WHAT ARE WETLANDS?
Wetlands are transition zones between land and aquatic systems where the water table is usually near or at the surface, or the land is covered by shallow water. Wetlands range in size from less than one acre to thousands of acres and can take many forms, some of which are immediately recognizable as "wet." Other wetlands appear more like dry land, and are wet during only certain seasons of the year, or at several year intervals. The U.S. Army Corps of Engineers reports that most U.S. wetlands lack surface water and waterlogged soils during at least part of each growing season.
Some of the more commonly recognized types of wetlands are marshes, bogs, and swamps. Marshes are low-lying wetlands with grassy vegetation. Bogs are wetlands that accumulate wet, spongy, acidic, dead plant material called peat. Shrubs, mosses, and stunted trees may also grow in bogs. Swamps are low-lying wetlands that are seasonally flooded; they have more woody plants than marshes and better drainage than bogs.
According to Thomas E. Dahl, in Status and Trends of Wetlands in the Conterminous United States 1998 to 2004 (2006, http://wetlandsfws.er.usgs.gov/status_trends/national_reports/trends_2005_report.pdf), there were an estimated 107.7 million acres of wetlands in the forty-eight coterminous states in 2004. This constituted about 5.5% of the total land area, whereas deepwater (rivers and lakes) constituted 1% and upland ("dry" land) constituted most of the total land area at 93.5%. (See Figure 7.1.) Of the wetland areas, 95% were freshwater and 5% were estuarine (coastal saltwater). (See Figure 7.2.) Dahl does not include data on Alaska and Hawaii, but the U.S. Environmental Protection Agency (EPA) notes in "Wetlands: Status and Trends" (February 22, 2006, http://www.epa.gov/OWOW/wetlands/vital/status.html) that in the 1980s an estimated 170 to 200 million acres of wetlands existed in Alaska—covering slightly more than half of the state—and Hawaii had 52,000 acres.
Hydrology and Wetland Formation
Wetlands are distributed unevenly, but occur in every state and U.S. territory. (See Table 7.1.) They are found wherever climate and landscape cause groundwater to discharge to the land surface or prevent rapid drainage from the land surface so that soils are saturated for some time.
In wetlands, when the soil is flooded or saturated, the oxygen used by the microbes and other decomposers in the water is slowly replaced by oxygen in the air, because oxygen moves through water about ten thousand times slower than through air. Thus, all wetlands have one common trait: hydric (oxygen-poor) soils. As a result, plants that live in wetlands have genetic adaptations in which they are able to survive temporarily without oxygen in their roots, or they are able to transfer oxygen from the leaves or stem to the roots. This anaerobic (without oxygen) condition causes wetland soils to have the sulfurous odor of rotten eggs.
Local hydrology (the pattern of water flow through an area) is the primary determinant of wetlands. Wetlands can receive groundwater in-flow, recharge ground-water, or experience both inflow and outflow at different locations. Figure 7.3 illustrates water movement in several different wetland situations. Part A in Figure 7.3 shows how wetlands do not always occupy low points and depressions in the landscape. They can occur in flat areas that have complex underground water flow.
Part B in Figure 7.3 shows a fen, which is a type of wetland that accumulates peat deposits like bogs do. Fens, however, are less acidic than bogs and receive most of their water from groundwater rich in calcium and magnesium. As part B shows, fens occur on slopes at groundwater seepage faces and are subject to a continuous supply of the chemicals that are dissolved in the groundwater.
Land along the sides of streams or rivers receives a continuous water supply and is ideal for wetland growth. It may also receive some groundwater discharge, as shown in part C in Figure 7.3. Bogs, shown in part D in Figure 7.3, are wetlands normally found on uplands or extensive flatlands. Most of their water and chemistry comes from precipitation.
Riverine (areas along streams, rivers, and irrigation canals) and coastal area wetlands are highly subject to periodic water level changes. Coastal area wetlands, for example, are affected by predictable tidal cycles. Other coastal and riverine wetlands are highly dependent on flooding and seasonal water level changes. Some examples are the floodplains of the Illinois and Missouri rivers.
TYPES OF WETLANDS
A wide variety of wetlands exist across the United States because of regional and local differences in hydrology, water chemistry, vegetation, soils, topography, and other factors. There are two large groups of wetlands: estuarine (coastal) and freshwater (inland). (See Figure 7.2.) Estuarine wetlands are linked to estuaries and oceans and are places where fresh- and saltwater mix, such as a bay or where a river enters the ocean. In estuaries the environment is one of ever-changing salinity and temperature. The water level fluctuates in response to wind and tide. Examples of estuarine wetlands are saltwater marshes and mangrove swamps.
|Location of various wetland types|
|Wetland type||Primary regions||States|
|Source: "Table 3. Locations of Various Wetland Types in the United States," in Wetlands: Their Use and Regulation, U.S. Congress, Office of Technology Assessment, March 1984, http://govinfo.library.unt.edu/ota/Ota_4/DATA/1984/8433.PDF (accessed January 11, 2007)|
|Inland freshwater marsh||Dakota-Minnesota drift and lake bed; Upper Midwest; and Gulf Coastal Flats||North Dakota, South Dakota, Nebraska, Minnesota, Florida|
|Inland saline marshes||Intermontane; Pacific Mountains||Oregon, Nevada, Utah, California|
|Bogs||Upper Midwest; Gulf-Atlantic Rolling Plain; Gulf Coastal Flat; Atlantic Coastal Flats||Wisconsin, Minnesota, Michigan, Maine, Florida, North Carolina|
|Tundra||Central Highland and Basin; Arctic Lowland; and Pacific Mountains||Alaska|
|Shrub swamps||Upper Midwest; Gulf Coastal Flats||Minnesota, Wisconsin, Michigan, Florida, Georgia, South Carolina, North Carolina, Louisiana|
|Wooded swamps||Upper Midwest; Gulf Coastal Flats; Atlantic Coastal Flats; and Lower Mississippi Alluvial Plain||Minnesota, Wisconsin, Michigan, Florida, Georgia, South Georgia, South Carolina, North Carolina, Louisiana|
|Bottom land hardwood||Lower Mississippi Alluvial Plain; Atlantic Coastal Flats; Gulf-Atlantic Rolling Plain; and Gulf Coastal Flats||Louisiana, Mississippi, Arkansas, Missouri, Tennessee, Alabama, Florida, Georgia, South Carolina, North Carolina, Texas|
|Coastal salt marshes||Atlantic Coastal Zone; Gulf Coastal Zone; Eastern Highlands; Pacific Moutains||All Coastal states, but particularly the Mid- and South Atlantic and Gulf Coast states|
|Mangrove swamps||Gulf Coastal Zone||Florida and Louisiana|
|Tidal freshwater wetlands||Atlantic Coastal Zone and Flats; Gulf Coastal Zone and Flats||Louisiana, Texas, North Carolina, Virginia, Maryland, Delaware, New Jersey, Georgia, South Carolina|
The most common location of freshwater wetlands is the floodplains of rivers and streams, the margins of lakes and ponds, and isolated depressions surrounded by dry land. Some examples of inland wetlands are the Florida Everglades, wet meadows, swamps, fens, bogs, prairie potholes, playa lakes, and wet tundra.
Wetlands are further divided by their vegetation. Emergent wetlands (marshes and wet meadows) are dominated by grasses, sedges, and other herbaceous (non-woody) plants. Emergent wetlands account for 73% of estuarine wetlands, even though they represent only 25.5% of freshwater wetlands. (See Figure 7.2.) Shrub wetlands (including shrub swamps and bogs), which are characterized by low-to-medium-height woody plants, make up 13% of estuarine wetlands and account for 17% of freshwater wetlands. Forested wetlands, mostly wooded swamps and bottomland hardwood forests, are dominated by trees and account for 51% of freshwater wetlands. (Bottomland hardwood forests are generally found along the edges of lakes and rivers and in sinkholes.)
HOW WETLANDS FUNCTION
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 all key players in the water, nitrogen, carbon, and sulfur cycles.
Wetland functions fit into several broad categories:
- High plant productivity
- Temporary water storage
- Trapping of nutrients and sediments
- Soil anchoring
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 how it functions. (A watershed is the land area that drains to a stream, river, or lake.) Other factors that affect wetland function 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 relationships 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. (See Figure 7.4.)
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 organisms. Fish of all sizes seek the warmer, shallow waters to mate and spawn, leaving their young to grow on the rich diet provided by the wetlands. Food and organic material that is flushed out of wetlands and into streams and rivers during periods of high water 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, decomposers such as bacteria in the water break them down to detritus (small particles of organic material). Algae that 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 7.5.) The EPA states in Wetlands Functions and Values (March 19, 2007, http://www.epa.gov/watertrain/wetlands/module08.htm) that "the fish and shellfish that depend on wetlands for food or habitat constitute more than 75% of the commercial and 90% of the recreational harvest."
Both estuarine and freshwater 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 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.
Wetlands' Role in Biodiversity
Wetlands are the source of many natural products, including furs, fish and shellfish, timber, wildlife, 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.
In "Wetlands and People" (February 22, 2006, http://www.epa.gov/owow/wetlands/vital/people.html), the EPA notes that "more than one-third of the United States' threatened and endangered species live only in wetlands." When wetlands are removed from a watershed or are damaged by human activity, the biological health of the watershed declines. Wetland health has a commercial impact as well. Dahl indicates that 75% of the fish and shellfish commercially harvested in the United States and up to 90% of the recreational fish rely directly or indirectly on wetlands for their survival. Dahl also notes that in 2004, 72% of freshwater mussels were imperiled and 39% of native freshwater fish species were at risk of extinction.
Waterfowl are birds such as ducks and geese that spend much of their lives in wetlands, lakes, rivers, and streams. The well-being of waterfowl populations is tied directly to the status and abundance of wetland habitats. According to the U.S. Fish and Wildlife Service's (USFWS) Division of Bird Habitat Conservation (December 12, 2006, http://www.fws.gov/birdhabitat/NAWMP/index.shtm), waterfowl are the most well-known and economically important group of migratory birds in North America. By 1985 (when waterfowl populations had decreased to record lows) approximately 3.2 million people were spending nearly $1 billion annually to hunt waterfowl. In addition, about 18.6 million people spent $2 billion on "waterfowl-watching" activities, such as observing and photographing them.
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 the directors of the three countries' national wildlife agencies. The Division of Bird Habitat Conservation notes that as of the end of 2006, $4.5 billion had been invested under the plan to protect, restore, and/or enhance 15.7 million acres of waterfowl habitat. North American Waterfowl Management Plan projects also target all wetland-associated species in their conservation efforts.
Wetlands absorb water, much like sponges. 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 slowing the flow of water permit it to percolate through the soil into groundwater, which recharges aquifers, and to move through the watershed with less speed and force.
Wetlands are particularly valuable in urban areas because 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 flooding by eliminating absorption of the peak flows and gradual release of floodwaters.
Nutrient and Sediment Control
Figure 7.6 shows how wetlands improve the quality of water. Wetlands act like natural water filters. 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. Furthermore, 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. If too much nutrient material reaches rivers, streams, lakes, and reservoirs, it can cause eutrophication, resulting ultimately in the death of many aquatic organisms. (See Figure 6.12 in Chapter 6 and the related discussion.)
Wetlands also play an important role in soil anchoring. The thick mesh of wetland vegetation and roots acts like a net and helps hold soil in place even during periods of relatively high water flow. Removing wetland vegetation the lines a stream or river leads to poorly anchored soil and an increased water flow, which carries away the soil. 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. At the same time, the plants and root masses cushion the force of wave action, retarding scouring of shorelines.
ECONOMIC BENEFITS OF WETLANDS
Appreciation of the economic value of wetlands has undergone a dramatic change since the 1970s. Before that time, wetlands were considered useless, good only for taking up space and breeding mosquitoes. The emphasis was on filling and draining wetlands to turn them into productive land for development and agriculture. In the mid-1970s the growing environmental movement with its emphasis on clean water led to a closer examination of wetlands and their role in watersheds and the global ecosystem. Wetlands are now valued not only for their ecological role but also for their contribution to the economy.
Some of the most popular recreational activities, including fishing, hunting, and canoeing, occur in and are dependent on healthy wetlands. The EPA notes in the fact sheet "Economic Benefits of Wetlands" (February 22, 2006, http://www.epa.gov/owow/wetlands/facts/fact4.html) that "more than half of all U.S. adults (98 million people) hunt, fish, birdwatch, or photograph wildlife." In 1991 spending on these activities amounted to $59.5 billion. Figure 7.7 shows that 42% of Americans who observed, fed, or photographed wildlife on trips away from home in 2001 visited wetlands for these activities.
An example of the value of these wetland-related recreational activities can be found in the USFWS's 2001 National Survey of Fishing, Hunting and Wildlife -Associated Recreation (October 2002, http://www.census.gov/prod/2002pubs/FHW01.pdf). The USFWS states that in 2001, 34.1 million people aged sixteen years and older went fishing and spent an average of $1,046 each; 28.4 million anglers went freshwater fishing and 9.1 million went saltwater fishing. Overall, anglers spent $35.6 billion in 2001 on fishing trips, $4.6 billion on equipment, $6 billion on food and lodging, and $3.5 billion on transportation. They spent nearly $5.3 billion on land-use fees, guide fees, equipment rental, boating expenses, and bait. Camping equipment, binoculars, and special fishing clothing accounted for $721 million in expenditures. Equipment such as boats, vans, and cabins cost $11.6 billion. Anglers spent $3.2 billion on land leasing and ownership and $860 million on magazines, books, membership dues and contributions, licenses, stamps, tags, and permits.
Jerry Leonard reports in Fishing and Hunting Recruitment and Retention in the U.S. from 1990 to 2005: Addendum to the 2001 National Survey of Fishing, Hunting, and Wildlife-Associated Recreation (February 2007, http://library.fws.gov/nat_survey2001_recruitment.pdf) that wetland-related activities are also important to children of all ages. In 2005, 64% of all first-time anglers were aged twenty or younger. In addition, 42% of children of any age living at home were exposed to fishing for the first time in 2005—49% of males and 35% of females.
The National Marine Fisheries Service notes in Fisheries of the United States 2005 (February 2007, http://www.st.nmfs.gov/st1/fus/fus05/fus_2005.pdf) that the value of the U.S. commercial fish landings (the part of the fish catch that is put ashore) in 2005 was $3.9 billion. Nearly 30% of the value of U.S. finfish landings was from species that are dependent on near-coastal waters and their wetlands for breeding and spawning. The EPA estimates in the National Coastal Condition Report II (2005) (December 2004, http://www.epa.gov/owow/oceans/nccr/2005/Chapt1_Intro.pdf) that "95% of commercial fish and 85% of sport fish spend a portion of their life cycles in coastal wetland and estuarine habitats. Adult stocks of commercially harvested shrimp, blue crabs, oysters, and other species throughout the United States are directly related to wetland quality and quantity."
Because wetlands function like sponges by absorbing and storing water, they help control flood waters and the resultant loss of life and property. In Economic Benefits of Wetlands (May 2006, http://www.epa.gov/OWOW/wetlands/pdf/EconomicBenefits.pdf), the EPA notes that floods in the United States cost about $2 billion annually. However, depending on their size, wetlands can store millions of gallons of water and then release the water slowly after the flood surge has passed, reducing flood damage. Wetlands can also buffer the effects of coastal tropical storms and hurricanes. The EPA suggests that if the Mississippi-Louisiana coastline had more wetland areas, then the effects of Hurricane Katrina would have been lessened.
HISTORY OF WETLANDS USE
Until well into the twentieth century wetlands were considered nature's failure, a waste in nature's economy. For this reason, people sought to increase the usefulness of wetlands. In the agricultural economy of that time, land unable to produce crops or timber was considered worthless. Many Americans began to think of draining these lands, an undertaking that required government funds and resources.
In the nineteenth century state after state passed laws to facilitate drainage of wetlands by the formation of drainage districts and statutes. When a number of landowners in an area petitioned for a drainage project, a hearing was held. A district encompassing the area affected could be created with the power to issue bonds, drain the area, and bill the landholders—petitioners and opponents alike. Coupled with an agricultural boom and technological improvements, reclamation projects multiplied in the late nineteenth and early twentieth centuries. The farmland under drainage doubled between 1905 and 1910 and again between 1910 and 1920. By 1920 state drainage districts in the United States encompassed an area larger than Missouri.
The earliest effective resistance came from hunters, sportsmen, and naturalist lobbies. Organizations such as the Izaak Walton League, the Audubon Society, and the American Game Protective Association deplored the destruction by drainage of wildlife habitats and began to press for protection of wetlands. These early conservation efforts met chilly receptions both from the public and the courts. A growing number of Americans, however, were beginning to sympathize with conservationists. Drainage projects were often disappointing—soils had proven to be poorer than expected, and the costs were generally greater than expected.
Lower Klamath Lake in Northern California became a striking example of reclamation's potential for creating wastelands far more desolate than those they replaced. The lake, a shallow sheet of water fringed by marshes, had been set aside by Theodore Roosevelt in 1908 as a waterfowl sanctuary. Nonetheless, in 1917 the water inflow was cut off to reclaim the land. The lakebed dried up and became prey to dust storms. The peat in the marsh bottom caught fire. Rather than being a reclaimed area of extraordinary fertility, the former wetlands became an ecological travesty. According to the USFWS's Klamath Basin National Wildlife Refuges (April 7, 2007, http://www.fws.gov/klamathbasinrefuges/history.html), even though time has helped reverse the damage, less than 25% of the historic wetland basin remains. In spite of this, the basin continues to support tremendous bird life on a smaller scale.
Efforts were made to help the Klamath Basin recover. According to the news release "President Bush to Propose Record-Level $3.9 Billion for Conservation Programs" (January 30, 2003, http://www.usda.gov/wps/portal/), the U.S. Department of Agriculture (USDA) reports that in the budget for fiscal year 2004, President George W. Bush proposed setting aside $8 million for water conservation and water quality enhancements in the Klamath Basin. The article "Federal Agencies Issue Final Mandates for Klamath Dams" (California Chronicle, January 30, 2007) notes that the Departments of Interior and Commerce mandated that fishways and fish ladders be operational in the area, making it economically favorable to remove dams that blocked water to the area, rather than relicensing the dams and providing fishways and ladders using alternative methods. This action led the way toward returning the Klamath River to being a productive salmon river.
Similarly, for many years Florida sought to drain the Everglades, a vast wetland region covering much of the southern part of the state. Efforts there resulted in lands prone to flooding and peat fires. Peat fires are particularly dangerous because they burn underground and can flare up without warning long distances from where they were originally ignited. Costs escalated, and the drainage district went broke. Across the nation the gap between the cost and the value of reclaimed land widened even more. The agricultural depression beginning in the 1920s increased the growing skepticism as to the value of reclamation. Nonetheless, during the Great Depression (1929–39), programs such as the Works Progress Administration and the Reconstruction Finance Corporation encouraged wetland conversion as a way to provide work for many unemployed people. By the end of World War II (1939–45) the total area of drained farmland had increased sharply.
Concern over Property Rights
Dispute over wetlands regulation reflects the nation's ambivalence when private property and public rights intersect, especially because three-fourths of the nation's wetlands are owned by private citizens. In recent years many landowners have complained that wetlands regulation devalued their property by blocking its development. They argued that efforts to preserve the wetlands have gone too far, citing instances where a small wetland precludes the use of large tracts of land. Many people believe this constitutes taking without just compensation.
The "takings" clause of the U.S. Constitution provides that when private property is taken for public use, just compensation must be paid to the owner. Wetland owners claim that when the government, through its laws, eliminates some uses for their land, the value is decreased, and they believe they should be paid for the loss.
In the 1970s and 1980s state courts and the lower federal courts frequently handed down contradictory rulings on the issue of compensation for wetland-related takings. In 1992 the U.S. Supreme Court, in Lucas v. South Carolina Coastal Council (505 U.S. 1003), resolved the issue of compensation when land taken for an accepted public good loses significant value.
David Lucas, a homebuilder, bought two residential lots on a South Carolina barrier island in 1986. He planned to build and sell two single-family houses similar to those on nearby lots. At the time he purchased the land, state law allowed house construction on the lots. In 1988 South Carolina passed the Beachfront Management Act to protect the state's beaches from erosion. Lucas's land fell within the area considered in danger of erosion; as a result, Lucas could no longer build the houses.
Lucas went to court, claiming that the Beachfront Management Act had taken his property without just compensation because it no longer had any value if he could not build there. Lucas did not question the right of the state of South Carolina to take his property for the common good. Rather, he claimed the state had to compensate him for the financial loss that resulted from the devaluing of the property.
The Supreme Court said that a state could stop a landowner from building on his or her property only if he or she was using it for a "harmful or noxious" purpose—for example, building a brickyard or a brewery in a residential area. This was not the case. Lucas had planned to build homes, a legitimate purpose that was neither harmful nor noxious. Although it was possible to define the planned buildings as harmful to South Carolina's ecological resources, this would not be consistent with earlier Court interpretations of "harmful." Only by showing that Lucas had intended to do something "harmful or noxious" with the land could the state take his land without compensation. This the state did not do, and, therefore, it owed him the money.
People are not the only ones who have dramatically altered wetlands. Nonnative—also called exotic—species have been as devastating to wetlands as humans by changing the nature of the ecosystem, thereby interfering with its function and the survival of native plants and animals. Plants and animals introduced either accidentally or deliberately can cause unexpected harm by displacing native species from their habitat or by placing stress, such as disease or predation, on a native species.
In 1899 the coypu (Myocastor coypus ) was introduced into California for the fur-farming trade. This animal is a beaverlike aquatic South American rodent that is bred for its fur. This introduction was originally viewed as a way to provide economic benefit. Subsequently, state and federal agencies as well as private interests were responsible for introducing the coypu into the wild in fifteen states to provide a new fur resource. In coastal states such as Maryland and Louisiana, the results have been disastrous.
Coypu live in fresh, intermediate, and brackish marshes and wetlands and feed on the vegetation. They eat all the vegetation in an area, changing a marsh to a barren mudflat. Coypu feed on the base of plant stems and dig for roots and rhizomes in the winter. Their grazing strips large patches of marsh, and their digging turns over the upper peat layer. This conversion of marsh to open water destroys valuable habitat for muskrat, wading birds, amphibians, reptiles, ducks, fish, crabs, and a host of other species, as well as causing erosion and siltation.
Invasive plant species can be as harmful as invasive animal species. Eurasian watermilfoil, phragmites (common reed grass), hydrilla, and purple loosestrife are introduced species that have disrupted wetland systems. Purple loosestrife (Lythrum salicaria ) is a good example. It is a perennial herb with reddish-purple flowers that may reach six feet in height under the right conditions. It was an important medicinal herb and ornamental as early as two hundred years ago on the East Coast and was probably introduced for this reason. It has no known North American predators and has a high reproductive capacity—up to three hundred thousand seeds per stalk. Because it can outcompete most native wetland plants, it can change the character and ecological function of a marsh. This is a serious threat because many wetland and other wildlife species are adapted to and depend on specific plants.
LOSS AND GAIN IN WETLAND ACREAGE
When the first Europeans arrived in North America, Thomas E. Dahl and Gregory J. Allord indicate in History of Wetlands in the Coterminous United States (March 7, 1997, http://water.usgs.gov/nwsum/WSP2425/history.html) that there were an estimated 221 million acres of wetlands in the lower forty-eight states. In his 2006 report, Dahl notes that in 2004 there were an estimated 107.7 million acres. In the intervening years, more than 50% of the wetlands in the lower forty-eight states had been lost. Wetlands had been drained, dredged, filled, leveled, and flooded to meet human needs. Although natural forces such as erosion, sedimentation, and a rise or drop in sea level may erase wetlands over time, most wetland losses have been caused by humans. Many of the nation's older cities, such as New York City, Baltimore, Philadelphia, New Orleans, and Charleston, are built on filled wetlands.
However, Dahl mentions that after decades of wetland losses there was an annual gain of thirty-two thousand acres of wetlands from 1998 to 2004. (See Figure 7.8.) Additionally, the loss of wetland shrank from 458,000 acres in the 1950s and 1970s to 290,000 acres in the 1970s and 1980s, which is a decrease of 37%. By the 1980s and 1990s the annual wetland loss had declined to 58,500 acres, which is a 79% decrease from the 1970s and 1980s.
Dahl details the reasons for wetlands losses during the 1998–2004 period. Figure 7.9 shows that the highest acreage lost annually was because of urban development. Dahl determines that urban and rural development accounted for an estimated 61% of wetland losses. In addition, 70,100 acres of wetlands were estimated to have been lost to deepwater habitats. Deepwater habitats are "environments where surface water is permanent and often deep, so that water, rather than air, is the principal medium in which the dominant organisms live." Some wetlands (18,000 acres) were lost to silviculture, the planting of trees.
Dahl also discusses the reasons for wetland gains. (See Figure 7.9.) More than 70,000 acres of wetlands were gained from wetland restoration and conservation programs converting agricultural land to wetlands, and nearly 350,000 acres were gained from these programs converting other types of land, such as prairie, forest, or scrub land, to wetlands.
WETLAND PROTECTIVE LEGISLATION AND PROGRAMS
Since the early 1970s conservationists have turned to the courts to challenge reclamation projects and protect wetlands. If drainage once seemed to improve the look of the land, beginning in the 1970s it was more likely to be seen as degrading it. Wetlands turned out to be not wastelands, but systems efficient in harnessing the sun's rays to feed the food chain and play an important role in the global cycle of water, nitrogen, carbon, and sulfur.
As the drainage movement once found support in state laws and federal policies, so did the preservation movement. In 1977 President Jimmy Carter issued an executive order instructing federal agencies to minimize damage to wetlands. In 1989 the EPA adopted a goal of "no net loss" of wetlands, meaning that where a wetland is developed for other uses, the developer must create a wetland elsewhere to maintain an overall constant amount of wetland acreage.
Clean Water Act
Section 404 of the Federal Water Pollution Control Act of 1972 is commonly called the Clean Water Act (CWA). The goal of the CWA is to "restore and maintain the chemical, physical, and biological integrity of the nation's water." Wetlands are considered part of the nation's water and are covered by the CWA.
The CWA authorizes the Army Corps of Engineers to be the primary federal authority for the protection of wetlands. The Corps' jurisdiction encompasses all navigable waters of the United States, plus their tributaries and adjacent wetlands, and includes ocean waters within three nautical miles of the coastline and isolated waters where the use, degradation, or destruction of these waters could affect interstate commerce or foreign commerce. The Corps evaluates the impact of proposed projects that involve wetlands by considering comments from the EPA, the USFWS, the National Marine Fisheries Service, and the affected states. Regulations established under the CWA require that any project affecting more than one-third of an acre of wetlands or five hundred linear feet of streams must be approved by the Corps.
CWA JURISDICTION QUESTIONED
Between 2000 and 2002 legal challenges arose over the extent of the Corps' authority under Section 404 of the CWA and the meaning of certain terms used in the act (such as "waters of the United States" and "navigable waters"). In the CWA information brief The Supreme Court's SWANCC Decision (August 2003, http://homer.ornl.gov/nuclearsafety/nsea/oepa/guidance/cwa/swancc_info_brf.pdf), the U.S. Department of Energy details one such challenge.
According to the brief, the Solid Waste Agency of Northern Cook County (SWANCC) wanted to develop a nonhazardous solid waste disposal facility on a site that contained isolated ponds and wetlands. The Corps denied SWANCC a Section 404 permit to fill those wetlands because they were used by migratory birds. Lower courts found in favor of the Corps, and SWANCC appealed the finding to the U.S. Supreme Court.
On January 9, 2001, the Supreme Court issued the decision Solid Waste Agency of Northern Cook County v. United States Army Corps of Engineers (531 U.S. 159). The Court determined that the Corps' authority under the CWA did not extend to isolated wetlands if they were not "adjacent" to navigable waters. It held that the Corps exceeded its statutory authority by asserting CWA jurisdiction over the ponds that SWANCC wanted to fill based solely on the use of those "non-navigable, isolated, intrastate" waters by migratory birds.
PROPOSED CLEAN WATER AUTHORITY RESTORATION ACT
The 2001 SWANCC decision narrowed the scope of wetlands, streams, lakes, and other waters protected under the CWA, which has prompted a move to restore protection. The proposed Clean Water Authority Restoration Act (CWARA) would restore the broad scope of protection to these water bodies and reestablish protections for "isolated" wetlands throughout the United States. The most recent version of the bill was introduced in both the U.S. House of Representatives and the U.S. Senate in May 2005. In early 2007 the bill was in the first stages of the legislative process in both chambers of Congress, having been referred to subcommittees for consideration.
Farm Bill of 1996
The 1996 Farm Bill reauthorized the Conservation Reserve Program and created the Wetlands Reserve Program. The two programs are designed to protect and restore wetlands.
WETLANDS RESERVE PROGRAM
The Wetlands Reserve Program (WRP) is a voluntary USDA program and has been implemented in forty-nine states. The program provides farmers with financial incentives, such as a fair market price for land, to retire marginal farmland and, in many cases, to restore and protect wetlands. In the key points sheet "Farm Bill 2002" (September 2004, http://www.nrcs.usda.gov/programs/farmbill/2002/pdf/WRPKyPts.pdf), the USDA notes that the program had enrolled nearly 1.5 million acres in 2004. Retiring cropland through the WRP has benefited the recovery of threatened or endangered species and has protected wetlands. The WRP was reauthorized under the Farm Bill of 2002 and is to extend through December 31, 2007.
CONSERVATION RESERVE PROGRAM
The Conservation Reserve Program (CRP) was originally authorized in the Farm Bill of 1985 as a soil conservation strategy that included paying farmers to retire marginal cropland from production for ten years. Its political support came from its potential to reduce expensive crop surpluses. The Natural Resources Conservation Service notes in "Conservation Reserve Program" (March 26, 2007, http://www.nrcs.usda.gov/Programs/crp/) that under the CRP the Farm Service Agency pays farmers to plant natural vegetation in their "highly erodible cropland or other environmentally sensitive acreage."
Unlike the WRP, farmers do not have to permanently retire their land under this program, but instead can do so for ten-year intervals. As a wetland protection and restoration strategy, the program has been successful in terms of the thousands of acres of cropland that have been restored to a natural state, which in many cases includes wetlands. The CRP was reauthorized in the Farm Bill of 2002, extending the program through December 31, 2007.
State Wetland Protection Programs
Many states have enacted their own state laws to protect wetlands. These laws may complement or be more stringent than federal regulations. For example, Maryland has had state laws to protect tidal wetlands since the early 1970s. In 1989 Maryland adopted its Nontidal Wetlands Act to provide the same protections to freshwater wetlands.
Besides using their CWA authority, states have included wetland protection in their water quality standards, passed laws protecting ecologically important wetlands such as the Dismal Swamp in Virginia and North Carolina, established mitigation banking, and created public education programs to increase public awareness of the value of wetlands. Several states have set up special funds to buy important wetlands.
As shown in Figure 7.8, wetland gains have been made in recent years. Figure 7.9 shows the reasons for these gains. Many efforts are ongoing at the private, local, state, and federal levels to protect existing wetlands and to create new ones. Wetland losses can be offset by restoring, creating, enhancing, replacing, or reallocating wetlands:
- Wetland restoration—the return of a wetland to a close approximation of its condition before disturbance, including reestablishment of its predisturbance aquatic functions and related physical, chemical, and biological characteristics.
- Creation—the construction of a wetland in an area that was not a wetland within the past one hundred to two hundred years and is isolated from other wetlands.
- Enhancement—the modification of one or more structural features of an existing wetland to increase one or more functions based on management objectives. Enhancement, while causing a positive gain in one function, frequently results in a reduction in another function.
- Replacement or reallocation—activities in which most or all of an existing wetland is converted to a different type of wetland and has the same drawback as enhancement.
Each of these approaches has benefits and drawbacks.
Many of the wetland areas in the United States are privately owned. A number of government programs, both regulatory and voluntary, exist to foster wetland protection, and some foster both restoration and enhancement. Some of the most successful wetland programs and projects are the result of private initiatives. Frequently, private organizations form partnerships with landowners to buy, lease, or create easements paid for with private, or a mix of private and public, funds.
Organizations such as the Nature Conservancy, Ducks Unlimited (DU), the Audubon Society, the Chesapeake Bay Foundation, and hundreds of others are working with private landowners, corporations, local communities, volunteers, and federal and state agencies in innovative projects to protect and restore wetlands. For example, the Nature Conservancy oversees many wetland restoration projects, including two on the Illinois River—Spunky Bottoms (2007, http://www.nature.org/initiatives/freshwater/work/illinoisriver.html) and Emiquon (2007, http://www.nature.org/wherewework/northamerica/states/illinois/preserves/art1112.html)—that aim to return more than eighty-five hundred acres of farmed land to their original wetland state.
In another example, the DU is working with the National Resources Conservation Service to implement the WRP in the Mississippi Alluvial Valley, which according to the DU, in "Mississippi Alluvial Valley" (2007, http://www.ducks.org/conservation/initiative21.aspx), historically comprised 24.7 million acres of hardwood bottom stretching from southern Illinois to Louisiana. In "Conservation in Mississippi" (2007, http://www.ducks.org/Page1666.aspx), the DU reports that it is working to conserve over 250,000 acres of waterfowl habitat throughout Mississippi by restoring hydrology and planting bottomland hardwood seedlings.
Constructed wetlands are marshes that are built to filter contaminated water. They consist of soil and drainage materials (such as gravel), water, plants, and microorganisms. Using constructed wetlands for wastewater treatment is a simple, economical, and environmentally friendly method that is being used more frequently than in the past.
Constructed wetland treatment systems are designed and built to use the natural processes involving wetland soils, vegetation, and their associated microbes to help treat wastewater. They are designed to take advantage of many of the same processes that occur in wetlands but in a more controlled manner. Even though some of these systems are operated solely to treat wastewater, others are designed with the multiple objectives of using treated wastewater as a source of water for the creation or restoration of wetland habitat for wildlife and environmental enhancement. The primary drawback to constructed wetlands for wastewater treatment is that they are land intensive; large land tracts are not always available at affordable prices.
There are two general types of constructed wetland treatments: subsurface flow systems and free water surface systems. Both types are usually built in basins or channels with a natural or human-made subsurface barrier to limit seepage. The subsurface flow systems keep water flowing through soil, sand, gravel, or crushed rock underground to minimize odors and other related problems. (See Figure 7.10.) Subsurface flow systems are also known as rock-reed filters, vegetated submerged bed systems, and root-zone systems. Free water surface systems are designed to simulate natural wetlands, with the water flowing over the soil surface at shallow depths.
The EPA's Office of Water reports in Constructed Treatment Wetlands (August 2004, http://www.epa.gov/owow/wetlands/pdf/ConstructedW.pdf) that approximately five thousand constructed wetland treatment systems have been built in Europe and about one thousand are operating in the United States.
Marsh construction and wetland rehabilitation as a method of disposing of dredged materials are another growing source of wetland construction. The Army Corps of Engineers has been using dredged material to restore or construct marshes since 1969. Dredged material is placed on shallow bay bottoms to build up elevations to an intertidal level, usually by pumping dredged material to the marsh construction site. If the site is exposed to high wind or wave action, protective structures such as rock or concrete breakwaters are built. Vegetation can be planted or the site may be left to develop naturally. Generally, within two to three years, these sites are indistinguishable from natural wetlands in appearance.
Restoration of the Florida Everglades
The Everglades is a premier wetland in the United States. It is designated as an International Biosphere Reserve, a World Heritage Site, and a Wetland of International Importance. According to the World Heritage Committee, the Everglades is the only U.S. site on the List of World Heritage in Danger (April 10, 2007, http://whc.unesco.org/en/danger/). Figure 7.11 shows the location of the Everglades and how it has been reduced to about half its former size.
According to the South Florida Ecosystem Restoration Task Force (April 6, 2007, http://www.sfrestore.org/), the Everglades is part of the South Florida Ecosystem, an eighteen-thousand-square-mile region extending from the Kissimmee River near Orlando to the Florida Keys. Originally a wide expanse of wetland, pine forests, mangroves, coastal islands, and coral reefs, in the twenty-first century it is one of the nation's most highly populated and manipulated regions. Its freshwater supply comes from rainfall in the Kissimmee River Basin and southward, mostly in May through October.
Slow and rain driven, the natural cycle of freshwater circulation feeding the Everglades historically built up in shallow Lake Okeechobee, which averages twelve feet deep and covers about 730 square miles. Thus began the flow of the wide, shallow "river of grass," as it was called by Native Americans. Fifty miles wide in places, one to three feet deep in the slough's center, and only six inches deep elsewhere, it flowed south at a rate of about one hundred feet per day across the saw grass of the Everglades to the mangrove estuaries on the Gulf of Mexico. A six-month dry season followed this flow. During the dry season water levels gradually drop. The plants and animals of the Everglades are adapted to the alternating wet and dry seasons.
During the past one hundred years an elaborate system of dikes, canals, levees, floodgates, and pumps was built to move water to agricultural fields, urban areas, and the Everglades National Park. Water runoff from agriculture and urban development brought excess nutrients into the Everglades, reducing production of beneficial algae and promoting unnatural growth of other vegetation. Ill-timed human manipulation of the water supply interfered with the natural water cycle, ruining critical spawning, feeding, and nesting conditions for many species.
The Florida legislature has enacted a number of laws to combat the growing water shortage in Florida, including the Everglades. The 1981 Save Our Rivers Act and the 1990 Preservation 2000 Fund authorized the water management districts to buy property to protect water sources, groundwater recharge, and other natural resources. The South Florida Water Management District (SFWMD; 2007, http://www.evergladesplan.org/pm/progr_land_aquisition.cfm), an agency that oversees flood protection and water supply, began buying out landowners in the eastern Everglades area in hopes of retaking thousands of acres of agricultural and residential property at an estimated cost of $2.2 billion. The action is aimed at restoring water flow to Everglades National Park.
In 1998 the Army Corps of Engineers and the SFWMD released their plan for improving Florida's ecological and economic health: the Comprehensive Everglades Restoration Plan (CERP). This plan covers the entire region and its water problems and focuses on recovering the major characteristics that defined the "river of grass." Specifically, the plan calls for:
- Reducing the freshwater flows into the Caloosahatchee River and the St. Lucie Canal, thereby restoring to the Everglades water now lost to the tide.
- Returning the water flow in the Kissimmee River to its former floodplain to achieve a more meandering river system.
- Restoring forty thousand acres of marshes for water storage and filtration to remove nutrients before water entering the Everglades.
- Modifying water deliveries through improved timing and distribution to mimic historic water conditions.
- Reestablishing historic flows and water levels to sloughs feeding into Florida Bay to restore natural estuarine salinity.
The Water Resources Development Act of 2000 approved the CERP. The SFWMD (2007, http://www.ever gladesplan.org/about/about_cerp_brief.aspx) states that this plan will take more than thirty years to carry out and will cost an estimated $7.8 billion. The primary goal of the project is to restore critical water flows to the Everglades and ensure adequate water supplies for cities, communities, and farmers in southern Florida well into the future. The cost of the project will be shared equally between the state of Florida and the federal government.
"Wetlands." Water: No Longer Taken For Granted. 2008. Encyclopedia.com. (May 29, 2016). http://www.encyclopedia.com/doc/1G2-3049800013.html
"Wetlands." Water: No Longer Taken For Granted. 2008. Retrieved May 29, 2016 from Encyclopedia.com: http://www.encyclopedia.com/doc/1G2-3049800013.html
The very word "wetland" indicates that water is the first requirement for wetlands to exist. A wetland can be identified by three basic factors: soil, vegetation, and water regime (hydrology). A wetland generally is described as an area where water is the dominant factor in determining the nature of soil development and the types of plant and animal communities living in the soil and on its surface. Specifically, a wetland is an area that is periodically or permanently saturated or covered by surface water or groundwater , that displays hydric soils (unique soils associated with extended saturation), and that typically supports or is capable of supporting hydrophytic (water-loving) vegetation.
Occurrence and Characteristics
Wetlands form in areas where one or more of the following occur:
- The land is flat and water runs off the surface very slowly;
- Water becomes ponded in land-surface depressions;
- Infiltration of precipitation into the soil is slow;
- Groundwater discharges to the land surface; or
- The water table (the upper surface of the saturated zone of an unconfined groundwater system) is at the land surface.
In the broadest sense, all wetlands fall between two extremes with respect to where they get their water. On the one hand, wetlands may receive most of their water supply from precipitation. On the other hand, they may receive most of their water from groundwater discharge. One type of wetland that receives most of its water from precipitation is a vernal pool. These temporary pools dry up when precipitation is lacking. A different type of wetland, and one that receives most of its water from groundwater discharge, is called a fen. Fens usually occur in low areas, such as at the base of hillsides or in land-surface depressions. Wetlands that are present along streams, called riverine or riparian wetlands, fall between these two extremes because the source of water to a stream depends on precipitation in upstream areas and groundwater inflow to the stream–wetland system.
Plant and Animal Communities.
The type of soil development and the plant and animal communities that are supported depend on the depth of water and the length of time it is available, known as its hydroperiod. Where water is present most of the year, wetlands tend to be dominated by softstemmed herbaceous vegetation and grasses. Where water will be present only for a brief time, and where soil surfaces will dry long enough to allow germination of woody species, brush or trees will be the dominant vegetation. In areas of restricted drainage, the accumulation of partially decomposed sphagnum moss and other acid-loving plants builds up deep peat layers.
Submerged aquatic plants, which are those that grow completely underwater, usually grow in the part of the wetland where the water is deepest. Emergent aquatic plants, which are those that have the lower part of the plant underwater but the upper part above the water, grow where the water is shallower. Some wetland plants near the edge of water do not actually grow in water, but they are considered to be wetland plants because their roots are in the saturated soils directly adjacent to the water.
Wetland plant communities generally have distinctive patterns called zones. For example, in the wetlands that are in land-surface depressions, such as in glacial terrain, the zones generally have a concentric pattern. The open-water zone where the submerged plants grow is in the middle. This is sometimes also called the deep-marsh zone. This zone is surrounded by the shallow-marsh zone where the emergent plants grow. Beyond the water's edge is the wet-meadow zone. Wetlands along streams have similar plant zones, also depending on water depth. Wetlands in flat, coastal areas generally do not have such distinct plant zones because the vast flat areas commonly are covered by only one plant type.
Just as water provides the basic conditions for the distribution of plant zones in wetlands, the plant zones themselves provide habitat for animals. Certain microorganisms and invertebrate animals can be found in some plant zones but not in others. Other animals, usually the more mobile ones, such as waterfowl, use all the plant zones, but for different reasons. For example, some plants provide food for waterfowl, but other plant zones provide shelter and nesting sites.
Wetlands contain some plants and aquatic animals that live only in wetlands and that are different from those living in upland areas. However, some upland animals occasionally use wetlands for food and shelter. For these reasons, wetlands are important ecosystems in their own right, and also are important parts of the larger natural environment.
"Marsh," "swamp," and "bog" are some names commonly used to identify wetlands. Other names for types of wetlands include bottomland, fen, mangrove, mire, moor, muskeg, peatland, playa, pothole, reedswamp, slough, swamp, vernal pool, wet meadow, and wet prairie.
To wetland scientists, these terms can be used to identify specific wetland types. But because of the diversity of wetlands, descriptive schemes based on landscape position have been developed to identify broad wetland systems. Wetlands at the ocean's edge are marine systems, whereas wetlands in estuaries (where rivers meet the ocean) are estuarine systems. Wetlands along the edges of rivers and streams are riverine or riparian systems, and wetlands along the edges of lakes are lacustrine systems. Upland wetlands not connected to rivers or lakes are palustrine wetlands.
Water supply, and consequently, vegetation and soils vary for each of the systems. Marine systems are dominated by tides. Estuarine systems are influenced by the interaction of tides and river flows. Riverine systems reflect the controlling role of flooding from high flows, while the water supply for lacustrine systems depends on the lake level and the water supply to the lake. Palustrine systems usually are dominated by rain and snowfall. As noted previously, groundwater may play an influential role in any of these systems, depending on the local geological situation.
The different types of places where wetlands can be found can be divided into six groupings of terrain: mountains, plateaus and high plains, playas, river valleys, coastal, and glacial and dune.
Mountains have small uplands and lowlands separated by large, steep valley sides. Wetlands in mountains generally form in the narrow flat uplands and in the narrow lowlands between the base of mountain slopes and the streams. Wetlands in the higher areas of mountains are among the best examples of those that are dependent on precipitation for their source of water. Wetlands in mountain valleys receive their water primarily from groundwater discharge at the base of mountain slopes and from nearby streams.
Plateaus and High Plains.
Plateaus and high plains have broad, extensive uplands and relatively small, narrow lowlands in the river valleys. Wetlands in plateau and high plains landscapes generally are restricted to the valley bottoms along streams. The wetlands in the valleys receive water from the stream and from groundwater discharge to the stream valley. If riparian wetlands are dependent primarily on the stream, they are dependent on precipitation in their upstream watershed. If wetlands in river valleys are dependent primarily on groundwater discharge, the size of the groundwater flow system is the primary consideration in determining their source of water.
Playas are extensive, flat lowlands that do not have streams draining them. The source of water to wetlands in playas is largely from stream flow that originates from precipitation in the surrounding uplands. Groundwater and precipitation directly on the wetland generally are much smaller contributors of water.
Large river valleys have relatively wide lowlands. The source of most water to the riparian wetlands along the river is the river itself, but groundwater also discharges to the wetlands along the river as well as to other wetlands across the valley bottom not connected to the river. Floods also cause some types of wetlands to form in river valleys. Even without floods, other types of wetlands form when high river levels cause water to move sideways through the stream banks and fill depressions in the floodplain. For these types of wetlands, changes in the level of the river cause changes in water level in the wetlands.
Many coastal plains have broad, flat areas. Perhaps the main reason why wetlands occur in flat coastal areas is the low slope of the land surface. Broad, flat lowlands commonly have slow runoff and also tend to have shallow water tables. Groundwater discharges to wetlands across the broad flatlands, but probably most groundwater discharges near the edge where the plains meet regional uplands. Wetlands in flat coastal areas are among the most extensive wetland systems in the world.
Glacial and Dune.
Glacial landscapes are parts of the Earth that were covered by glaciers during the last Ice Age. Glacial landscapes have isolated depressions that can have a wide variety of shapes and sizes. Wetlands often form in the depressions in such landscapes if the water table is close to land surface. Most of the depressions do not have streams entering or leaving them; therefore, streamflow generally is not a major source of water to the wetlands. Wetlands in these areas receive their water supply from precipitation and/or groundwater discharge. In glacial terrain, some wetlands have no groundwater input; some receive groundwater inflow through part of their bed and lose water to groundwater through other parts; and some receive groundwater inflow throughout their bed. Wetlands of all three types can be present on regional uplands as well as on regional lowlands.
Conserving and Mitigating Wetlands
Wetland ecosystems play varied but important roles in the landscape. Depending on their type and location, wetlands can moderate and influence the timing of flows (including flood flows) in streams and rivers. Wetlands play important roles in helping to maintain streamflow and groundwater supplies because they hold water that otherwise would run off the land surface and be "lost" to a downstream watershed. The stored water can be slowly released to streams and to underlying groundwater systems. Wetlands can improve water quality by trapping and removing sediments and nutrients; in fact, wetlands are so effective that some natural and artificial wetlands are used to treat wastewater.
Wetlands are critical habitats for a variety of plant and animal species. About one-third of the species federally listed as endangered or threatened in the United States depend on wetlands. Wetlands provide resting and nesting habitat for more than half of the nation's migratory bird species.
Society is beginning to appreciate the ecological values of wetlands in supporting a variety of wildlife species. In addition to the direct economic benefits from flood control and water quality improvement, wetlands offer significant recreational and educational benefits.
Despite these recent realizations, wetlands historically have been viewed as not valuable, and have been drained or filled at an alarming rate, often to allow agricultural development. Over half of the wetlands in the United States's lower 48 states were lost to various land uses between the late 1700s and the mid-1980s. Although the trend has slowed, wetland loss still continues.
Mitigating Wetland Loss.
Despite ongoing losses of wetlands in the United States (and worldwide), many strides have been made in conserving and restoring wetlands both through private efforts and through legislation and government programs. The Swampbuster provision of the 1985 U.S. Farm Bill, the Conservation Reserve Program, the Wetlands Reserve Program, and the North American Wetlands Conservation Act have helped protect and restore hundreds of thousands of wetland acres across the country. Another help in slowing the loss has been the federal Clean Water Act, which requires that permits be obtained from the U.S. Army Corps of Engineers for developments that will significantly affect wetlands. Wetland mitigation guidelines often include replacement specifications that require replacing an acre of wetland lost to development by one or more acres of wetland developed or enhanced in another area.
If a mitigation offset cannot be accomplished as a part of the development, a developer may "buy" wetland acres at a "wetland bank" in or near the same watershed, to offset the loss at the development site. The wetland bank is an area of restored, constructed, or enhanced wetland maintained specifically for such banking purposes. Under exceptional circumstances, acres of an existing wetland that are specifically preserved from development can be used as "bank credit."
Depending on the situation, the replacement may be a direct replacement, or it may be a "paper" replacement. Direct replacement may involve restoring a wetland lost to other uses, or it may involve converting an area lacking significant wetland characteristics to an area capable of playing the role of a wetland in the landscape. A paper replacement might involve an agreement to protect appropriate amounts of wetland acreage in another area, or it might involve the use of appropriate acreage from a wetland bank.
The best solution to conserving wetlands is to reduce or prevent wetland loss in the first place. Educating the public about the value of wetlands is one step in protecting these natural ecosystems. With 75 percent of the nation's wetlands in private ownership, the future of wetland protection will increasingly rely on volunteer involvement. The key to furthering wetland protection is to motivate communities to value the unique environmental, social, and economic values wetlands provide, and to create practical solutions to protect, enhance, or restore those values.
see also Everglades; Fish and Wildlife Issues; Land-Use Planning; Watershed, Restoration of a; Wetlands.
Thomas C. Winter (terrains)
N. Earl Spangenberg (mitigation)
Dahl, T. E., and C. E. Johnson. Wetlands—Status and Trends in the Conterminous United States Mid-1970s to Mid-1980s. Washington, D.C.: U.S. Department of Interior, Fish and Wildlife Service, 1991.
Kauffman, S. C. Water Matters, Vol. 1. Arlington, VA: National Science Teachers Association, 1994.
Mitsch, William J., and James G. Gosselink. Wetlands, 3rd ed. New York: John Wiley & Sons, 2000.
National Academy Press. Compensating for Wetland Losses Under the Clean Water Act. Washington, D.C.: National Academy Press, 2001.
National Geographic Society. Our Disappearing Wetlands. National Geographic Magazine vol. 182, no. 4 (1992):2-45.
"America's Wetlands: Our Vital Link Between Land and Water." U.S. Environmental Protection Agency. <http://www.epa.gov/OWOW/wetlands/vital/toc.html>.
National Wetlands Inventory. U.S. Fish and Wildlife Service. <http://wetlands.fws.gov>.
National Wetlands Research Center. U.S. Geological Survey. <http://www.nwrc.usgs.gov>.
ACID BOGS AND ANCIENT CONDITIONS
Bogs are wetlands characterized by the presence of saturated organic soil (peat) and acidic water. The acidity and anoxic (lowoxygen or no-oxygen) conditions of bogs help preserve organic materials (e.g., plants and animals) for hundreds, even thousands of years. Many human artifacts also have been discovered in these wetlands. Human bodies estimated to be 2,000 years old have been found so well preserved that the color of the hair and eyes could be determined, as well as the last meal eaten.
Scientists use anoxic bogs as natural record-keepers that indicate changes in plant communities over time. The sediments can be dated, and preserved pollen and other plant components can be used to examine changes in plant communities and in the overall environment. Data from bogs also are useful to climate modelers interested in reconstructing past climate-change scenarios over thousands of years.
WHAT (AND WHO) DEFINES A WETLAND?
What is legally considered a wetland has particular importance with respect to the requirements for wetland preservation. On the one hand, agricultural interests, developers, and others want more freedom to develop and drain both seasonally wet regions and permanent wetlands; hence, they desire a restrictive definition of wetlands. On the other hand, environmentalists and natural resources managers want a more inclusive definition that affords protection for more lands. As a result of decades of debate, many definitions of wetlands have been developed by scientists and policymakers. The definition used often depends on the requirements of the user.
Winter, Thomas C.; Spangenberg, N. Earl. "Wetlands." Water:Science and Issues. 2003. Encyclopedia.com. (May 29, 2016). http://www.encyclopedia.com/doc/1G2-3409400348.html
Winter, Thomas C.; Spangenberg, N. Earl. "Wetlands." Water:Science and Issues. 2003. Retrieved May 29, 2016 from Encyclopedia.com: http://www.encyclopedia.com/doc/1G2-3409400348.html
Wetlands are low-lying ecosystems that are saturated with water at or close to the surface. (An ecosystem consists of all the animals, plants, and microorganisms that make up a particular community living in a certain environment.) The most common types of wetlands are swamps, marshes, and bogs. Wetlands provide habitats for an incredibly wide variety of plants and animals. They also are important because they absorb heavy rainfalls and prevent flooding. In addition, wetlands protect the ground water humans depend on for drinking by capturing and neutralizing surface pollutants.
However, wetlands are rapidly disappearing because they are being drained and filled for farming and urban growth. Wetlands also are being destroyed by pollution, especially the runoff of agricultural fertilizers and sewage dumping.
Swamps are shallow bodies of water in a low-lying, poorly drained area. These wetlands support a wide range of plant life, especially trees and high shrubs. In southeastern North America, swamp forests are typically dominated by such tree species as bald cypress, water tupelo, swamp
tupelo, and eastern white cedar. More northern temperate swamps are usually dominated by red maple, silver maple, American elm, and green or swamp ash.
Swamps provide a habitat for numerous species of animals. For example, swamps of bald cypress provide dwelling for the pileated woodpecker, red-shouldered hawk, Carolina wren, and many other small birds. These swamps also provide a nesting habitat for colonies of wading birds such as herons and egrets. Mammals supported by cypress swamps include swamp rabbits, white-tailed deer, and panthers. Many species of amphibians and reptiles—including the American alligator—live in cypress swamps.
Marshes are large wetlands dominated by rushes, sedges, and low-lying grasses. Typical plants of North American marshes include cattails, reeds, bulrushes, and saw-grass. Marshes can support relatively large populations of birds and certain mammals such as muskrats. Relatively small, fringing marshes around lakes and ponds are common in the prairies of North America. The borders of these marshy areas, called potholes, have historically provided major breeding habitats for surface-feeding ducks such as mallards, pintails, and blue-winged teals.
Words to Know
Biodiversity: Existence of a variety of plant and animal species in an ecosystem.
Ecosystem: The collection of plants, animals, and microorganisms in an area considered together with their environment.
Peat: Soil composed chiefly of decaying plant matter.
Primary succession: Natural replacement over time of one plant community with another more complex one.
Bogs are areas of wet spongy ground composed chiefly of peat (soil composed chiefly of decaying plant matter). The water underneath the surface-floating peat contains very little oxygen and other nutrients. It is also very acidic. As a result, bogs are dominated by acid-loving vegetation such as sphagnums (an order of mosses), sedges, and heaths.
Wetlands are dynamic ecosystems that are in transition between land and water habitats. Over time, most wetlands gradually fill in, a natural process known as primary succession. All wetlands were originally lakes or other bodies of water. Tons of plants, animals, and insects grow and die each year. The decaying material from these organisms gradually accumulates in small lakes. After a while, the lake becomes a wetland. The process continues with the wetland filling in more and more. Eventually, the wetland becomes a meadow, which in turn becomes a forest.
Wetlands also are delicate ecosystems. The biodiversity (the existence of a variety of plant and animal species in an ecosystem) of a particular wetland is maintained by the conditions that exist in that wetland. The plants and animals that thrive in a specific wetland have done so by adapting to the soil, water, nutrient supply, and other conditions found there. 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 large populations of plants and animals. This is commonly the case for marshes, which are among the most productive natural ecosystems on Earth. In contrast, wetlands with low supplies of nutrients, such as bogs, sustain only small populations of plants and animals.
All wetlands have great value as natural ecosystems, and they all support species of plants and animals that occur nowhere else. Their usefulness in providing essential habitat for fish, birds, and other wildlife cannot be overstated. Similarly, humans gain from wetlands, which control floods and erosion, cleanse the water that flows through them, and extend supplies of water for drinking or irrigation. In addition, wetlands have an aesthetic (beauty) value that is priceless.
Unfortunately, most of the world's wetlands are being lost rapidly. Land developers drain and fill them in. Since the beginning of European settlement in America, more than 65 million acres have been lost. Often, wetlands are used for the disposal of municipal solid wastes and sewage. Run-offs of chemical pollutants from farmland further pollute wetlands, disturbing their delicate soil-water balance and endangering their many plant and animal species.
[See also Biodiversity; Water ]
"Wetlands." UXL Encyclopedia of Science. 2002. Encyclopedia.com. (May 29, 2016). http://www.encyclopedia.com/doc/1G2-3438100668.html
"Wetlands." UXL Encyclopedia of Science. 2002. Retrieved May 29, 2016 from Encyclopedia.com: http://www.encyclopedia.com/doc/1G2-3438100668.html
"Wetlands" is the collective term for habitats that are too wet to be upland and not wet enough to be fully aquatic. They occur in areas of transition between dry upland and open water or in low areas where drainage water collects or the water table is at the ground's surface.
Wetlands are characterized by:
- the presence of surface water, at least part of the year
- unique soils that differ from adjacent uplands (due to the influence of waterlogging)
- plants adapted to wet soil conditions (hydrophytic vegetation)
There are many types of wetlands, differing in water chemistry, hydrology, soils, topography, climate, and vegetation. The broadest categories are coastal and inland wetlands. Coastal wetlands experience periodic flooding by saltwater or brackish water, and include estuaries (tidal marshes), mud flats, and mangrove swamps. They are nurseries for crustaceans, such as shrimp, and many fish species, and are also important habitat for birds and other wildlife. The presence of coastal wetlands can reduce inland erosion and other damage from hurricanes and winter storms.
Inland wetlands are freshwater wetlands and occur throughout the interior of a continent. These wetlands include: cattail marshes and wet meadows dominated by grasses, sedges, and herbs; swamps dominated by woody vegetation such as shrubs and trees; and peatlands (fens and bogs) that contain a buildup of peat, which forms as plants die and fall into the water and are not completely decomposed. The Florida Everglades are a vast inland wetland system.
A key factor determining what kind of soil and plant community develops in a wetland is the depth and duration of waterlogging and its effect on oxygen (O2) in the soil. Soils that are waterlogged for any length of time become depleted of O2 because soil microbes and plant roots use it during cellular respiration. The oxygen is not quickly replaced by O2 from the atmosphere because O2 diffuses very slowly through water. The anoxic (low oxygen) conditions influence soil development. Decomposition of plant litter and other organic matter is slowed in absence of O2 and the wetland soils become high in organic matter. If decomposition is much slower than the production of plant matter, peat will form. Peatlands typically occur in northern climates where low average temperatures further slow decomposition.
Since O2 availability is a limiting factor for plants growing in wetlands, most wetland plants have structural adaptations that increase gas exchange. Some have spongy tissues, called aerenchyma, in their stems and roots that conduct O2 within the plant from the aboveground shoot down to the roots. Others produce adventitious roots above the anoxic zone or have prop roots with pores that let in oxygen from the atmosphere.
In the past, many people viewed wetlands as mosquito-infested wastelands needing to be drained. More than one-half of the original wetlands of the United States have been drained or otherwise altered. Now there is a public consciousness that wetlands are important and valuable natural resources. Wetlands improve water quality by removing and retaining nutrients from surface waters and trapping sediments. They reduce flood and storm damage, and act to control erosion of shorelines. They provide important habitat for fish, crustaceans, and other wildlife and produce natural products such as blueberries, cranberries, rice, mink, and beaver. They support hunting and fishing activities and provide other recreational and educational opportunities.
see also Estuaries; Global Climate Change; Limnologist
Mitsch, William J., and James G. Gosselink. Wetlands. New York: Van Nostrand Reinhold, 1986.
Williams, Michael, ed. "Understanding Wetlands." In Wetlands: A Threatened Landscape. Cambridge, MA: Basil Blackwell, Inc., 1990.
U.S. Environmental Protection Agency. America's Wetlands: Our Vital Link Between Land and Water. Washington, DC: U.S. Environmental Protection Agency, 1988.
Phillips, Martha. "Wetlands." Biology. 2002. Encyclopedia.com. (May 29, 2016). http://www.encyclopedia.com/doc/1G2-3400700468.html
Phillips, Martha. "Wetlands." Biology. 2002. Retrieved May 29, 2016 from Encyclopedia.com: http://www.encyclopedia.com/doc/1G2-3400700468.html
WETLANDS are any of an array of habitats—including marshes, bogs, swamps, estuaries, and prairie potholes—in which land is saturated or flooded for some part of the growing season. According to the U.S. Fish and Wildlife Service, wetlands contain water-loving plants (hydrophytes) and hydric soils. They serve many ecological and practical purposes. Wetlands provide habitat and breeding sites for fish, shellfish, birds, and other wildlife; help maintain biological diversity (biodiversity); reduce the effect of floods by diverting and storing floodwaters;
provide protection from storm waves and erosion; recharge ground waters; and improve water quality by filtering out sediments, excess nutrients, and many chemical contaminants. Wetlands provide recreational, research, and aesthetic opportunities such as fishing, boating, hunting, and observing and studying wildlife.
Since 1780 human activity has destroyed more than half the wetlands of the United States, which now make up only 5 percent of the land surface of the contiguous forty-eight states, or 104 million acres. Nevertheless, they are extremely productive, exceeding even the best agricultural lands and rivaling rain forests in quantity and diversity of plant and animal life. More than half of the saltwater fish and shellfish harvested in the United States—and most of the freshwater sport fish—require wetlands for food, reproduction, or both. At least half of the waterfowl that nest in the contiguous states use the midwestern prairie potholes as breeding grounds. Wetland dependent animals include bald eagles, ospreys, beaver, otter, moose, and the Florida panther.
Few people recognized the value of wetlands until the 1970s. Before then, most people considered wetlands to be wastelands. In an effort to make them more productive—primarily through agriculture or development—people destroyed them by draining, ditching, diking, or filling. Early legislation, such as the Swamp Lands acts of 1849, 1850, and 1860, allowed fifteen states on the Mississippi River to "reclaim" wetlands for cultivation. By the mid-twentieth century, accumulating evidence, including U.S. Fish and Wildlife wetlands inventories in 1954 and 1973, made clear that destruction of wetlands was causing declines in fish and waterfowl. Federal, state, and local laws—notably the federal Clean Water Act of 1972 and amendments in 1977—attempted to regulate destruction.
Development, agriculture, and increasing pollution still threaten U.S. wetlands. One-third of wetland losses have occurred in midwestern farmbelt states. All but three states (Alaska, Hawaii, and New Hampshire) have lost more than 20 percent of their wetlands. Biologists and economists agree that preserving wetlands is less expensive than attempting to restore those that have been damaged, and experts still argue whether it is even possible to restore wetlands and how scientists might measure restoration. The economic and biological feasibility of restoration is debated each time a developer seeks permission to build on a wetland, thus destroying it, and offers (or is required) to attempt to rehabilitate a second site in return. Many biologists feel that because damaged sites cannot be returned to their previous states, it may not be acceptable to allow this tradeoff.
Council of Environmental Quality. Environmental Trends. Washington, D.C.: Executive Office of the President, 1989.
National Research Council. Restoration of Aquatic Ecosystems: Science, Technology, and Public Policy. Washington, D.C.: National Academy Press, 1992.
Susan J.Cooper/c. w.
"Wetlands." Dictionary of American History. 2003. Encyclopedia.com. (May 29, 2016). http://www.encyclopedia.com/doc/1G2-3401804526.html
"Wetlands." Dictionary of American History. 2003. Retrieved May 29, 2016 from Encyclopedia.com: http://www.encyclopedia.com/doc/1G2-3401804526.html
wetlands, low-lying ecosystem where the water table is always at or near the surface. It is divided into estuarine and freshwater systems, which may be further subdivided by soil type and plant life into bogs, swamps, and marshes. Because wetlands have poor drainage, the area is characterized by sluggish or standing water that can create an open-water habitat for wildlife. Wetlands help to regulate the water cycle, filter the water supply, prevent soil erosion, and absorb floodwaters. More significantly, however, wetlands serve as spawning and feeding grounds for nearly one third of the endangered animal and plant species in the United States, and their ecological value in most other countries is comparable.
Many wetlands were destroyed by urban growth and farming before their value was recognized. More than half of U.S. wetlands in the lower 48 states have been lost since colonial times. Federal wetlands policy today is based on the wetlands provisions (1987) of the Clean Water Act. The working concept is that of "no net loss," a concept that has been interpreted in various ways by each federal administration. Although the U.S. Fish and Wildlife Service has estimated that more than one million acres (about 400,000 hectares) of wetlands were lost in the decade from 1985 to 1995, this assessment was down from nearly 3 million acres (1.2 million hectares) lost in the previous decade, before the wetlands preservation policy was in force. As part of the "no net loss" policy, developers who fill wetlands may create new ones, but a 2001 National Academy of Sciences report found that new wetlands were not always created and when they were they were often of lesser value, both to the environment and to people, than the wetlands they replaced. The report recommended that replacement wetlands be designed to recreate the function of the developed wetlands.
Because of the restrictions wetlands protection can place on development and agriculture, it has become a political battleground between property rights activists and environmentalists. In 2001 the Supreme Court ruled that the Clean Water Act did not authorize federal regulation of so-called isolated wetlands (wetlands that do not abut navigable waters or their tributaries); as much as a fifth of the nation's wetlands are potentially affected by the ruling.
"wetlands." The Columbia Encyclopedia, 6th ed.. 2016. Encyclopedia.com. (May 29, 2016). http://www.encyclopedia.com/doc/1E1-wetlands.html
"wetlands." The Columbia Encyclopedia, 6th ed.. 2016. Retrieved May 29, 2016 from Encyclopedia.com: http://www.encyclopedia.com/doc/1E1-wetlands.html
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|
|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.
see also Aquatic Ecosystems; Aquatic Plants; Carnivorous Plants; Endangered Plants; Invasive Species; Peat Bogs.
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.
Buchsbaum, Robert. "Wetlands." Plant Sciences. 2001. Encyclopedia.com. (May 29, 2016). http://www.encyclopedia.com/doc/1G2-3408000319.html
Buchsbaum, Robert. "Wetlands." Plant Sciences. 2001. Retrieved May 29, 2016 from Encyclopedia.com: http://www.encyclopedia.com/doc/1G2-3408000319.html
MICHAEL ALLABY. "wetlands." A Dictionary of Plant Sciences. 1998. Encyclopedia.com. (May 29, 2016). http://www.encyclopedia.com/doc/1O7-wetlands.html
MICHAEL ALLABY. "wetlands." A Dictionary of Plant Sciences. 1998. Retrieved May 29, 2016 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O7-wetlands.html
MICHAEL ALLABY. "wetlands." A Dictionary of Ecology. 2004. Encyclopedia.com. (May 29, 2016). http://www.encyclopedia.com/doc/1O14-wetlands.html
MICHAEL ALLABY. "wetlands." A Dictionary of Ecology. 2004. Retrieved May 29, 2016 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O14-wetlands.html
"wetland." World Encyclopedia. 2005. Encyclopedia.com. (May 29, 2016). http://www.encyclopedia.com/doc/1O142-wetland.html
"wetland." World Encyclopedia. 2005. Retrieved May 29, 2016 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O142-wetland.html
wet·land / ˈwetˌland; -lənd/ • n. (also wetlands) land consisting of marshes or swamps; saturated land.
"wetland." The Oxford Pocket Dictionary of Current English. 2009. Encyclopedia.com. (May 29, 2016). http://www.encyclopedia.com/doc/1O999-wetland.html
"wetland." The Oxford Pocket Dictionary of Current English. 2009. Retrieved May 29, 2016 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O999-wetland.html
"wetland." Oxford Dictionary of Rhymes. 2007. Encyclopedia.com. (May 29, 2016). http://www.encyclopedia.com/doc/1O233-wetland.html
"wetland." Oxford Dictionary of Rhymes. 2007. Retrieved May 29, 2016 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O233-wetland.html