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Wetlands - Many Roles Of Wetlands

water plants waterfowl fish

Wetlands provide essential ecological functions that benefit people and the ecological systems surrounding the wetlands, as well as the wetland itself. The plants, microbes, and animals in wetlands are part of the global cycles for water, nitrogen, and sulfur. Wetlands also store carbon in their plant communities and soils instead of releasing it into the air as carbon dioxide, making them part of the global cycle for carbon.

Wetland functions fit into several broad categories (see Figure 7.2 and discussion below):

  • High plant productivity
  • Temporary water storage
  • Trapping of nutrients and sediments
  • Soil anchoring

FIGURE 7.2
Ecological value of wetland processes
SOURCE: "Figure 4. Relationship between Wetland Processes and Values," in Wetlands: Their Use and Regulation, U.S. Congress, Office of Technology Assessment, 1984

Not all wetlands perform all functions, nor do they perform all functions equally. The location of the wetland in the watershed and its size determine its functions. A watershed is the land area that drains to a stream, river, or lake. Other factors that will affect wetland functions and their performance are weather conditions, quality and quantity of water entering the wetland, and human alteration of the wetland or the land surrounding it. The values of wetland functions to human communities depend on the complex relationship between the wetland and the other ecosystems in the watershed. An ecosystem consists of all the organisms in a particular area or region and the environment in which they live. The elements of an ecosystem all interact with each other in some way and depend on each other either directly or indirectly.

Wetlands—Nursery, Pantry, and Way Station

Wetlands are diverse and rich ecosystems, which provide food and shelter to many different plants and animals. The combination of shallow water, high nutrient levels, and primary productivity (plant growth and reproduction) is perfect for the development of organisms that form the base of the food chain. The water, dense plants, their root mats, and decaying vegetation are food and shelter for the eggs, larvae, and juveniles of many species. Smaller animals avoid predators by hiding among the vegetation while they wait to prey on still smaller life forms. Fish of all sizes seek the warmer, shallow waters to mate and spawn, leaving their young to grow on the rich diet provided by wetlands. Food and organic material that is flushed out of wetlands and into streams and rivers during periods of high flow feed downstream aquatic systems, including commercial and sport fisheries.

Estuarine marshes, for example, are among the most productive natural ecosystems in the world. They produce huge amounts of plant leaves and stems that make up the base of the food chain. When the plants die, they break down in the water and form detritus. Algae that FIGURE 7.3
Coastal wetlands produce detritus that support fish and shellfish
SOURCE: "Figure 6-8. Coastal Wetlands Produce Detritus That Support Fish and Shellfish," in National Water Quality Inventory: 1998 Report to Congress, U.S. Environmental Protection Agency, June 2000
grow on plants and detritus are the principal foods for shellfish such as oysters and clams, crustaceans such as crabs and shrimp, and small fish. Small fish are the food for larger commercial species such as striped bass and bluefish. (See Figure 7.3.) According to the EPA's Functions and Values of Wetlands (March 2002), 75% of commercially harvested fish are wetland-dependent. When shellfish species are factored in, the number increases to 95%.

Both estuarine and palustrine wetlands also serve as way stations for migrating birds. The Central Flyway extending from south-central Canada through the north-central United States and into Mexico, for example, provides resting places and nourishment for more than 400 of some 800 species of protected migratory birds (which individually number in the millions) during the migration season. Without this stopover area, the flight to their Arctic breeding grounds would be impossible. Chesapeake Bay with its extensive tidal and freshwater marshes on the East Coast Atlantic Flyway gives winter refuge to thousands of ducks and geese.

Wetland Biodiversity

Wetlands are the source of numerous natural products, including furs, fish and shellfish, timber, wild-life, and wild rice. A wide variety of species of microbes, plants, insects, amphibians, reptiles, fish, birds, and other animals make their homes in or around wetlands because of the availability of water. For others, wetlands provide important temporary seasonal habitats. Physical and chemical features such as landscape shape (topology), climates, and abundance of water help determine which species live in which wetland.

According to the EPA's Wetlands and People (http://www.epa.gov/owow/wetlands/vital/people.html, March 23, 2005), more than one-third of the United States' threatened and endangered species live only in wetlands, and nearly half use wetlands at some point in their lives. When wetlands are removed from a watershed or are damaged by human activity, the biological health of the watershed declines. Many species of plants and animals are lost to the watershed or decline in number. The EPA estimated that approximately two-thirds of freshwater mussels (67%) and crayfish (65%) were rare or imperiled and more than one-third of freshwater fish (37%) and amphibians (35%) dependent on aquatic and wetland habitats were at risk. Nearly one-fifth (18%) of dragonflies and plants were at risk. Forty-six percent of the threatened and endangered species listed by the USFWS rely directly or indirectly on wetlands for survival. All (100%) amphibians, fish, clams, and crustaceans listed as threatened or endangered rely directly or indirectly on wetlands for their survival.

According to the Division of Bird Habitat Conservation of the United States Fish and Wildlife Service, waterfowl remain the most prominent and economically important group of migratory birds on the North American continent. By 1985 (when waterfowl populations had decreased to record lows) approximately 3.2 million people were spending nearly $1 billion annually to hunt waterfowl. Interest in waterfowl and other migratory birds also had expanded in other areas. About 18.6 million people observed, photographed, and otherwise appreciated waterfowl and spent $2 billion on waterfowl-related activities.

Numbers of people who regularly engage in wildlife watching activities are even larger. According to the 2001 National Survey of Fishing, Hunting, and Wildlife-Associated Recreation, which is conducted every five years, in 2001 some sixty-six million people spent $38.4 billion on wildlife-watching activities.

The well-being of waterfowl populations is tied directly to the status and abundance of wetland habitats. Populations of ducks in North America dropped from 1955 through 1993, primarily because of declining wetland acreage. New wetland protection measures, however, are beginning to help reverse the trend. Under the Conservation Reserve Program (CRP) during the period 1986 to 1990, farmers enrolled 8.2 million acres of cropland within the Prairie Pothole Region, a vast area of the north-central United States and Canada from which almost 70% of North America's ducks originate. Nearly 13,000 square miles (an area approximately the size of Maryland) was converted to dense nesting cover under the CRP.

In the early 1990s, when the prolonged drought in the northern Great Plains ended, prairie potholes that had been dry for more than ten years filled with water. The prairie came to life and great numbers of waterfowl occupied the potholes. Surrounded by the CRP grass, nesting ducks were no longer easy targets for predators. Prior to the CRP, nest success of only 10% was common, that is, the rate of successful breeding was exceeded by the waterfowl mortality rate. With FIGURE 7.4
How wetlands improve water quality
SOURCE: "Figure 5-3. Water Quality Improvement Functions in Wetlands," in 2000 National Water Quality Inventory, U.S. Environmental Protection Agency, August 2002, http://www.epa.gov/305b/2000report/ (accessed May 5, 2005)
abundant grass and wetlands, spring survey numbers of ducks began to climb. In 1993, 26.3 million ducks were reported in the area, 32.5 million in 1994, and 36.9 million in 1995. Studies by the USFWS and Ducks Unlimited, Inc., a hunting and conservation group dedicated to protecting ducks and their habitat, showed that the CRP has tripled nest success throughout the Prairie Pothole Region.

Additional measures to preserve and protect the waterfowl population include the North American Waterfowl Management Plan. A joint strategy adopted by the governments of the United States, Canada, and Mexico, the Plan established an international committee with six representatives from each of the three countries. Its purpose is to provide a forum for discussion of major, long-term international waterfowl issues and to make recommendations to directors of the three countries' national wildlife agencies. It approves the formation of Joint Venture partnerships and reviews and approves Joint Venture implementation and evaluation plans. The Committee is responsible for updating the Plan, considering new scientific information and national and international policy developments, and for identifying the need to expand or diminish activities carried out on behalf of the Plan.

Water Storage

Wetlands function like sponges, absorbing water. By temporarily storing runoff and flood waters, wetlands help protect adjacent and downstream property owners from flood damage. Wetland plants slow the flow of water, which contributes to the wetland's ability to store it. The combined effects of storing and conveying (in this case, to carry and slow the flow) permit water to percolate through the soil into groundwater recharging aquifers, and to move through the watershed with less speed and force.

Wetlands are particularly valuable in urban areas because the paved and other impermeable surfaces shed water, increasing the rate, velocity, and volume of runoff so that the risk of flood damage increases. Loss or degradation of wetlands indirectly intensifies the flooding by eliminating their ability to absorb the peak flows and gradually release floodwaters, thereby helping to maintain stream flow, particularly at times of runoff or low flow.

Nutrient and Sediment Control

Figure 7.4 shows how wetlands improve the quality of water. Wetlands are natural filters that cleanse water. When water is stored or slowed down in a wetland by the plants and root masses that grow there, sediment settles out and remains in the wetland so that the water leaving the area is much less cloudy than the water that entered. The loss of cloudiness or turbidity has important consequences for both human health and the ecological health of the watershed. Turbidity has been implicated in disease outbreaks in drinking water. Turbid water bearing silt has been responsible for smothering plants and animals in rivers, streams, estuaries, and lakes.

Wetlands can also trap nutrients (phosphorous and nitrogen) that are dissolved in the water or attached to the sediment. Nutrients are either stored in the wetland soil or used by the plants to enhance growth. Studies in the Chesapeake Bay watershed, for example, have shown that some forested streamside wetlands are capable of removing 80% of the phosphorous and 90% of the nitrogen from water.

Too much nutrient, such as silt, reaching rivers, streams, lakes, and reservoirs can affect both human and ecological health. Too much nitrogen in drinking water can cause "blue baby syndrome" in infants and young livestock. Too much nutrient can cause eutrophication (depletion of dissolved oxygen by aquatic plant growth) in estuaries, lakes, rivers, and streams.

Soil Anchoring

Wetlands also play an important role in soil anchoring. The thick mesh of wetland vegetation and roots acts like a net and helps to hold soil in place even during periods of relatively high water flow such as a major storm. If the wetlands lining a stream or river are removed, this leads to poorly anchored soil and an increased water flow to carry it away. The result can be severe erosion and changes to the contours of channels, making them deeper and flatter. As a result, aquatic communities at the erosion location are disrupted or eliminated, and downstream aquatic systems are damaged by silt.

Marsh plant fringes in lakes, estuaries, and oceans protect shorelines from erosion in a similar fashion. The plants reduce soil erosion by binding the soil in their root masses as a function of anchoring the marsh. At the same time, the plants and root masses cushion the force of wave action, retarding scouring of shorelines.

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