Over the twentieth century, concern about environmental trends - particularly, widespread chemical pollution -sparked the development of indicators to enforce environmental laws and regulations. Initially, these indicators centered on measures of administrative activity, pollution-control technologies, and effluent flows. Eventually, five major classes of indicators came into use, reflecting the dominance of toxicology (specifically, the concepts of stressor, exposure, and response) and a focus on chemical pollutants.
• Administrative indicators. Number of pollution discharge permits issued, enforcement actions, planning exercises, and pollution-control grants.
• Technological indicators. Application of technologies to reduce effluents or implementation of best management practices (stormwater detention, conservation tillage).
• Stressor indicators. Measurements of effluent reduced through a particular technology or land-use practice.
• Exposure indicators. Concentration of pollutants in water, soil, or air; measures of physical habitat, altered nutrient dynamics, rates of sedimentation.
• Response indicators. Measures of biological condition, such as taxa richness; population demographics; thermodynamics, including plant productivity and emergy (contraction of embodied energy, equivalent to the amount of energy consumed to produce something, for example, the total amount of fossil fuel-derived fertilizers, mechanized and human labor, and sunlight needed to grow corn); multimetric indexes that integrate multiple biological attributes.
Exposure and response indicators arose as environmental managers and regulators came to recognize that the first three groups of indicators neither assessed real ecological condition nor ensured that legally mandated goals (e.g., protecting ecological integrity) were attained. Only exposure and response indicators directly measure ecological endpoints and might thus appropriately be called ecological indicators.
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