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Although they account for only about 6.4 percent of the earth's land surface, wetlands are extremely important ecosystems. Wetlands are ecotones, or transitional environments between deeper water systems and terrestrial uplands. They are characterized by the presence of water at or near the surface, unique soil conditions, and hydrophytes (plants suited to wet conditions). Several formal definitions have been written during the past twenty years for regulatory purposes, including the oftencited U.S. federal definition: "The term wetlands means those areas that are inundated or saturated by surface or ground water at a frequency and duration su.cient to support, and that under normal circumstances do support, a prevalence of vegetation typically adapted for life in saturated soil conditions." Thus, wetlands include marshes, bogs, swamps, fens, wet meadows, peatlands, playas, and a number of other aquatic ecosystems that fall under a variety of names.
Wetlands can be categorized into five main types: marine, estuarine, lacustrine, riverine, and palustrine. The latter three types occur in abundance throughout the Great Plains. Indeed, this region includes two of the most important wetland areas in North America: the expansive Prairie Potholes region of the Prairie Provinces and north-central United States, and the Nebraska Sandhills. Scientists further group wetlands into subsystems and classes based upon their seasonal hydrology (e.g., intermittent or permanent standing water), bottom substrate material (e.g., rock or soft sediment), and the predominant plants (fifty-six subcategories in all). Lacustrine wetlands are typically associated with ponds and lakes, forming the shallow areas along the margins, while riverine wetlands occur along rivers or streams and are often fed by floodwaters. Palustrine wetlands include the remaining shallow water systems that are typically identified as wetlands, including marshes, swamps, and bogs.
Wetlands are dynamic systems, exhibiting major changes in vegetation during the growing season, as surface and subsurface water levels change, sometimes dramatically. Consequently, they provide habitat for a large number of plants and animals, some adapted strictly to aquatic existence and others capable of living in the water for short periods of time during their life cycle. Many aquatic insects, for example, require water for their preadult stages, then emerge as winged adults: dragonflies, damselflies, midges, and many other insects are common members of the wetland community. Aquatic plants, being less mobile, must have adaptations that allow them to live underwater or in saturated sediments, adaptations such as air chambers in their leaves and stems for gas exchange to roots anchored in oxygen-depleted muds.
On a per acre basis, wetlands produce more plant and animal biomass than any other ecosystem on earth. The vast majority of this production is hidden to the untrained eye: hydrophytes in the wetland bottom in the form of roots, rhizomes, and other storage structures, and microscopic plants called algae, which are attached to hydrophytes or bottom mud or occur free-floating in the water. The propensity of wetlands to produce plant and animal biomass is a reflection of their ability to rapidly take up and cycle basic nutrients such as phosphorus and nitrogen. Consequently, wetlands are also now recognized as ideal systems for treating municipal waste, storm water runoff, agricultural runoff, and animal waste. Many wetlands are being constructed each year for this purpose, and natural and restored wetlands are also used. Thus, wetlands have been termed biological filters or the "kidneys" of the landscape. They serve a variety of other important functions as well, including flood control, sediment trapping, erosion control, and groundwater recharge, in addition to their recreational benefits for camping, hunting, canoeing, fishing, and bird watching.
Perhaps the most significant role of wetlands is in maintaining biodiversity. It has been estimated that one-half of the fish, onethird of the birds, and one-sixth of the mammals on the U.S. threatened and endangered species list occur in wetlands. The Prairie Potholes region, for example, is home to twelve of the thirty-four species of breeding ducks in North America. Nevertheless, wetlands continue to be lost at an alarming rate. Conversions of wetlands to croplands and urban development have resulted in a cumulative loss of 53 percent of all wetlands in the continental United States. South Dakota and Nebraska have lost approximately 35 percent of their wetlands. Federal efforts to preserve and restore many wetlands have increased in the past ten years in the Great Plains, but the decline continues.