There are a variety of methods for assessing the condition of individual organisms that can provide evidence of environmental degradation. These include measurements of body burdens of various compounds (e.g., polychlorinated biphenyls, mercury), the prevalence of cancers or deformities, and the concentrations of enzymes that are synthesized in response to environmental contaminants. Other types of environmental impacts can be detected from studies of the size or structure of populations. These latter measures can be particularly useful for monitoring the status of harvested populations such as fish or trees.
Like the closely related effluent bioassays described above, ambient bioassays expose test organisms to a stimulus. In effluent bioassays, the stimulus is an effluent. In ambient bioassays, the stimulus is usually water from a polluted or potentially polluted source, such as a river. Like effluent bioassays, ambient bioassays measure the survival, growth, or reproduction of test organisms. Whereas effluent bioassays are used to assess the toxicity of particular effluents, ambient bioassays assess the cumulative toxicity of point and nonpoint sources of pollutants after their dilution by, for example, a body of water. Stewart (this volume) describes the insights yielded by ambient bioassays and the considerations that are necessary when evaluating ambient bioassay data.
Particular species have long been used as "indicators" of ecosystem condition. Indicator species are organisms whose sensitivity to pollution makes them useful as a tool for detecting polluted sites. The concept is useful, but reliance on a particular species makes for a crude measure; the indicator species is either present or absent. Absence is not necessarily a result of local conditions; the species may never have had the opportunity to colonize the site. In addition, particular indicator species may help detect particular environmental impacts, but they are not likely to be suitable for detecting a wide range of impacts.
Information on the presence or absence of indicator species has frequently been supplemented with various basic ecological measurements such as species richness (e.g., the number of fish species in a particular section of a river), the abundance of various organisms, or more formal ecological measures of species diversity. Most of these measures assume that analysts have good information on the characteristics of relatively undisturbed reference sites in the region of inter-