchical classification for wetlands that identifies four major systems—a complex of wetlands and deepwater habitats with shared hydrologic, geomorphic, chemical, and biological traits having many subsystems and classes.
The four major wetland systems are estuarine, riverine, palustrine, and lacustrine. Estuarine wetlands are coastal wetlands, like salt marshes and mangrove forests, influenced by tides and fluctuating salinity from the mixing of saline ocean waters and freshwater river flows and runoff. Riverine wetlands are freshwater wetlands associated with river and stream channels. Riverine wetlands may be tidal, with fluctuating water levels; perennial, non-tidal with continuous water flow; or intermittent, having no flow for part of the year. Riverine wetlands are sometimes also called riparian wetlands when they occur in a stream or river FLOODPLAIN.
Palustrine wetlands are inland freshwater wetlands that are not associated with a stream channel but may be bounded by uplands or associated with lakes less than 20 acres (8 hectares) in size. Palustrine wetlands are among the most common worldwide and include marshes and forested wetlands such as swamps and pocosins. Lacustrine wetlands are freshwater wetlands associated with lakes greater than 20 acres (8 hectares) in size. Lacustrine wetlands are commonly associated with the near-shore of lakes, the LITTORAL zone, and are dominated by emergent plants like cattail that are rooted below water but have upright stems that rise above the water surface.
Some experts consider a hierarchical, system-based wetland classification too complicated for everyday use and instead rely on a system based on the growth form of dominant wetland plants: grasses and herbs, shrubs or trees. Marshes are wetlands dominated by nonwoody plants like graminoids, grasses and grasslike plants such as sedges and rushes. Shrub wetlands are dominated by low-growing multi-stemmed woody plants, shrubs like azalea or blueberry. Forested wetlands, swamps, are dominated by trees.
A second simple classification system focuses on soil type and identifies wetlands as either mineral or organic soil wetlands. Mineral soil wetlands include swamps, marshes, PLAYAs (marshlike ponds in the southwestern United States), potholes (marshlike ponds in the Dakotas and Canadian prairie provinces), wet meadows, and wet prairies, among others. Organic soil wetlands include bogs, fens, mires, moors, and muskegs (boglike wetlands of CANADA and ALASKA), inclusively known as peatlands because of substantial accumulation of peat (dead plant matter) in the soil. Bogs and fens are common organic soil wetlands in cool climates. They differ in hydrology, nutrient content, pH, and peat sources.
Wetland area has declined considerably in most regions of the world owing to alteration, especially hydrologic alteration, and destruction by humans. Some estimates put worldwide wetland loss at 50 percent, with some countries loss of wetland area exceeding 80 percent.
Wetlands were historically viewed as wastelands, malodorous swamps spewing miasmas that bred malaria and other infectious diseases. In North America and Europe, wetlands were extensively drained for agriculture, with drainage often promoted by the government. For example, the Swamp Lands Acts of 1849 and 1850 in the United States transferred 59 million acres (24 million hectares) of wetlands to the states, which sold them to private citizens to promote drainage and agriculture. Of the estimated 3.4 million square mi (8.9 million square km) of wetlands in the United States in 1780, over 50 percent had been lost by the early 2000s.
Efforts to identify and quantify the ecological value of wetlands have helped raise awareness of wetland loss and degradation and have fueled conservation and restoration initiatives and protective legislation at local, regional, national, and international levels. For example, the Convention on Wetlands of International Importance—the Ramsar Convention, adopted in Iran in 1971—provides a global framework for protection of wetlands for migratory species, especially birds, and human populations dependent on wetlands.
The value of wetlands as natural nurseries for wildlife, fish and shellfish for commercial harvest, recreation, and aesthetics and their role in flood moderation, water quality improvement, and as sources and sinks in global material cycles are unparalleled.