ABSTRACT
The goal of this entry is to discuss the characteristics of two categories of species mentioned in the preceding text. First, those species that provide signifi cant insights into the state of the ecological systems in which they reside, often indicating impending state changes and generically referred to as indicator species. Second, those species with highly signifi cant functional roles in the structuring and stability of ecological systems, referred to as keystone species.[3]
INDICATOR SPECIES
I view the indicator species concept broadly-that is, they are species whose status and trends provide insights into environmental conditions that extend beyond their own
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measurement.[3,4] Indicator species can be used as surrogates for ecosystem processes or conditions that are too diffi cult or costly to measure directly. The primary role of ecological indicators, in general, and indicator species in particular, is to measure the response of the ecosystem to anthropogenic disturbance.[5] Selection of indicator species is a critical aspect of most environmental monitoring programs. For example, indicator species with known stressresponse relationships are often chosen as early warning sentinels of impending stressor effects so as to avoid undesirable or irreversible environmental outcomes.
