The value of a wetland is a measure of its importance to society. Wetland functions are valued to various degrees by society, but there is no precise, general relationship between wetland functions and the value of wetlands to society, and values can be difficult to determine objectively. Wetlands also have aesthetic values. A wetland's value can be weighed directly or relative to other uses that could be made of the site; thus, the location of a wetland affects its value to society. For example, wetlands in urban settings might have higher value for recreation and education or for alternative uses of the site than wetlands in undeveloped lands or far from population centers. Assessing the value of wetlands involves methods of social science, especially economics.

Decisions about whether to protect wetlands or how much to spend on wetland protection are policy decisions based in part on the value society places on wetlands. Part of that value depends on a sound scientific knowledge of what wetlands do, i.e., a knowledge of wetland functions. Indeed, some groups have suggested the creation of a national scheme that would designate wetlands of high, medium, and low value based on some general assessment of wetland functions that does not require field evaluation. Examples of categories that have been proposed as having low value include wetlands of under 10 acres (4 ha) or of some other specific size; fastlands, which are wetlands maintained behind dikes or levees; wetlands within industrial complexes or in intensely developed areas; wetlands affected by anthropogenic disturbance; artificial wetlands; frequently farmed wetlands; and regionally abundant types of wetland. As described above, however, it is usually not possible to relate such categories in a reliable way to objective measures of wetland functions, in part because the relationships between them are variable and in part because we do not have enough scientific knowledge. Wetlands of any of the categories can have a variety of wetland functions based on objective measurements (Bostwick, 1992). Examples include the prairie potholes of the upper midwest, which are isolated wetlands generally smaller than 10 acres (4 ha), but which provide vital wildlife habitat; and the wetlands of the Lake Calumet area in heavily industrialized southeast Chicago, which provide habitat for more than 170 bird species, of which 11 are listed by the state of Illinois as threatened or endangered (Kendall, 1990). The remainder of this chapter deals with the scientific assessment of wetland functions and its usefulness in planning.

Important concepts in functional assessment of wetlands include functional capacity, predictors of function (indicators), and thresholds for functions. Functional capacity is the ability of an ecosystem to perform a function. An example would be kilograms of sediment removal per hectare per year. A predictor is an observable condition, the state of which is related to the capacity or ability of a wetland to perform a function. For example, morphometry of a wetland might