Biological indicators are defined here as a numerical value(s) derived from actual measurements, has known statistical properties, and conveys useful information for environmental decision making. It can be a measure, an index of measures, or a model that characterizes an ecosystem or one of its critical components. The primary uses of an indicator are to characterize current status and to track or predict significant change. With a foundation of diagnostic research, an ecological indicator may also be used to identify major ecosystem stress."

see also: Indicators_Environmental_Science

Source: About Biological Indicators. In: Biological Indicators of Watershed Health. EPA