386 Wetlands Ecosystems reed grass (Phragmites) and other herbaceous species in parts of Europe and Africa Fens dominated by woody species may be called swamps, forested fens, or forested wetlands In the strictest sense, bogs are peat-accumulating wetlands that receive water only from precipitation However, the term also is applied to poor fens, which superficially resemble true bogs, and more loosely to any wet vegetated environment that offers less than firm footing Numerous formal classifications of the different types of wetlands have been developed They give some indication of the vast diversity of wetland ecosystems but cannot be used to enumerate that diversity Because they have been developed for different purposes and from different scientific perspectives, they vary in the criteria used for classification and in the level of resolution to which they classify wetlands Nonetheless, the existence of literally dozens of wetland classifications worldwide implies the existence of such a high diversity of types that they must be grouped into similar classes for practical reasons of inventory, mapping, management, and scientific communication The high diversity of wetland ecosystems also is revealed by considering the many different bases for wetland classification: geomorphologic setting in the landscape, genesis (how they formed), shape or form (e.g., surface morphology, basin morphometry, morphology of the underlying mineral terrain), hydrology (e.g., flooding frequency and duration), sources of water supply, physiognomy of the vegetation, species composition of the vegetation, soil type, water chemistry, and various combinations of these factors Given that each of these variables represents not a single factor but rather a continuum, the extremely large number of possible combinations becomes apparent Thus, most classifications in wide use today are broadly based, interdisciplinary, and hierarchical (i.e., consisting of several nested levels with increasing detail only at lower levels in the hierarchy) Species composition, the basis for diversity within wetlands and the focus of many older classifications, is usually not a defining characteristic except at the lowest levels in current classifications Rather, similarities in hydrology and geomorphology are more likely to form the highest levels of classification, reflecting the importance of the interaction of water with the landscape in determining the biotic characteristics of wetlands Nonetheless, the terms bog, fen, marsh, and swamp persist in both scientific and popular literature because of their long history of use and the powerful images they evoke These terms are not distinguished primarily on the basis of geomorphology or hydrology; nor are they distinguished on any single criterion, nor used consistently Perhaps the key distinction among them in terms of species diversity is their source or sources of water, which determine hydrologic regime and water chemistry Greater surface water inputs generally lead to greater water level fluctuations and higher concentrations of base cations and nutrients in marshes and swamps on mineral soils than in bogs and fens In this chapter, these terms follow current American usage (see Glossary) The current classification systems used in the United States (Cowardin et al., 1979) and Canada (National Wetlands Working Group, 1997) reveal much about the diversity of wetlands in these two countries For example, the U.S classification is hierarchical, recognizing major wetland systems and 10 subsystems, each of which is divided further into to classes A system is defined as ‘‘a complex of wetlands and deepwater habitats that share the influence of similar hydrologic, geomorphologic, chemical, or biological factors’’ (Cowardin et al., 1979, p 4) The five systems are the marine, estuarine, riverine, lacustrine, and palustrine The term class ‘‘describes the general appearance of the habitat in terms of either the dominant life forms of the vegetation or the physiography and composition of the substrateFfeatures that can be recognized without the aid of detailed environmental measurements.’’ Classes can be further divided into dominance types based on dominant plant species for vegetated sites or on the dominant sedentary or sessile animal species where vegetation is not the dominant cover Special modifiers are used for water regime (type and duration of flooding), water chemistry, soils, and human alterations (e.g., excavated, impounded, diked) Fifty-five classes emerge before one ever gets to the level of dominant species, water regime, or water chemistry Most of Canada’s wetlands fall within just one system (palustrine) of the U.S classification Canada’s classification differentiates many types of palustrine wetland, especially peatlands This classification also is hierarchical but with just three levels: class, form, and type Five classes are recognized: bog, fen, marsh, swamp, and shallow water Forms are defined on the basis of surface morphology of the wetland, position in the landscape, morphology of the underlying terrain, tidal effects, and proximity to surface water bodies The Canadian system recognizes 16 bog forms and subforms, 12 fen forms and subforms, marsh forms with 18 subforms, swamp forms with 19 subforms, and shallow water forms with 24 subforms The 18 types based on the general physiognomy of the vegetation (e.g., hardwood treed, coniferous treed, low shrub) modify these 121 forms and subforms Differences in species composition are left to individual users Only the wetlands of western Europe are as well documented as those of the United States and Canada Specific sites on other continents have received considerable attention but, with few exceptions, comparative regional treatments began to emerge in the English language only 15 years ago, with most appearing since 1990 (see References) The exceptions include Chapman’s booksFfor example, Salt Marshes and Salt Deserts of the World (1960), Mangrove Vegetation (1976), and Wet Coastal Ecosystems (1977) However, despite the general lack of scientific studies in many areas of the world, as of 1991, 62 countries from all continents had signed the Convention on Wetlands of International Importance Especially as Waterfowl Habitat (Ramsar Convention) and had designated 527 sites covering more than 30 million as being of international importance Five international wetland symposia have been organized since 1980 by INTECOL (The International Society of Ecology), with a sixth planned for the year 2000 in Quebec City Major Regional Wetland Ecosystems Efforts to classify wetlands into unique entities often obscure, either spatially or temporally, critical connections among parts of what might be viewed as integrated ecological systems