Interior Wetlands

Interior wetlands occur from the Arctic Circle to the equator and support a vast array of life forms. There are six major types of interior wetlands: swamp, marsh, fen, bog, wet meadow, and shallow water (aquatic). In all wetlands the soil, plants, and animals are adapted to an environment of flooding. The distinctive soils are often anoxic, because water contains less oxygen than air, because oxygen diffuses slowly through water, and because oxygen is rapidly consumed by soil microorganisms. The plants in wetlands often have hollow stems to permit movement of atmospheric oxygen downward into the soil; simultaneously, gases such as methane may move from the soil back into the atmosphere. Wetland plants often grow rapidly, supporting numerous animal species including invertebrates (plankton, shrimp, clams), fish, amphibians (salamanders, frogs), reptiles (turtles, snakes, alligators), birds, and mammals.

Interior wetlands occur in the interior of continents, and, being isolated from the saline oceans, normally have freshwater. (Occasionally, in arid regions where evaporation exceeds rainfall, such as in western North America, northern Africa, and central Eurasia, interior wetlands may become saline.) These six types listed above are produced by different combinations of flooding, soil nutrients, (see Figure 1) and climate. Within each type of wetland, smaller differences in conditions produce further differences in plant and animal communities. Hence the more kinds of conditions, the greater the biological diversity. The other factors that increase biological diversity are large area, low latitude, seasonal changes in water levels, low nutrient levels, and the effects of fire or wild grazing animals. In landscapes heavily altered by humans, diversity also declines with loss of adjoining forests and stabilized water levels, or with increased nutrients (eutrophication).

Wetlands in the northeastern United States. The soil, plants, and animals found in wetlands are adapted to flooding. (USQS/W. C. Rasmussen)

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