Max Wade and Elena Lopez-Gunn
Wetlands represent only 6 per cent of the Earth’s surface, but it is believed that in 1900 this percentage might have been twice as much (Barbier et al. 1994). Wetlands include a wide array of habitats, ranging from fens and marshes to mangrove forests and rice paddies, and are considered one of the most threatened landscapes in the world (Gardiner 1994). A simple definition is ‘land with soils that are permanently flooded’ (Williams 1990: p. 1). The Ramsar Convention, an international treaty to conserve wetlands, defines wetlands 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 six metres.’ This encompasses a wide range of habitats, the main types being shown in Box 20.1 based on just one recognised classification (Gleich 1993). According to the Ramsar classification, there are marine, coastal, inland and man-made types, subdivided into thirty categories of natural wetland and nine humanmade ones, such as reservoirs, barrages, and gravel pits (Dugan 1993). The International Wetlands Research Bureau has established a wetland database. In relation to natural wetlands, the World Conservation Monitoring Centre (Groombridge 1992) summarises the extent of different types relative to latitude (Figure 20.1). Surveys have established the extent of wetlands, both past and present, in different areas of the world, leading to the compilation of inventories of wetland sites, particularly for plants, birds and mammals, and investigations into physical, chemical and biological processes. Collectively, these have developed a real insight into wetland ecology.
Wetlands epitomise the problem of trying to classify ecosystems and habitats. While mangroves, for example, meet the criteria set for wetlands, they also meet those for both forest and coastal systems (Groombridge 1992), indicating the integral role that wetlands play in the broader
A classification of wetland habitats
Marsh A frequently or continually inundated wetland characterised by emergent herbaceous vegetation adapted to saturated soil conditions. In European terminology, a marsh has a mineral soil substrate and does not accumulate peat.
Swamp A wetland dominated by trees or shrubs (US definition). In Europe, a forested fen would easily be called a swamp. In some areas, wetlands dominated by reed grass are also called swamps.
Fens A peat-accumulating wetland that receives some drainage from surrounding mineral soil and usually supports marsh-like vegetation.
Bogs A peat-accumulating wetland that has no significant inflows or outflows and supports acidophilic mosses, particularly spaghnum.
Peatland A generic term for any wetland that accumulates partly decayed plant matter.
Source: Gleick 1993.