An Introduction to Wetlands

A great egret hunts in the Los Cerritos Wetlands as land developers and wildlife conservationists compete for the vanishing wetland near Long Beach, California. 2007 Getty Images

Wetlands are areas of land that are covered with fresh water or saltwater and feature species adapted to life in a saturated environment. They are shallow and allow the growth of rooted or anchored plants such as water lilies but also free floating plants like duckweed.

Wetlands represent the meeting of two habitats (land and water) and are therefore some of the most biodiverse areas in the world (some say more than rainforests) with many land and water species, and some that are unique only to the wetlands.

Currently, wetlands exist on all the world's continents except Antarctica, but because of increasing pollution and a reduction in open land, they are all threatened. Examples include the Mahavavy-Kinkony Wetlands in Madagascar, and the Everglades in Florida.

Wetland Formation

Wetlands can also form when a river overflows its banks or when changes in sea level make once dry areas saturated. Additionally, climate can impact wetland formation as high rainfall in normally dry areas with poor drainage causes the ground to become saturated.

Once wetlands form, they are constantly changing. Just as growing sediment and debris levels cause the wetlands to form, they along with roots and dead plant matter, can cause the wetland to become more shallow, eventually to the point where the upper layers rise above the water table and dry out. When this happens, terrestrial plant and animal species can colonize the area.

Types of Wetlands

Coastal wetlands are along the coastlines of mid to high latitude areas worldwide, but they are most common along the Atlantic, Pacific, Alaskan and Gulf Coasts. Coastal wetlands form near estuaries, the area where a river meets the sea, and are prone to varying levels of salinity and water levels because of tidal action. Because of the varying nature of these locations, most tidal wetlands consist of unvegetated mud and sand flats.

Some plants however, have been able to adapt to such conditions. These include the grasses and grass-like plants of the tidal salt marshes on the coasts of the United States. In addition, mangrove swamps consisting of salt loving trees or shrubs are common in tropical coastal areas.

By contrast, inland wetlands are along rivers and streams (these are sometimes called riparian wetlands), in isolated depressions, along the edges of lakes and ponds, or in other low-lying areas where the groundwater meets the soil's surface or when runoff is significant enough to allow formation. Precipitation can also sometimes saturate the soil and create bogs or temporary wetlands called vernal pools.

Unlike coastal wetlands, inland wetlands are always comprised of freshwater. They include marshes and wet meadows that are filled with herbaceous plants and swamps dominated by shrubs and wooded swamps full of trees.

Significance of Wetlands

Estuarine and marine fish and shellfish, and some mammals must have wetlands to survive as they are breeding grounds and/or provide a rich source of food via decomposing plant matter. Some of the species that live in wetlands include wood ducks and muskrats. Other fish, mammals, reptiles and birds visit wetlands periodically because they provide food, water and shelter. Some of these are otters, black bears and raccoons.

In addition to being unique ecosystems, wetlands also act as a filter for pollution and excess sediment. This is important because rainwater runoff is normally laden with dangerous pesticides and other pollutants. By going through a wetland prior to reaching open water, this is filtered out and often, excess sediment naturally builds up in the wetland instead of in rivers or other water bodies.

Wetlands also aid in flood protection as they act as sponges that absorb rain and floodwater. Furthermore, wetlands are significant to the reduction of coastal erosion as they can act as a buffer between land and the sea- an important thing to have in areas prone to storm surges and hurricanes. Inland wetlands also prevent erosion because the roots of the wetland's vegetation hold soil in place.

Human Impacts and Conservation

To view wetland locations across the United States, visit the National Wetlands Inventory.