Wetlands

Wetlands are habitats characterized by saturated (waterlogged) soils for at least part of the year and plants that are adapted to grow under wet conditions. They may be completely covered by water or the water may be just below the ground. There are many different types of wetlands, such as swamps (wetlands dominated by trees), marshes (wetlands dominated by nonwoody plants such as grasses and sedges), wet meadows, bogs, fens, flood-plain forests, lakes, and ponds.

Wetlands are to a large extent the product of the topography of the land. They develop wherever there is a depression in the land that brings the water table (groundwater) close to or even above ground. The type of wetland that will develop in a particular area depends on the rate of water flow, the length of the season of soil saturation, latitude (polar versus temperate versus tropical), proximity to the coast (marine versus freshwater wet-lands), and the surrounding geology.

Plant Adaptations to Wetlands

It is challenging for a plants to grow in constantly damp conditions. The saturated soil contains little or no oxygen compared to upland soil, therefore the roots of wetland plants require special adaptations to enable them to survive. Water lilies have air channels that run from their leaves, which are in constant contact with the air, to their roots under water. Water lilies also have their stomata only on the upper surface of their leaves (most plants have stomata on the lower surface) so that water can not enter when these pores open to allow carbon dioxide in for photosynthesis. Salt marsh grasses (Spartina species) also transport oxygen to their roots, where it may be excreted into the surrounding soil and create a small oxygenated zone around their roots. Plants that grow completely underwater, such as seagrasses and pondweeds, use the oxygen created as a by-product of photosynthesis to aerate their roots. Most wetland plants have also adapted to wet conditions through changes in their metabolism. Non-wetland plants, for example, typically produce an alcohol (ethanol) as a breakdown product of sugar metabolism when the soil is saturated with water. Ethanol is toxic to plants. Wetland plants have different enzymes that prevent the formation of the alcohol.

Although only a limited number of species can thrive in the constantly saturated soils of wetlands, those plants that have adapted are often extremely productive. As anyone who has planted a garden knows, one of the major factors limiting the growth of terrestrial plants is water. Having adapted to life in constantly damp conditions, wetland plants never have to worry about getting water. As a result, their growth rates can be very high.

The Value of Wetlands to Humans

Wetlands are very important features in the landscape and provide humans with some tangible benefits. They act like a sponge helping to reduce the impacts of floods by absorbing water and serving as a reservoir for groundwater. As water flows through a wetland, pollutants such as excess silt and harmful nutrients are trapped; thus, the wetland acts as a filter of pollutants and helps to maintain clean water. Wetlands serve as a vital habitat to many different species of wildlife, including many that are very rare and in danger of extinction.

The value of wetlands has not always been appreciated. A conservative estimate is that over 30 percent of the original wetlands in the United States have been lost forever. These were filled in the past to make way for farms, houses, highways, businesses, and other human activity. Since the early 1970s, the attitude toward wetlands has changed. Not only are there now strong efforts in most states to protect the remaining wetlands, many environmental agencies and land conservation groups are working to restore wetlands damaged by past human activities.

Current Threats to Wetlands

Even with a greater sense of the value of wetlands among much of the public, there are still pressures on these habitats. The ability of wetlands to

GROWTH HABITS OF WETLAND PLANT		S
Growth Habit	Examples
Completely submerged	Sea grasses, pondweeds, water plantain, water milfoil, elodea
Floating plants, unrooted in substrate	Duckweeds, bladderworts, water hyacinth
Floating leaves, rooted in substrate	Water lilies, lotus, floating hearts, water chestnut
Emergent perennials: roots in substrate under water, leaves and stems above water	Cattails, common reed, purple loosestrife, salt marsh grasses, tule, saw grass, wild rice
Emergent shrubs	Buttonbush, alders, leather leaf, sweet gale
Trees: constantly submerged	Mangroves, bald cypress, black spruce
Trees of floodplains: tolerate periodic flooding	Cottonwood, willow, silver maple, black ash

absorb pollutants is not unlimited. Excessive amounts of pollution entering a wetland over a long period of time is likely to cause long-term changes in the wetland. One of the world's most famous wetlands, the Everglades of southern Florida, has suffered for years from pollution from fertilizers used by farms upstream from it. The pollution has resulted in some major changes in the plant community and suspected declines in the diversity of animals it supports.

Another major threat to wetlands is changes in hydrology (the flow of water). Water is the lifeblood of wetlands. If too much water is removed for human consumption or to irrigate cropland, the wetland may be degraded into a less-valuable habitat or even disappear completely. The Florida Everglades has to compete with the farms and rapidly growing cities of southern Florida for this precious resource and has suffered as a result. Not only is the quantity of water important to maintaining wetlands, but so is the timing. Many wetlands depend on seasonal flooding followed by a dry period. This type of natural cycle may be altered by dams, which may hold back the water during the wet season.

Direct filling of wetlands, although less common than it was twenty years ago, still occurs, particularly with smaller wetlands that may be perceived as less valuable than larger ones. Some small wetland types contain rare species of animals precisely because they are too small and temporary in existence to support fish predators, pointing out that size is not always a good indicator of value.

Finally, many wetlands throughout the world are threatened by invasions of nonnative plant species. Purple loosestrife, a European garden plant, has taken over many freshwater marshes in the northeastern United States. West Coast salt marshes are threatened with being overrun by tall cord-grass, an East Coast salt marsh species. These are only two examples of a very widespread problem.

Robert Buchsbaum

Bibliography

Mitsch, William J., and James G. Gosselink. Wetlands, 3rd ed. New York: John Wiley & Sons, 2000.

Niering, William A., and Charles Elliot, eds. Wetlands. National Audubon Society Nature Guides, 1983.

Tiner, Ralph W. In Search of Swampland: A Wetland Sourcebook and Field Guide. New Brunswick, NJ: Rutgers University Press, 1998.