Introduction

Like all the chapters of this book, this is a co-operative work, written by the two main authors, with a contribution on geographical distribution by I.M. Raspopov, and with the help of comments from colleagues and the three editors.

The term ‘wetland’ came into use in the 1970s and was soon generally accepted. There is, however, not much agreement about its limits, which are sometimes denned very broadly, and sometimes very narrowly. Generally speaking, wetlands may be denned as habitats where the water table is situated at or near the ground surface, bearing a vegetation adapted to more or less continuous waterlogging (cf. Sjörs, 1948; Ratcliffe, 1964). ‘Near’ may mean either under or above the ground level.

This definition proves to be vague if we try to fix the limits of wetlands. There is no doubt that littoral habitats and marshes belong to wetlands, but there is less agreement about open water, coastal habitats, mangrove swamps, wet heaths, peat bogs, carrs, and habitats which are waterlogged for only certain periods. One difficulty is that wetlands are very often successional stages. Succession seres often start from open water, under which bottom deposits accumulate gradually causing the community to change over long periods.

Another difficulty arises where the water level changes considerably within brief periods, and the habitat may be waterlogged or flooded only for several short periods per season, or not at all in some years, or as in desert climates, it may be dry for even longer.