However, use of these indices as a sole basis for quantifying ecological status was soon abandoned both because natural systems vary substantially in both of the key diversity components of richness and evenness and because little consensus emerged regarding the broader ecological significance of observed differences in diversity values. High diversity values were interpreted as implying better ecological status than lower diversity values. Two potential advantages of these indices are that they had a theoretical foundation and did not require specific identification of taxa (just the ability to assign individuals into unique taxa – e.g., morphospecies). With the emergence of theoretical ecology in the 1970s, aquatic scientists promoted the use of these indices, which measure aspects of alpha diversity (i.e., at a site) and whose values are jointly influenced by variation in both taxa richness and evenness. The first of the assemblage-wide indices used in bioassessments were species diversity indices, such as the Shannon-Wiener index. As a consequence, bioassessment programs typically favor the use of indices that assess the overall condition of entire assemblages. However, the response of single species cannot quantify overall ecological status because they occupy only a small part of the overall niche space represented by an entire ecological assemblage. Indicator or sentinel species are those species that are highly sensitive to environmental disturbance and are sometimes used in aquatic biological assessments to provide an early warning that more severe water quality conditions are likely to occur unless mitigation measures are taken. Regardless of the measurement endpoint used, the assessment of ecological status is usually based on the degree to which index values observed at an assessed site deviate from values observed at one or more reference sites considered to be in high, or desirable, ecological condition (see the section on Benchmarks and baselines for details). Six broad approaches emerged over time: the use of indicator or sentinel species, diversity indices, biotic indices, multimetric indices, measures of taxa loss or shifts in taxa composition, and functional indices. Several different methods have been developed to do so, the choice of which was driven by specific sociopolitical needs coupled with the current state of scientific understanding of aquatic ecosystem structure and function.
Regardless of the indicator group(s) used in assessments, managers need measurement endpoints that quantify ecological status.
Carlisle, in Reference Module in Earth Systems and Environmental Sciences, 2021 Indices
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