The concept of wetlands is not easy to define, due in part to the enormous variety of wetland types, its highly dynamic nature, the difficulty of precisely defining its limits, and their great variation in size, location, and human influence. The definitions that are available are abundant and can be confusing or contradictory. However, it is important to define its meaning both for the scientific aspects as well as for the proper management of these systems. One of the first formal definitions for the term “wetland” was made in 1956 by the U.S. Fish and Wildlife Service (USFWS), in a publication that is referred to as Circular 39 (Shaw and Fredin, 1956) where it is defined as follows:
The term ‘wetland’ refers to lowlands covered with shallow and sometimes temporary or intermittent waters. They are referred to by such names as marshes, swamps, bogs, wet meadows, potholes, sloughs, and river overflow lands. Shallow lakes and ponds, usually with emergent vegetation as a conspicuous feature, are included in the definition, but the permanent waters of streams, reservoirs, and deep lakes are not included. Neither are water areas thar are so temporary as to have little or no effect on the development of moist-soil vegetation.
Canada has a vast area of wetlands, so the Canadians have developed a definition for their country. In 1979, Tarnorai presented a definition that is used today by the Canadian Wetlands Inventory and defines wetlands as “lands that have a water table at, near or above the land surface which is saturated for a long enough period to promote wetland or acquatic processes as indicated by hydric soils, hydrophytic vegetation, and various kinds of biological activities which are adapted to the wet environment”. This latter definition is considered the definition for wetlands in Canada after being formally presented in the book Wetlands of Canada in 1988.
At the first meeting of the Convention on Wetlands of International Importance Especially as Waterfowl Habitat, better known as the Ramsar Convention, the International Union for the Conservation of Nature and Naturar Resources (IUCN) adopted an extremely broad proposal to determine wetlands that come under their protection. In this proposal, wetlands are defined as “Areas of marsh, fen, peatland or water, whether natural or artificial, permanent or temporary, with water that is static or flowing, fresh, brackish or salt, including areas of marine water, the depth of which at low tide does not exceed 6 meters.”
Moreover, this definition, adopted in 1971, states that wetlands “may incorporate riparian and coastal zones adjacent to the wetlands and islands or marine deeper than 6 meters at low tide lying within the wetlands.
Perhaps the most comprehensive definition of wetlands was adopted by wetland scientists in the U.S. Fish and Wildlife Service in 1979, after several years of review. This definition was presented in the report entitled “Classification of Wetlands and Deepwater Habitats of the United States” (Cowardin et al., 1979), and defines wetlands as “lands transitional between terrestrial and aquatic systems where the water table is usually at or near the surface of the land is covered by shallow water……. Wetlands must have one of the following three attributes: (1) at least periodically, the land supports predominantly hydrophytes; (2) the substrate is predominantly undrained hydric soil; and (3) the substrate is nonsoil and is saturated with water or covered by shallow water at some time during the growing season of each year.”
Even today, this definition is one of the most accepted, and is designed for both scientists and manager. It is broad, flexible, and comprehensive, and includes descriptions of vegetation, hydrology, and soils.
It is difficult to find a definition that is final and satisfactory to all users. This variation is the natural result of the unique focus of each person, depending on their field of study and the different ways in which each of the disciplines relate to the wetlands. It has to do with the objectives and discipline of interest of each user.
Value of Wetlands: Wetlands are among the most important ecosystems on the planet. While their value to the protection of fisheries and wildlife, in general, is recognized, some of their other benefits have not been identified until recently. Mitsch and Gosselink (1993) describe wetlands as “kidneys of the landscape” for the role they play within the chemical and hydrological cycles and because they function as the downstream receivers of water and waste, of both natural or human sources . They have been found to cleanse polluted waters, prevent floods, stabilize the shoreline, and serve as groundwater recharge. Furthermore, these authors consider that wetlands are valuable as sources, sinks, and transformers of a multitude of chemical, biological, and genetic materials. They also serve as stabilizers of local climatic conditions, particularly rainfall and temperature.
Mitsch and Gosselink (1993) state that they have also been called “biological supermarkets” because of the extensive food chain and rich biodiversity that they support, and provide a unique habitat to a wide variety of species of flora and fauna.
Classification System: DUMAC developed this classification system based on the Classification of Wetlands and Deepwater Habitats of Cowardin (1979), used by the National Wetlands Inventory of the U.S. Fish and Wildlife Service, to which some modifications were made according to the needs that were presented for Mexico.
Traditionally, the term “wetland” does not include deepwater habitats, however, for the purpose of this classification, both must be considered in an ecological approach to classification. Deepwater habitats include permanently flooded lands lying below the deepwater boundary of wetlands. It includes environments where surface water is permanent and often deep, so that water, rather than air, is the principal medium within which the dominant organisms live (Cowardin et al., 1979).
Within this classification, six systems are considered, five of which correspond to different wetlands and deepwater habitats: Marine, Estuarine, Lacustrine, Palustrine and Riverine, and a sixth system corresponding to Uplands.
Source: Carrera, E. and G. de la Fuente 2003. Wetlands Inventory and Classification in Mexico. Part 1. Ducks Unlimited de Mexico AC, Mexico. 239pp.