Wetlands as natural habitats

Wetlands are transitional areas between aquatic and terrestrial habitats. These water-influenced areas represent one of the most biologically unique and productive of all natural habitats.

Background

In unaltered states, wetlands are used by a variety of wildlife species. These habitats also have the ability to take up and store water during floods, and their soils and plants have the ability to remove nutrients and heavy metals from water. The recognition of these values has helped to slow the rate of wetlands loss to such uses as agricultural development and urban expansion. A desire to protect remaining areas has led to a significant movement for wetlands preservation.

Definition of Wetlands

Ecologists recognize wetlands as a type of “ecotone.” Ecotones are unique areas that represent a transition from one type of habitat to another. Often, these transitional areas have characteristics of both habitats. Wetlands are transitional areas located between aquatic, water-based habitats and dry land. Since they are located at the edge of an aquatic habitat, wetlands are always influenced to some degree by water. They are not always underwater, as are aquatic habitats, and they are not always dry, as are terrestrial habitats.

The most important environmental factor in wetlands is the periodic or frequent occurrence of water. This presence of water influences both the nature of the soil and the biota. Soils which experience periodic coverage with water become anoxic, develop a dark color, and give off an odor of hydrogen sulfide. These soil characteristics differ from those of upland soils and give wetland soils their unique “hydric” nature. In these soils influenced by water, only flood-tolerant “hydrophyte” species can exist. Hydrophytic plants vary in their tolerance to flooding from frequent (bald cypress) to infrequent (willows).

In defining a particular area as a wetland, often all three of the components listed above are used: water, hydric soils, and hydrophytic plants. However, the presence of water is not always a reliable indicator, because water rarely covers a wetland at all times. Often, a wetland is dry during a period of low river flow or during a low tide. For this reason, only hydric soils and hydrophytic plants should be used as reliable indicators of a wetland.

Classification of Wetlands

The broadest classification of wetlands includes two categories: freshwater and saltwater wetlands. Freshwater wetlands occur inland at the edges of rivers, streams, lakes, and other depressions that regularly fill with rainwater. Saltwater wetlands occur along the coast in bays where and mix and wave energy is reduced. Of the two categories, freshwater wetlands are by far the most common.

Freshwater wetlands are subdivided into two categories: tree-dominated types and grass-dominated types. Tree-dominated freshwater wetlands include areas that are frequently covered with water (cypress swamps) and those that are only occasionally covered with water (bottomland forests). Grass-dominated types include freshwater marshes, prairie potholes, and bogs. While freshwater marshes are widespread, prairie potholes and bogs occur regionally in the United States. Prairie potholes are located in the central portion of the United States, while bogs are found in the Northeast and Great Lakes regions.

Saltwater wetlands are also subdivided into tree-dominated and grass-dominated types. Tree-dominated types include tropical mangrove swamps, which occur in the contiguous United States only in southern Florida. Grass-dominated types can be further subdivided into salt marshes and brackish marshes. Salt marshes occur in bays along the coast where salt water and fresh water mix in almost equal proportions. Brackish marshes occur farther inland than salt marshes do; their mix is less sea water and more fresh water. Both grass-dominated types are common in bays along the Gulf and East Coasts of the United States.

The Biota of Wetlands

The most noticeable feature of all wetlands is the abundance of plant life. A variety of plant species thrive in wetlands, but each occurs only in a particular kind of habitat. Freshwater wetlands that are frequently flooded provide a favorable habitat for water-tolerant trees, such as bald cypress and water tupelo, and water-tolerant herbaceous plants, such as cattail, arrowhead, bulrush, spike rush, water lily, and duckweed. Less frequently flooded freshwater areas support trees, such as willow, cottonwood, water oak, water hickory, and red maple. Seawater areas in tropical bays favor the development of mangroves, while temperate bays favor the development of cordgrass.

Wetland plants provide a habitat for a variety of animals. Cypress swamps and cattail marshes support a large assortment of animals, including alligators, ducks, crayfish, turtles, fish, frogs, muskrat, wading birds, and snakes. Likewise, mangrove prop roots provide attachment sites for a variety of invertebrates and shelter for numerous small fish, while upper branches provide roosting and nesting sites for birds. In salt marshes, mussels live among cordgrass roots, while snails, fiddler crabs, oysters, and clapper rails live among plant stalks. When water covers cordgrass at high tide, plant stalks shelter small fish, crabs, and shrimp seeking refuge from large predators.

The Value of Wetlands

The amount of plant material produced in wetlands is higher than that produced in most aquatic and terrestrial habitats. This large amount of plant material supports an abundance of animal life, including commercially important species such as crayfish, ducks, fish, muskrat, shrimp, and crabs.

The biotic value of wetlands is well recognized, but it represents only a part of their total value. Wetlands provide “services” for other areas that often go unrecognized. For example, freshwater wetlands are capable of storing large amounts of water during periods of heavy rainfall. This capability can be important in minimizing the impact of flooding downstream. Saltwater wetlands along the coast are an effective barrier against storms and hurricanes. These natural barriers hold back the force of winds, waves, and storm surges while protecting inland areas. Wetlands are also capable of increasing water quality through the trapping of sediment, uptake of nutrients, and retention of heavy metals. Sediment trapping occurs when moving water is slowed enough by grass and trees to allow suspended sediment particles to settle. Wetland plants take up nutrients, such as nitrates and phosphates, from agricultural runoff and sewage. For this reason, wetlands are used as a final treatment step for domestic sewage from some small cities. Wetland soils are capable of binding heavy metals, effectively removing these toxic materials from the water.

Wetlands Loss and Preservation

It is estimated that the United States once contained more than 80 million hectares of wetlands. Less than half this amount remains today. Once considered wastelands, wetlands were prime targets for “improvement.” Extensive areas of freshwater wetlands and prairie potholes have been drained and filled for agricultural development. Saltwater wetlands have been replaced by urban or residential development and covered with dredge spoil. Wetlands loss rates have slowed, but an estimated 25,000 hectares continue to be lost each year in the United States. The loss of wetlands habitat threatens the survival of a number of animal species, including the whooping crane, American crocodile, Florida panther, manatee, Houston toad, snail kite, and wood stork.

Beginning in the 1970s the rate of wetlands loss slowed for several reasons. One was the passage of federal and state laws designed to protect wetlands; another was the efforts of organizations. At the federal level, the single most effective tool for wetlands is Section 404 of the Clean Water Act. Section 404 requires that a permit be issued before the release of dredge or fill material into U.S. waters, including wetlands. At the state level, Section 401 of the Clean Water Act, allows states to restrict the release of dredge or fill material into wetlands. There are several conservation organizations that support wetlands preservation, including Ducks Unlimited, the National Audubon Society, the National Wildlife Federation, and the Nature Conservancy. These organizations keep the public informed regarding wetlands issues and are active in wetlands acquisition.

In 2024, the U.S. Fish and Wildlife Service issued a report to Congress. This report showed that the lower 48 states have lost fifty percent of their wetlands since the 1780s. Additionally, the report showed that the United State's longstanding goal of "no net loss of wetlands" had failed. Experts expect this trend to continue, particularly in regards to the continued reductions in wetlands protection that occurred in the early 2020s.

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