An indicator is a numeric value derived from actual measurements of a pressure, state or ambient condition over a specified geographic domain, whose trends over time represent or draw attention to underlying trends in the condition (or "health") of the environment. Key indicator criteria are (see EPA's Report on the Environment):
- the indicator is useful;
- the indicator is objective;
- the indicator is transparent and reproducible;
- the underlying data is characterized by sound collection methodologies, data management systems to protect its integrity, and quality assurance procedures;
- data are available to describe changes or trends; and
- the data are comparable across time and space, and representative of the target population.
Therefore, biological indicators are defined 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.
Using Bioindicators in Regulatory Programs
Assessing the condition of biological communities provides a basis both to determine ecological potential and to measure success in achieving that potential. Biological indicators and data are the hub for:
- setting protection or restoration goals;
- determining what to monitor and how to interpret what is found;
- prioritizing stressors and choosing control measures; and
- assessing and reporting the effectiveness of management actions.
The U.S. Environmental Protection Agency has endorsed biological integrity, through bioassessments, as an indicator of ecological health. It is unique among currently used indicators in that:
- it uses information gathered directly from the aquatic organisms and the biological community of which they are a part;
- the biota that biological integrity is concerned with, is shaped by all environmental factors to which it is exposed over time, whether chemical, physical, or biological; and
- it combines multiple, community level, biological response characteristics into an indicator of cumulative environmental impacts.