Defining biological integrity and incorporating it into philosophical, policy, scientific, and legal constructs is but the first step toward using the concept. For credibility in any of these arenas, practitioners need tools for translating the subjective concept into something objective; they need tools both to quantify and to describe. Scientists and managers need formal methods for sampling the biota, evaluating the resulting data, and clearly describing the condition of sampled areas. A multimetric measurement system, called the index ofbiological integrity (IBI), developed in 1981, has helped fill this need.
The complexity of biological systems and the varied impacts humans have on them require a broadly based index composed of multiple measures that, like IBI, integrates information from individual, population, assemblage, and landscape levels. If properly used, such a multimetric index enables practitioners to evaluate sites and rank them according to how far the sites diverge from integrity along a gradient of biological condition.
Multimetric indexes, including conventional economic indexes such as the index of leading economic indicators, provide a convenient way to measure the status of a complex system. Such indexes typically include a variety of measurements, or metrics, that characterize the system being measured, such as housing starts or manufacturers' shipments, inventories, and orders. Besides measuring the economy, multimetric indexes have also been developed to assess human health. Physicians apply the multimetric concept when they rely on a battery of tests to diagnose illness. Similarly, the Apgar test, developed in 1952 by anesthesiologist Virginia Apgar, assesses the condition of newborn babies on the basis of five simple criteria, including heart rate and respiration. A newborn's scores for each criterion are summed to give an overall Apgar score ranging from 0 to 10, which describes the baby's overall condition just after birth. All multimetric indexes require a baseline state against which changing conditions are assessed. For an ecosystem, that baseline - biological integrity - is the condition at a site with a biota that is the product of evolutionary and biogeographic processes in the relative absence of the effects of modern human activity.
Multimetric biological indexes integrate multiple dimensions of living systems to measure and communicate biological condition. Robust measures of the biological dimensions of site condition have by now been applied in basic science, resource management, engineering, public policy, law, and community participation on every continent except Antarctica and in developing as well as developed nations. One advantage of a robust multimetric biological index is that ecological knowledge reinforced by empirical data supports it; its use does not require resolution of all higher-order theoretical debates in contemporary ecology.
Initial work to develop a multimetric approach to biological indicators concentrated on streams, with fish as focal organisms, but the underlying conceptual framework has now been applied to diverse environments (streams, large rivers, wetlands, lakes, coastal areas, coral reefs, riparian corridors, sagebrush steppe, and others) and with varied taxonomic groups (fish, aquatic and terrestrial invertebrates, algae and diatoms, birds, and vascular plants). Several US states have incorporated biological measures, or biocriteria, into state water-quality standards (Ohio, Florida, Maine, Vermont), and biological assessment is now a key component of implementing the Clean Water Act and the European Union's Water Framework Directive.
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