Wetlands

I

Introduction

Wetlands, areas where shallow water dominates the environment. This may be because the water table is at or near the land surface, or because the land is covered by shallow water. Wetland waters may be fresh, brackish, or salty, including areas of marine water whose depth at low tide does not exceed 6 m (20 ft). Wetlands occur in every climate and country, except Antarctica, and it is estimated that they cover about 6 per cent of the Earth's land surface, about 570 million hectares (1,408 million acres) or 5.7 million sq km (2ƒ million sq mi). About 30 per cent are peatlands (or bogs), 26 per cent marshlands (or fens), 20 per cent swamps, 15 per cent floodplains, and 2 per cent lakes. Mangroves cover some 240,000 sq km (92,640 sq mi) of tropical coast, while coral reefs extend over an estimated 600,000 sq km (231,600 sq mi) worldwide.

Wetlands vary widely because of regional and local differences in soils, topography, climate, hydrology, water chemistry, and vegetation. They can be natural or man-made, and temporary or permanent in nature. Natural wetlands include river margins, lakes, coastal lagoons, mangroves, peatlands, bogs, fens, mires, and coral reefs. Man-made habitats include fish and shrimp ponds, farm ponds, irrigated agricultural land, salt pans, reservoirs, gravel pits, sewage farms, and canals. Wetlands are among the most productive ecosystems in the world, comparable to rainforests and coral reefs. They have been called “cradles of biological diversity” supporting a rich variety of species. Many birds, mammals, reptiles, amphibians, fish, and invertebrates are found in wetlands, as are numerous plants, including rice—the staple food plant of about half the world's population.

The climate in Britain is suitable to support a large number of wetlands. Many are estuaries, such as Slimbridge and Bridgwater Bay on the River Severn and the marshes of the Humber estuary. Several are reservoirs, such as Rutland Water, and others are natural lakes, such as Lake Bala (Llyn Tegid) and Llyn Idwal in the Snowdonia National Park in Wales. Many more wetland areas are found in Scandinavia, Canada, Alaska, and Russia. In North America the Prairie Pothole Region has a significant number of small wetlands. Major wetland areas are found in warmer climates, such as the Sudd in southern Sudan, one of the largest remaining wetlands in the world, and the Okavango Delta and swampland in north-western Botswana. Many of these swamp systems are extensive and support large numbers of wildlife.

II

Plants and Animals

The plants and animals of wetlands are highly adapted to the environmental conditions. Oxygen may be in short supply, and many plant species including willow (Salix) and mangrove (Rhizophora) have pore systems to allow oxygen to diffuse from the aerial parts of the plants to the roots. In coastal wetlands, plants and animals must adjust to changes in salinity in order to survive. This includes invertebrate animals such as bivalves, worms, and crustaceans, as well as fish. Further adaptations are seen in all organisms found in seasonal wetlands where they have to cope with the extremes of waterlogging and drying out.

III

Ecological Value of Wetlands

The ecological value of wetlands arises from their high primary productivity and rich biodiversity. Many species rely on wetlands for food, water, and shelter. Wetland organisms are important in global water and nutrient cycles. Large quantities of carbon (see Carbon Cycle) are stored in these ecosystems, so they help to moderate global climatic conditions. Scientists now realize that wetlands may play an important part in the maintenance of natural cycles on Earth and in the atmosphere.

Many species are endangered because of the vulnerability of these ecosystems. Wetlands are probably best known for their abundance of birds, and more than 50 per cent of the 800 protected species in the United States rely on wetlands. Several species of ducks (Anas) and geese (Branta) are common along with waders, dippers, and divers. While some are permanent residents others only visit at certain times of the year, during breeding and migration.

Wetlands act as natural sponges, absorbing surplus rainwater, snowmelt, and floodwaters. During dry periods they help to keep rivers flowing, and replenish groundwater, especially in warmer climates. In the United States, the wetlands of the Sand Hills of Nebraska help to replenish the Ogallala aquifer. Wetlands also slow down floodwaters, reducing both the risk of flooding and the damage done by flood erosion. River basin managers now use wetlands in flood control. Wetlands can also aid coastal protection by acting as a buffer to storm tides, hurricanes, tropical storms, and even tsunamis. They absorb much of the energy of incoming waves and are, therefore, used to protect low-lying coastal areas.

IV

Economic Value of Wetlands

There is great economic value in the natural resources of wetlands and many local and indigenous people depend on wetlands for their food and livelihood. Commercial fish species, such as sea trout, bass, and flounder, breed in coastal waters, while shrimps, oysters, clams, and crabs inhabit wetlands. The skins of alligators and crocodiles in tropical areas and the furs of animals like beaver, muskrat, and mink in North America are of high economic value.

Wetland areas are important globally for their timber products. Many tropical hardwoods come from wetland areas, and in South East Asia extensive logging of the swamp forests is occurring.

Wetland vegetation traps sediments carried in rainwater run-off that can affect fish and other water life. This function provides an economic benefit as the productivity of the fish and water life species is maintained and the cost of dredging waterways is reduced. The filtering of impurities (including pollution) is an important economic function of wetlands. A study has shown that without the Congaree Bottomland Hardwood Swamp in South Carolina, in the United States, the area would need a US$5 million waste water treatment plant.