Wetlands

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Wetlands

Wetlands are low-lying, depressional ecosystems that are permanently or periodically saturated with water at or close to the surface. The vegetation of wetlands must be adapted to the physical and chemical stresses associated with flooded substrates. The most common types of wetlands are swamps, marshes, shallow open waters, and mires, the latter consisting of peat-accumulating fens and bogs. Wetlands vary greatly in their productivity, mostly because of intrinsic differences in the rate of supply of nutrients . 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 nutrient loading, which causes eutrophication , and 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.

Types of wetlands

Wetlands can be characterized on the basis of their hydrology , morphology, water chemistry , and vegetation. All of these factors can vary regionally and locally, depending on the climate, character of the surrounding watershed , and the species that are present (that is, the biogeographic region). The major kinds of wetlands are described below, with an emphasis on North American types.

Swamps

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 , alluvial and floodplain swamp forests are sometimes extensive, and are typically dominated by such tree species as bald cypress (Taxodium distichum), water tupelo (Nyssa sylvatica), swamp tupelo (N. sylvatica), and eastern white cedar (Chamaecyparis thyoides). More northern temperate swamps are usually dominated by red maple (Acer rubrum), silver maple (Acer 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 used to support the now-extinct American ivory-billed woodpecker (Campephilus principalis principalis), and they still provide habitat for the pileated woodpecker (Dryocopus pileatus), red-shouldered 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).

Marshes

Marshes are a relatively productive wetland in which the vegetation is dominated by tall, emergent, graminoid (that is, grass-like) 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). Marshes dominated by these plants are relatively productive, because they have access to nutrients dissolved in their slowly flowing water. Wet meadows are less productive types of marshes, and are dominated by shorter graminoid plants such as sedges (Carex spp.) or a grass known as blue-joint (e.g., Calamagrostiscanadensis). Salt marshes are brackish because they are periodically inundated by oceanic water. Temperate salt marshes are dominated by species of cordgrass (e.g., Spartina alterniflora).

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, fringing marshes around bodies of open water, such as 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 (these are known as "dabbling" ducks), such as mallard (Anas platyrhynchos), pintail (Anas acuta), and blue-winged teal (Anas discors). Unfortunately, most prairie 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. Consequently, 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 in boreal and sub-arctic regions provide important breeding habitat for geese , especially snow goose (Chen caerulescens) and Canada goose (Branta canadensis). Historically, these migratory waterfowl wintered in extensive temperate marshes farther to the south, but now many of these birds spend much of the winter foraging for unharvested grain in agricultural fields.

Shallow open water

This is a heterogenous wetland type, transitional from deeper open-water habitats such as lakes, and more completely vegetated wetlands such as marshes. 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, and free of emergent plants, but often having floating-leaved vegetation. These wetlands and their fringing marshes can support relatively large populations of waterfowl, amphibians, and other animals. For example, the open-water portions of prairie potholes provide habitat for populations of diving ducks, such as lesser scaup (Aythya affinis), canvasback (Aythya valisneria), and redhead (Aythya americana), along with coot (Fulica americana), western grebe (Aechmorphorus occidentalis), and other species of birds. Beaver (Castor canadensis) can also be abundant in shallow, open-water wetlands, and in fact they often create such habitat by damming streams.

Fens

Fens are a type of mire, or a peat-accumulating wetland, and are most commonly found in boreal and subarctic regions. Because surface water percolates slowly through fens (that is, they are minerotrophic), they are relatively well supplied with nutrients and alkalinity, and are only slightly acidic. Consequently, fens are relatively productive, and they are non-acidic or only slightly acidic. Fens have a plant community dominated by short-statured graminoids, especially sedges and rushes , some shrubs, and species of peat-moss (Sphagnum spp.) that do not prefer acidic habitats. Because they are not very productive, fens support relatively small populations of animals, although their associated open-water habitat may be used by breeding ducks, geese, and cranes .

Bogs

Bogs are another type of peat-accumulating wetland, but they only receive very small inputs of nutrients and alkalinity, entirely from atmospheric deposition associated with rain, snow, and particulates (that is, bogs are ombrotrophic, or "fed from the clouds"). As a result of their nutrient-poor status, bogs are highly unproductive and acidic, with a pH less than about 4.5 and as low as 3.5. The surface of the most ombrotrophic bogs is often raised above the level of the surrounding terrain, occurring as blanket- or raised-bogs, which can have peat depths of more than 30-50 ft (10-15 m). The vegetation of bogs is typically dominated by acid-loving species of peat-moss (these are known as acidophilous species, and they are different Sphagnum spp. than the ones found in fens), along with various shrub species in the heath family (Ericaceae) . Because they are so acidic and unproductive, not many animals breed in bogs or their associated, brown-water pools.

Wetland ecology

Wetlands are dynamic ecosystems, transitional between terrestrial and aquatic habitats. Over time , most wetlands gradually in-fill because of the cumulative deposition of sediment and peat. Consequently, wetlands are most numerous in places where geological forces, such as glaciation or the migration of oxbow rivers, periodically create conditions that are favorable to their formation.

The ecological conditions of wetlands are, of course, dominated by the influences of permanent or
temporary waterlogging. Clearly, the availability of water to sustain plant growth is not a problem in wetlands, as it is in many terrestrial ecosystems. 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. Many herbaceous plants, such as cattails and bulrushes, have spongy, air-filled stem and root tissues, called aerenchyma, which helps to facilitate the transport of oxygen to underwater tissues. Some trees, such as bald cypress and black mangrove (Avicennia nitida), have specialized woody structures called pneumatophores, that extend from roots into the air, and have extensive intercellular spaces that are useful in supplying oxygen to below-water tissues.

The anaerobic 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 assimilate, 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, of course, a highly variable character. Some wetlands are permanently flooded, while others are only waterlogged some of the time, usually seasonally. These dynamics are highly influential on the types of plants that can occur 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 transitional 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.

Losses of wetlands

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 sub-arctic regions of northern North America, these types of wetlands are uncommon farther south, where they usually occur as relict, post-glacial ecosystems. 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. As such, any proposals to "develop" these wetlands into agricultural or urbanized lands are highly controversial, because these conversions would cause an irretrievable loss of natural values.

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 required 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, and this also contributes to their value as an ecological resource.

Unfortunately, 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, which can only be resolved 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.

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, non-governmental organizations such as the World Wildlife Fund, The Nature Conservancy, the 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 activities of all of these agencies are important and useful, but much more needs to be done to give wetlands and their species the degree of protection that they require.

Resources

books

Barbour, M.G., and W.D. Billings. North American Terrestrial Vegetation. Cambridge: Cambridge University Press, 1988.

Barbour, M.G., J.H. Burk, and W.D. Pitts. Terrestrial Plant Ecology, 2nd ed. Ontario: Benjamin/Cummings Pub. Co., Don Mills, 1987.

Finlayson, M., and M. Moser. Wetlands. New York: Facts on File, 1991.

Hamblin, W.K., and E.H. Christiansen. Earth's Dynamic Systems. 9th ed. Upper Saddle River: Prentice Hall, 2001.

Hancock, P. L., and B. J. Skinner, eds. The Oxford Companion to the Earth. Oxford: Oxford University Press, 2000.

Mitsch, W.J., and J.G. Gosselink. Wetlands. John Wiley and Sons, 1997

Sobel, Jack. Marine Reserves: A Guide to Science, Design, and Use. Washington, DC: Island Press, 2003.

Vileisis, A. Discovering the Unknown Landscape: A History of America's Wetlands. Island Press, 1997

periodicals

"Satellite Remote Sensing of Wetlands." Wetlands Ecology And Management 10, no. 5-5 (2002): 381-402.

Bill Freedman

KEY TERMS

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Alkalinity: —The amount of alkali in a solution. In fresh water, alkalinity is mainly associated with bicarbonates, carbonates, and hydroxides, and it is generally measured by titration with acid to a fixed end point.
Anaerobic: —Environments in which oxygen is not present, or only present in a very small concentration.
Eutrophication: —An aquatic ecosystem process by which increased productivity results from an increase in the rate of nutrient input. Excessive eutrophication and its symptoms are regarded as a type of ecological degradation.
Graminoid: —A generic term for plants with a grass-like growth form, such as grasses (Poaceae), sedges (Cyperaceae), rushes (Juncaceae), and cattails (Typhaceae).
Hydrology: —The study of the distribution, movement, and physical-chemical properties of water in Earth's atmosphere, surface, and near-surface crust.
Minerotrophic: —This refers to wetlands that receive much of their nutrient supply as substances dissolved in water draining from a part of the watershed that is higher in altitude.
Ombrotrophic: —This refers to wetlands with no input of nutrients from ground water or surface water, so that all of the nutrient supply arrives from the atmosphere with precipitation and dust.
Watershed: —The expanse of terrain from which water flows into a wetland, waterbody, or stream.