What are the trends in the extent and condition of wetlands and their effects on human health and the environment?
Importance of Wetlands
Wetlands include marshes, swamps, bogs, and similar areas that are periodically saturated with or covered by water. They provide food and habitat for a diverse array of plants and animals, act as buffers to flooding and erosion, and serve as key links in the global water cycle.
Because of their sponge-like ability to absorb water, wetlands can slow the momentum of flood waters or of a coastal storm surge. Wetlands' highly developed root systems hold the soil in place and filter pollutants, naturally improving water quality (including water that is eventually used for drinking).
Wetlands are among the most biologically productive ecosystems in the world. Their microbial activity enriches the water and soil with nutrients. Plant growth in wetlands provides a “sink” for many chemicals including atmospheric carbon.
Wetlands support a wide range of human activities such as commercial fishing, shellfishing, and other industries, as well as recreation, education, and aesthetic enjoyment.
Wetlands currently cover 5.5 percent of the land in the 48 contiguous states. An estimated 95 percent of these wetlands are freshwater; the rest are marine or estuarine.1 Wetland extent can be affected by a variety of natural stressors, such as erosion, land subsidence, droughts, sea level change, and storms. However, the vast majority of wetland losses and gains over the last few centuries have occurred as a result of human activities.
For years, people have drained or filled wetlands for agriculture or development, causing habitat loss as well as a decline in many other important wetland functions. Human activities can also increase the extent of wetlands—for example, by creating shallow ponds or re-establishing formerly drained wetlands on farmlands.
The condition of wetlands reflects a combination of physical, chemical, and biological attributes. It can be influenced by many natural and human-induced stressors.
- Hurricanes, sea level change, and certain agricultural and forestry practices can increase erosion or sedimentation.
- Human modifications such as pipes and channels can alter wetland types, elevation, or hydrology.
- Withdrawal of ground water or surface waters can reduce the flow of water into wetlands.
- Pollutants in ground water and fresh surface waters that flow into wetlands can be toxic to plants and animals, and they can accumulate in wetland sediments.
- Invasive species can alter the composition of wetland communities.
- Wetland loss can add stress to remaining wetlands. For example, if fewer wetlands are available to filter pollutants from surface waters, those pollutants could become more concentrated in the remaining wetlands. Wetland loss can also decrease habitat, landscape diversity, and connectivity among aquatic resources.
- Conversion from one wetland type to another—for example, by cutting down trees in forested wetlands—can have a major ecological impact by changing habitat types and community structure.
The ROE includes one wetland indicator, which focuses on extent. This indicator provides a comprehensive look at changes in the acreage of different types of wetlands since the 1950s, and classifies the causes of these changes.
The ROE indicator does not describe spatial patterns of wetland change—whether large wetlands are being left intact, or whether they are being fragmented into smaller pieces that are less connected and more isolated, and therefore less able to perform ecological functions.
A larger gap is the lack of an indicator on wetland condition. Condition is difficult to measure because it is made up of many different attributes, and each wetland has its own unique baseline condition, function, hydrology (water flow), and combination of plant and animal species. Efforts are underway to conduct a comprehensive national survey of wetland condition, which could lead to more indicators in the future.
 Dahl, T.E. 2011. Status and trends of wetlands in the conterminous United States 2004 to 2009. Washington, DC: U.S. Department of the Interior, Fish and Wildlife Service.