Selecting Metrics

Metrics incorporated into a multimetric biological index are chosen because they reflect specific and predictable responses of the biota to human activities across landscapes (Figure 1). These responses are similar to dose-response curves used by toxicologists, which measure how an organism's response varies with dosage of a toxic compound. Because they provide an integrative measure of the cumulative impacts of all human activities in a region or watershed, a multimetric index and its component metrics can be viewed as ecological dose-response curves. The metrics in a multimetric index are validated empirically, and therefore they (1) are biologically and ecologically meaningful, (2) increase or decrease as human influence increases, (3) are sensitive to a range of stresses, (4) distinguish stress-induced variation from natural and sampling variation, (5) are relevant to societal concerns, and (6) are easy to measure and interpret.

Total taxa richness

Clinger taxa richness

% Legless individuals

Low High

Human influence gradient

Dominance (3)

Low High

Human influence gradient

Figure 1 An example of the dose-response curves for a hypothetical four-metric IBI for a study of Japanese streams. The four metrics are benthic invertebrate taxa richness (top left); taxa richness of clinger taxa (top right); percentage of sampled individuals in sample that are legless, such as snails and worms (bottom left); and dominance, measured using the three most abundant taxa (bottom right). Black triangles represent values from reference steams considered to be minimally influenced by humans. Gray triangles represent values from highly disturbed streams. The black ovals represent a sampling site that has an IBI of 16 (3 + 3 + 5 + 5). Modified from Karr JR and Chu EW (1999) Restoring Life in Running Waters: Better Biological Monitoring. Washington, DC: Island Press Copyright © 1999 by Island Press. Reproduced by permission.

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