Wetlands occur throughout the world and support a wide variety of plants and animals. Wetlands are areas, other than lakes or rivers, whose soils are saturated with water for indefinite or prolonged periods of time. The water level remains near or above the surface of the ground for most of the year. Often these wetlands result from the surface exposure of the groundwater ( the water that has percolated down through the soil and accumulated and built up on a layer of rock or other material). The upper boundary of the groundwater, called the water table, rises or falls as the amount of groundwater varies.
Swamps, marshes, and bogs are wetlands in which the water table is at or, near, or above the surface of the land. Plant growth in these areas is limited to species that can withstand having their roots submerged for a long periods of a time. The water in these areas may be tea-colored or dark brown because of the presence of organic acids, such as tannic acid, derived from decaying vegetation. Although they have been studied for decades, coastal wetlands still pose a challenge to scientist who observe their complex interrelationship.
Depending on many factors, including climate, mineral content, and the permanency of surface water, wetlands may be mossy, grassy, shrubby, or wooded. The major types of wetlands include bogs, fens, marshes, and swamps. Bogs are a distinctive wetland type, usually characterized by evergreen trees and shrubs and underlain by deep PEAT deposits. Bogs will develop in former glacial lakes by a gradual accumulation of organic matter falling from beneath a floating mat of vegetation advancing out over the water. Peat deposits 20 to 40 ft. deep are not uncommon.
A characteristic of marshes is the absence of trees and shrubs; marsh plants tend to be soft-stemmed or herbaceous. Marshes often include areas of open water surrounded by plants such as cattails, rushes, arrowheads, pickerel weed, and bur reed, all of which grow with their stems and leaves partly submerged. Water lilies are rooted in the bottom, with their leaves floating on the surface. Floaters, such as the tiny duckweed, water lettuce, and water hyacinth, are more restricted in distribution; some occur only in the open waters of southern marshes and swamps.
In marshes animal life is highly diverse and includes an array of aquatic insects, some of which spend only their early stages in the water and then become terrestrial adults, such as dragonflies and damselflies; others are permanently bound to the water. Common amphibians include spring peepers and the leopard, pickerel, and green frogs, along with painted and spotted turtles, and the snapping turtle in deep water marshes. In the Florida Everglades, a region that is dominated by saw grass (sedge), alligators are especially common in the sloughs during the dry season.
The anhinga, or snake bird, which actually spears its fish as it dives beneath the open water areas of the marshes, is also common to the Everglades, as well as wading birds, such as the great blue heron, bittern, and egret. Muskrats frequent cattail marshes, where they feed on the roots of the plant. Large groups of muskrats have been known to clear an entire area of cattails. Another type of marsh is a salt marsh. Salt marshes are restricted to temperate coastal areas. They occur from the intertidal zone to slightly above high tide.
Here Spartina, or cord grass, tends to dominate, forming extensive grassy swards. Salinity and duration of flooding are among the complex of factors that determine the belting pattern or mosaic of vegetation types found in salt marshes. Marsh snails, ribbed mussels, fiddler crabs, and amphipods are conspicuous among the animal life. Salt marshes are an important waterfowl breeding habitat, as are freshwater marshes. Marshes around the world are recognized as duck factories, serving as breeding and feeding areas and resting places on the long migration routes of these birds.
Swamps are dominated by woody plants, primarily trees and shrubs. In the northeastern United States, red maple is a distinctive swamp tree, and in the southeastern United States, the bald cypress and gums dominate. Shrub swamps represent another swamp type, and willow, alder, or buttonbush can from pure stands. These wetlands can also develop floodplain forests along major river courses, as in the southeastern United States, or as isolated cypress domes in central Florida.
Swamps can occur in wet basins or can be dissected by stream courses. Forested wetlands often show a vertical stratification in vegetation structure, with tree, shrub, and herbaceous layers. Shrubs such as highbush blueberry, swamp azalea, spice bush, and sweet pepperbush may form a continuous stratum. Beneath the shrubs, skunk cabbage may be an important member of the ground cover, along with a diversity of wetland species, including the purple-fringed orchid, cardinal flower, jewelweed, and marsh marigold.
Many of the animals that inhabit marshes also live in swamps; additions include the raccoon and beaver. Wood ducks are a distinctive species of swamp forest, because they depend on hallow trees for nesting sites. Wetlands play an important role in nature. In addition to being a huge habitat for many plants and animals (particularly many endangered species) wetlands help to control flooding because they retain large amounts of water. The water stored by wetlands also serve as a source of replenishment for ground water supplies.
In addition, people use wetlands for fishing, hunting, and natural studies. Many wetlands throughout the world have been destroyed by human activities. Almost half the wetlands in existence at the time, the United States have been destroyed. Since the early 1970s, however, awareness of the ecological and economic value of wetlands has steadily increased. In the United States, various federal and state programs have been designed to preserve the remaining wetlands. But, a significant number of wetlands continue to be destroyed.