Multimetric biological indexes have many applications, including setting priorities for conservation, diagnosing the likely cause of damage at degraded sites, and evaluating the effectiveness of ecological restoration efforts.
To determine a locale's index, practitioners collect samples of invertebrates, fish, plants, or other taxa. They sort, identify, and count organisms in the sample and calculate relevant metrics, such as taxa richness or the relative abundance of species groups differentiated by pollution tolerance, taxonomic composition, functional feeding group, behavioral habit, or numerical dominance. As in the Apgar test for newborns, combining metrics into a multimetric index for bioassessment requires conversion of individual metric values into unitless numbers, or scores, which are then summed to yield a single index value. The scores are defined by comparing the locale's metric scores with the scores expected under reference conditions, that is, at a relatively undisturbed or natural site of the same type in the same geographic region (see Figure 1).
Under the IBI as originally developed, metrics can have a score of 5, 3, or 1, depending on whether the metric is comparable to, deviates somewhat from, or deviates strongly from 'undisturbed' reference condition. The sum of metric scores reflects the locale's biological condition. The lowest index indicates the most-disturbed sites in poor biological condition, and the highest scores indicate relatively undisturbed sites in robust biological condition. For example, for rivers in the midwestern United States, an IBI based on 12 metrics could range from a low of 12 in areas with no fish to 60 in areas with diverse fish faunas typical of pristine locales.
The benthic invertebrate IBI (B-IBI) for streams contains 10 metrics, including seven measures of taxa richness (total number of taxa; number of mayfly, stone-fly, and caddisfly taxa; number of clinger, long-lived, and intolerant taxa); two relative abundance measures (predators and tolerant taxa); and dominance (relative abundance of the three most abundant taxa). B-IBI, then, ranges from 10 to 50 and defines five classes of stream conditon. In western Washington State, for example, recent work has taken two important steps in how these classes are applied: the work (1) connects the numeric
B-IBI, and the biological condition it reflects, to regulatory language under the Clean Water Act and (2) casts this language in terms of creatures the regional populace cares about - the Pacific Northwest's iconic salmon (Table 1). This effort defines a stream as impaired under the act when B-IBI declines below 35, a level indicating that a stream can no longer support a healthy anadromous (migratory) salmon population. It defines a stream whose B-IBI is over 35 but under 46 as compromised but not impaired under the act.
Finally, a few studies have applied the IBI approach to assessing the condition of terrestrial systems. In the shrub-steppe environments of eastern Washington and Idaho, two IBIs - one based on terrestrial invertebrates and the other on plants - were able to detect the biological effects of human actions on the resident biota. In Washington, sites with a minimal history of human disturbance had higher IBIs than all other categories of disturbance, even when that disturbance was no longer occurring (physical, waste dumping, and agricultural: Figure 2). Agricultural disturbances, whether past or present, yielded the lowest IBIs, or the poorest biological condition. A companion study in Idaho showed that biological condition was also influenced by livestock grazing and that carefully planned restoration programs increased IBIs over those at similar, unrestored sites.
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