Home


The Nation's Wetlands


by
Bill O. Wilen
U.S. Fish and Wildlife Service
The national interest in wetlands is set forth in the findings of the Emergency Wetlands Resources Act of 1986:
The Congress finds that wetlands play an integral role in maintaining the quality of life through material contributions to our national economy, food supply, water supply and quality, flood control, and fish, wildlife, and plant resources, and thus to the health, safety, recreation, and economic well-being of all citizens of the Nation.
The act requires the Secretary of the Interior to map the nation's wetlands, develop a national digital wetlands data base, and report to Congress on the status and trends of wetlands within the conterminous United States. The U.S. Fish and Wildlife Service (USFWS) has delivered three reports to Congress (Frayer et al. 1983; Dahl 1990; Dahl and Johnson 1991). The reports show that half of the nation's wetlands have been converted to uplands since colonial times (Dahl 1990), and that although the rate of conversion has slowed, wetland losses continue to outdistance gains (Frayer et al. 1983; Dahl and Johnson 1991).
The quality of the remaining wetlands continues to be an unanswered question. Presidential candidate George Bush's 1988 No-Net-Loss campaign promise was adopted by the federal government as a policy goal. It was expanded by President Clinton in his August 25, 1993, policy statement, "Protecting America's Wetlands: A Fair, Flexible, and Effective Approach," to include a long-term goal of increasing the quality and quantity of the nation's wetlands resource base. Here we present a brief overview of wetlands, their definition, distribution and abundance, dynamics, functions, values, and future.

Wetland Descriptions and Definitions
The United States encompasses an area of about 931 million ha (2.3 billion acres) extending from above the Arctic Circle to the Virgin Islands and spanning the North American continent, and includes the Hawaiian Islands as well as Puerto Rico. Within this broad area, regional variations in climate, topography, hydrology, geology, soils, and vegetation create diverse wetland habitats ranging from the tundra in Alaska to the tropical rain forests of Hawaii to isolated wetlands in the arid Southwest.
Cowardin et al. (1979) defined wetlands as
lands where saturation with water is the dominant factor determining the nature of soil development and the types of plant and animal communities living in the soil and on its surface. The single feature that most wetlands share is soil or substrate that is at least periodically saturated with or covered by water. The water creates severe physiological problems for all plants and animals except those that are adapted for life in water or in saturated soil. (p. 3)
There are three widely used definitions of wetlands. All use three parameters: hydrology, hydric soil (wetland soils), and hydrophytic vegetation (wetland plants). The USFWS's definition is ecological whereas the definitions used by the U.S. Environmental Protection Agency, the U.S. Army Corps of Engineers, and the Soil Conservation Service are regulatory. All three, however, endorse and use the same interagency wetland plant list, National List of Plant Species That Occur in Wetlands (Reed 1988), and wetland soils list, Hydric Soils of the United States (SCS 1991).
Regulators are concerned with establishing a definitive line to delineate wetlands from uplands and with placing the wetlands into administrative or regulatory categories. In contrast, the USFWS and the National Biological Service (NBS) are concerned with ecological characterization and mapping the biological extent of both vegetated and nonvegetated wetlands found on soils and substrates. The biological extent of wetlands should be established by scientists using biological criteria. Likewise, policy makers should establish regulations for the subset of wetlands that needs regulating. The subset of wetlands to be regulated and the degree of regulation have changed and will change over time based on our understanding of the functions and values of wetlands, wetlands scarcity, our ever-changing social values, and the political climate.
The USFWS classification system was developed to provide uniformity in concepts and terminology for wetlands. It is hierarchical, moving from systems at the broadest level through subsystems, classes and subclasses, to modifiers describing hydrology (water regime), soils, and water chemistry, and special modifiers relating to human activities.
These categories are used to form wetland types for mapping. More than 2,500 wetland types are commonly used on National Wetlands Inventory maps nationwide. Counties will have from 10 to 400 types, with an average of 100. These wetland types describe ecological units that have certain homogeneous natural attributes. The USFWS's National Wetlands Inventory maps are available for 84% of the conterminous United States, 28% of Alaska, and all of Hawaii.

Distribution and Abundance
The distribution of wetlands has changed dramatically since the 1780's (Figs. 1 and 2). In addition, the percentage of the landscape occupied by wetlands varies markedly from state to state (i.e., Alaska, where 43.3% of the landscape is covered by wetlands as compared with nine states where 1% or less of the landscape is covered by wetlands). The wetland areal loss by states tells one story (Fig. 3) and the percentage of the wetland base lost by states tells another (Fig. 4). Wetlands occupy 11.9% of the landscape of the United States, which is about 5% of the conterminous United States, 43% of Alaska, and 1% of Hawaii.

Fig. 1. Surface-area percentage of wetlands in each state: 1780's (Dahl 1990).

Wetland Dynamics
The three status and trends reports to Congress provide estimates of net wetland gains or losses; they do not examine wetland quality as a result of disturbance. Wetlands are constantly being disturbed. Even when a wetland is not converted to upland, its successional stage is often pushed back to an earlier stage. For example, between the mid-1970's and mid-1980's, forested wetlands suffered tremendous loss from agriculture and "other" uses. (The category of "other" includes all wetland areas converted to upland where the ultimate land use could not be determined.) Thousands of hectares of forested wetlands were converted to emergent, scrub-shrub, and nonvegetated wetlands. Likewise, thousands of hectares of scrub-shrub wetlands were converted to the "other" category and the agricultural land-use category. These losses were nearly offset by the conversion of forested wetlands to scrub-shrub wetlands. Despite these gains to the scrub-shrub category, however, there was an overall net loss of scrub-shrub wetlands during the study period.

Fig. 2. Surface-area percentage of wetlands in each state: 1980's (Dahl 1990).
The net gain of thousands of hectares of freshwater emergent wetlands is similarly deceptive. The thousands of hectares that were lost to agricultural, "other," and urban land uses were more than offset by the conversion of forested wetlands and scrub-shrub wetlands to freshwater emergent wetlands. The area of nonvegetated wetlands (primarily ponds) increased by several million hectares. Most of these gains, however, resulted from construction of ponds on uplands not used for agricultural production, but additional thousands of hectares were built on former agricultural lands. This category also experienced gains from converted forested wetlands and scrub-shrub wetlands.

Fig. 3. Wetland acreage loss by state (Dahl 1990).

Functions and Values
The functions and values of the nation's wetlands are nearly as diverse as the wetlands themselves (Table), but include flood protection and plant, fish, and wildlife habitat. Table. Major wetland functions and values documented in the National Wetlands Inventory "Wetlands Values Database."
Functions and values Examples
Biogeochemical processes Carbon cycling, sulfur cycling
Food chain Detritus production, food source, nutrient cycling, nutrient export, primary production
Habitat Amphibians, fish, furbearers, insects, mammals, nongame birds, reptiles, shellfish, shorebirds, waterfowl, endangered species
Hydrology Erosion control, flood control, flow stabilization, groundwater discharge, groundwater recharge, saltwater intrusion prevention, storm dampening
Socioeconomic Aesthetics, agricultural crops, aquaculture, archaeological, commercial harvest, cultural, educational, energy source (peat), food, forage, heritage, hunting and trapping, indicator species, medicinal, open space, natural products, recreation, research, timber, wastewater treatment, water supply
Water quality Chemical and nutrient absorption, pollution filtering, oxygen production, sediment trapping
All wetlands do not perform all functions. Some functions tend to be compatible, such as flood control and water purification. Other functions tend to be incompatible, such as flood control and food chain support. In addition, wetlands of a given type do not have the same effectiveness in performing a given function. For example, the effectiveness of a given forested wetland for flood control depends on its size, shape, location in the watershed, and so forth. Because wetlands are constantly being affected by disturbance, their effectiveness in performing functions constantly changes. Thus, the effectiveness of a wetland area as wildlife habitat can be improved or degraded by the creation, maintenance, or destruction of vegetated corridors; the ratio of vegetated wetland to upland areas; buffer zones; and plants that provide for wildlife food and habitat. Uplands can and do perform some of the functions performed by wetlands, such as sediment trapping. But because wetlands are situated in the low points of the landscape or are adjacent to streams, rivers, lakes, and oceans, they are more able to perform these functions. In many cases, wetlands are the last line of defense for the protection of surface water quality.

Fig. 4. Surface-area percentage of wetland base loss by state (Dahl 1990).
Some wetland functions and values can be replaced by artificial substitutes; for example, flood-control values of wetlands can be replaced by dams, ditches, levees, floodwalls, and reservoirs. Other wetland functions, however, cannot be performed by uplands or replaced by artificial substitutes. An especially important function of wetlands is supporting rich plant diversity. Although wetlands occupy only about 5% of the surface area of the conterminous United States, 6,728 plant species (31% of the U.S. flora) occur in wetlands (Reed 1988). Of these plants, half are restricted to, or usually occur in, wetlands. Thus, wetlands provide critical habitat for a high percentage of the U.S. flora.
Some argue that we cannot afford to maintain the remaining 40 million ha (99 million acres) of wetlands in the conterminous United States because of our increasing population, living standards, and competition for resources. Others argue that wetlands must occupy a greater percentage of the nation's landscape. In the conterminous United States, non-federal rural land occupies nearly 75% of the landscape and contains more than 75% of the nation's wetlands (USDA 1989). Wetlands comprise nearly 6% of the rural non-federal landscape. Specifically, wetlands occupy roughly 1% of cropland, 2% of rangeland, 5% of pastureland, 12% of forestland, and 31% of other rural land (USDA 1989).

Future
Although our understanding of wetlands is imperfect, it is clear we have more information upon which to make public policy decisions on wetlands than we have for many other ecosystems. The challenge for policy makers is to avoid ecologically irreversible choices that would diminish the wealth of future generations while promoting economic development and improving income distribution.
For further information:
Bill O. Wilen
U.S. Fish and Wildlife Service
National Wetlands Inventory Project
4401 N. Fairfax Dr.
Rm. 400
Arlington, VA 22203
References
Cowardin, L.M., V. Carter, F.C. Golet, and E.T. LaRoe. 1979. Classification of wetlands and deepwater habitats of the United States. U.S. Fish and Wildlife Service FWS/OBS-79/31. 131 pp.

Dahl, T.E. 1990. Wetland losses in the United States: 1780's to 1980's. U.S. Fish and Wildlife Service, Washington, DC. 21 pp.

Dahl, T.E., and C.F. Johnson. 1991. Status and trends of wetlands in the conterminous United States: mid-1970's to mid-1980's. U.S. Fish and Wildlife Service, Washington, DC. 28 pp.

 Frayer, W.E., T.J. Monahan, D.C. Bowden, and F.A. Graybill. 1983. Status and trends of wetlands and deepwater habitats in the conterminous United States: 1950's to 1970's. Department of Forest and Weed Sciences, Colorado State University, Fort Collins. 32 pp.

Reed, P.B., Jr. 1988. National list of plant species that occur in wetlands: national summary. U.S. Fish and Wildlife Service Biological Rep. 88(24). 244 pp.

SCS. 1991. Hydric soils of the United States. USDA Soil Conservation Service Miscellaneous Publ. 1491. n.p.

USDA. 1989. Summary report, 1987 National Resources Inventory U.S. Department of Agriculture Statistical Bull. 790. 37 pp.


Home