Wetlands cover about six percent of the Earth's land surface. They come in all shapes and sizes, from inconspicuous sinkholes to vast expanses covering hundreds of square kilometers. They're found on every continent, save for Antarctica, and in climates that range from tropical to arctic. Although wetlands are a widespread feature of the landscape, they still lack a standard definition - a fact that can pose problems for conservationists and government regulators trying to stake out zones worthy of protection. Yet wetlands do have some identifiable characteristics, such as the presence of standing water, unique soils formed by the decomposition of plant matter, and vegetation adapted to chronically wet conditions. Simply put, they serve as intermediaries between aquatic and terrestrial realms, sharing some features of each.
Marshes, swamps, and bogs - the most common types of wetlands - can be classified by the prevailing vegetation. Marshes are filled with grasses and can be found in both coastal and inland regions. Freshwater marshes make up more than 90 percent of the total wetland area in the continental United States. Trees and bushes take root in swamps, with mangrove trees dominating in the world's subtropical and tropical coastal areas. Bogs are peat-accumulating wetlands that support the growth of mosses. Bogs dominated by Sphagnum moss store about one-third of the carbon locked up in the world's soils. In addition, there are a number of lesser known wetland varieties with colorful names such as bayous, bottomlands, fens, muskegs, moors, peatlands and sloughs. Two of the most unusual found in the United States are prairie potholes and vernal pools.
Prairie potholes are crater-like depressions - carved out by glaciers and filled with either fresh or salty water - that are scattered about the American Great Plains and the Canadian provinces of Alberta, Manitoba and Saskatchewan. Less than half of the original 16 million or so acres of prairie potholes remain in this country. Many were turned into farmland; others were sacrificed during road construction. Although relatively few humans appreciate the value of the potholes, millions of animals depend on them for survival. For example, more than half the ducks born in North America each year rely on these holes in the ground for feeding, breeding, nesting and resting.
Vernal pools - intermittently flooded meadows that are mainly found in California - are another underappreciated ecosystem. More than 93 percent of the state's vernal pools have been drained, leveled or plowed over. This devastation has proceeded with limited protest, perhaps because the pools are little more than puddles that dry up completely during certain months of the year. But much is at stake, as these unheralded wetlands provide oases for dozens, if not hundreds, of plants and animals found nowhere else in the world.
More than half of the wetlands in the United States have been lost to human activities and as much as 90 percent of the wetlands in states such as California and Ohio have been wiped out.