Wetlands are low-lying ecosystems that are permanently or periodically saturated with water at or close to the surface. The vegetation of wetlands is adapted to survival under flooded conditions. The most common types of wetlands are swamps, marshes, shallow open waters, and mires (which contain peat-accumulating fens and bogs).

Wetlands provide important habitat for a wide variety of plants and animals. However, wetlands are rapidly disappearing because they are being drained and in-filled for agricultural, urbanization, and industrial purposes. Wetlands are also being degraded by the influx of excessive nutrients; the resulting explosive growth of microorganisms depletes the oxygen from the water (eutrophication). Wetlands are also being degraded by pollution associated with inputs of toxic chemicals and organic materials. Losses of wetlands and the biodiversity that they support are an extremely important aspect of the environmental crisis.

Swamps are forested or shrub-dominated wetlands, usually associated with low-lying, periodically or permanently flooded areas around streams and rivers. Water flows through swamps, although the movement can sometimes be imperceptible. In southeastern North America, swamp forests can be extensive and are typically dominated by tree species such as bald cypress (Taxodium distichum), water tupelo (Nyssa sylvatica), swamp tupelo (N. sylvatica), and eastern white cedar (Chamaecyparis thyoides). More northern swamps are usually dominated by red maple (Acer rubrum), silver maple (A. saccharinum), American elm (Ulmus americana), and green or swamp ash (Fraxinus pennsylvanica). Freshwater tropical swamps can support much more diverse species of angiosperm trees, while tropical mangrove swamps support only a few tree species that are tolerant of the brackish water.

Swamps provide habitat for numerous species of animals, many of which have a specific requirement for this type of habitat. For example, swamps of bald cypress in southeastern North America provide habitat for the pileated woodpecker (Dryocopus pileatus), redshouldered hawk (Buteo lineatus), prothonotary warbler (Protonotaria citrea), Carolina wren (Thryothorus ludovicianus), and many other small birds. These swamps also provide nesting habitat for wood duck (Aix sponsa) and for colonies of wading birds such as herons and egrets (e.g., great blue heron, Ardea herodias, and common egret, Casmerodius albus), and wood stork (Mycteria americana). Cypress swamps also support mammals, including swamp rabbit (Sylvilagus aquaticus), white-tailed deer (Odocoileus virginianus), and panther (Felis concolor), along with many species of amphibians and reptiles, including the American alligator (Alligator mississippiensis).

A marsh is a wetland in which the vegetation is dominated by tall, grasslike plants. Typical plants of North American marshes include cattails (e.g., Typha latifolia), reeds (e.g., Phragmites communis), bulrushes (e.g., Scirpus validus), and saw grass (Cladium jamaicense).

Wetlands support a diversity of wildlife and plantlife. In coastal areas, they also serve as protection against storms and flooding. (NPS/Patrick Myers) (NPS/Patrick Myers)

Because they are rather productive, marshes can support relatively large populations of certain mammals, such as muskrat (Ondatra zibethicus). Birds can also be abundant in marshes. This is true of large, extensive marshes, and also of relatively small marshes around lakes and ponds. For example, small ponds are common in the prairies of North America where they are called potholes. The marshy borders of potholes have historically provided important breeding habitat for most of the continent's surface-feeding ducks. However, most potholes have been drained or filled to provide land for agriculture. This ecological conversion has increased the importance of the remaining potholes as habitat for declining populations of ducks, other animals, and native plants. Further losses of this habitat type are vigorously resisted by the conservation community, even though agricultural interests continue to encourage the drainage of these important wetlands.

Farther to the north, extensive salt marshes and freshwater fringing marshes provide important breeding habitat for geese, especially the snow goose (Chen caerulescens) and Canada goose (Branta canadensis).

Shallow, open-water wetlands are known locally by names such as ponds, sloughs, and potholes. These are small bodies of surface water, less than about 7 ft (2 m) in depth that often contain floating-leaved vegetation. These wetlands and their fringing marshes can support relatively large populations of waterfowl, amphibians, and other animals.

Wetlands are dynamic ecosystems, transitional between terrestrial and aquatic habitats. Over time, most wetlands gradually fill in because of the ongoing deposition of sediment and peat. Consequently, wetlands are most numerous in places where geological forces such Page 4780  |  Top of Articleas glaciation periodically create conditions that are favorable to their formation.

The ecological conditions of wetlands are dominated by the influences of permanent or temporary waterlogging (the saturation of soil with water). The availability of water to sustain plant growth is not a problem in wetlands. However, waterlogged soil or sediment are usually lacking in oxygen, a factor that inhibits respiration by plant roots. To cope with this stressful environmental condition, some plants have evolved specific adaptations to supply oxygen to their roots. For example, cattails and bulrushes have spongy, air-filled stem and root tissues (aerenchyma), which helps in the transport of oxygen to underwater tissues. Some trees, such as bald cypress and black mangrove, have specialized woody structures called pneumatophores that extend from roots into the air, and they have extensive intercellular spaces that are useful in supplying oxygen to below-water tissues.

The anaerobic (oxygen-free) nature of wetland substrates also causes other chemical changes that can pose important problems for plants, by affecting their nutrition and exposing roots to toxic chemicals. For example, access to certain nutrients can be difficult under anaerobic conditions. This is because the nutrients may not be present in a chemical form that is easy for roots to take up, or because roots cannot sustain the oxygen-demanding respiratory demands required for the active uptake of nutrient ions. Anaerobic conditions also encourage the solubilization of certain potentially toxic metals, such as manganese. In addition, anaerobic metabolism within root tissues can lead to excessive accumulations of alcohols, possibly causing toxicity. In general, wetland plants are well adapted to these conditions, although they may nevertheless be physiologically stressed if these factors are severe enough.

Wetland hydrology is highly variable. Some wetlands are permanently flooded while others are only waterlogged some of the time, usually seasonally. These dynamics greatly influence the types of plants that can exist in particular wetlands and on the communities that they develop. Tolerance of permanent or frequent flooding, as occurs, for example, in salt marshes, mangroves, and some swamps, requires highly adapted species of plants. In comparison, wetlands that are only occasionally flooded are, in some respects, a more ephemeral transition between truly aquatic and terrestrial environments. In these situations, plants must only be tolerant of the stresses of sporadic events of flooding, while growing relatively freely when the water recedes and the soil is drier.

Another highly influential environmental factor in wetlands is the supply of plant nutrients. In general, wetlands that are well supplied with phosphorus (in the form of phosphate), and to a lesser degree nitrogen (as nitrate or ammonium), sustain relatively high rates of plant productivity and, consequently, large populations of animals. This is commonly the case for marshes, which are among the most productive natural ecosystems on Earth. In contrast, wetlands with restricted supplies of nutrients, such as ombrotrophic bogs, sustain only small productivities of plants and animals.

All wetlands have great intrinsic value as natural ecosystems, and they all support species of plants and animals that occur nowhere else. Consequently, wetlands have great value in terms of biodiversity.

Sometimes, the biodiversity-related importance of particular types of wetlands is a matter of their relative abundance, in the regional context. For example, although bogs and fens can be extremely abundant in boreal and subarctic regions of northern North America, these types of wetlands are uncommon farther south. In these southern regions, the few bogs and fens that occur have great conservation value as scarce and unusual ecosystems and because most of their species of plants and arthropods are regionally or locally rare. Any proposals to convert these wetlands into agricultural or urbanized lands are usually controversial because these conversions would cause an irretrievable loss of natural values.

One of the largest wetlands in the U.S. is the Florida Everglades, which has been named a World Heritage Site and is also the largest tropical wilderness in the U.S. (NPS) (NPS)

Wetlands also provide essential habitat for species of birds and mammals that are hunted, and this gives them economic value. Waterfowl such as ducks and geese occur primarily in marshes and swamps. During the past century, the populations of some of these hunted waterfowl were greatly decreased as a combined effect of overhunting and loss of natural habitats. Consequently, there are now substantial efforts to regulate hunting and to preserve or enhance the marshes and swamps that are Page 4781  |  Top of Articlerequired as habitat by these birds. Some species of waterfowl are responding well to these conservation measures, and their populations are increasing.

Wetlands are also important because they offer other ecological goods and services, in addition to those previously described. For example, wetlands maintain some control over hydrology, helping to prevent extremes of water flow. This service moderates the risks of flooding caused by heavy rain or the spring flush of snowmelt in northern regions. It also helps to extend supplies of water for drinking or irrigation longer into the drier seasons of the year. Wetlands also provide important services by cleansing the water that flows through them of pollutants, including nutrients and toxic chemicals, such as metals and certain pesticides. Furthermore, wetlands are useful in protecting shorelines from erosion, controlling sedimentation, and providing essential habitat for fish, birds, and other wildlife. Wetlands have good aesthetics, which contributes to their value as an ecological resource.

However, wetlands are being rapidly lost in most of the world. The most important causes of the destruction of wetlands are drainage and in-filling to provide dry land for agriculture, urbanization, and industrialization. Wetlands are also sometimes used as convenient places for the disposal of mine tailings, municipal solid wastes, and sewage. In some cases, wetlands are degraded or lost because economically useful products can be mined from them, especially peat from bogs, and wood from forested swamps. Wetlands are also degraded if they are subjected to large inputs of nutrients through the runoff of agricultural fertilizers or by sewage dumping. These nutrient inputs can cause eutrophication, with a consequent loss of the original ecological values of the wetland.

All of these disturbances, stresses, and ecological conversions result in net losses of wetlands. The ecological consequences include endangerment of natural wetland ecosystems, endangerment of their species of plants and animals, and the loss of many important services that wetlands can provide. The loss of wetlands is an important environmental issue that can be resolved only by protection of those wetlands that still survive, and in some areas where the losses have been especially severe, by the active restoration of wetlands. In a 2012 study, a researcher from the University of California, Berkeley, analyzed wetland restoration projects and determined that restored wetlands do not achieve the same qualities of original wetlands even after 100 years. Restored wetlands have significantly lower biodiversity among plants, animals, and insects than original wetlands. Overall, restored wetlands are, on average, 25 percent less productive than original wetlands.

In 2020, wetlands and the ecosystems that they support continue to be under threat from a variety of factors. Wetlands are considered natural filters for the environment, taking in pollutants and either absorbing them or using them as nutrients for the plants and animals that live in the wetland. However, that system can be easily overwhelmed if too much pollution is in the water, and the plants and animals will begin to die. Many industrial discharges, sewage, and even household wastes are dumped into wetlands. Aside from direct human-made impacts on the wetlands, people are also indirectly destroying wetland habitats with global climate change. Due to rising sea levels, coastal wetlands are being drowned beneath the ocean waters; due to extreme droughts in other areas, some wetlands are drying up.

The protection and conservation of wetlands is an important activity of many governments and private organizations. In the United States and Canada, wetlands are among the highest-priority natural habitats for protection by governments at all levels (national, state or provincial, and local). In addition, nongovernmental organizations such as the World Wildlife Fund, Nature Conservancy, Nature Conservancy of Canada, and Ducks Unlimited have made the conservation and protection of wetlands a high priority in their activities. Internationally, the Convention on Wetlands of International Importance, Especially as Waterfowl Habitat (also known as the Ramsar Convention, after the city in Iran where it was negotiated) is an intergovernmental treaty that provides a framework for worldwide cooperation in the conservation of wetlands. The main protections in the United States fall under the Migratory Bird Act, the Swampbuster provision in the Food Security Act, and the Clean Water Act.