Several properties of multimetric biological indexes make them particularly useful for evaluating ecosystem condition:
1. focus on biological endpoints to define condition;
2. use of reference condition (no disturbance or minimal disturbance) as a benchmark;
3. organization of sites into classes (e.g., large streams, small streams, wetlands), each with a select set of environmental characteristics;
4. assessment of change caused by human activities;
5. standardized sampling, laboratory, and analytical methods;
6. numerical scoring of sites to reflect site condition;
7. definition of condition classes, representing degrees of degradation; and
8. numerical and verbal expressions of biological condition that can be easily understood by scientists, citizens, and policy makers.
Unlike single-attribute chemical measures of water quality, analytical tools such as multimetric indexes enhance practitioners' ability to measure condition in a manner that communicates the severity and extent of biological impairment. When combined with knowledge of human activities in a study region, they also provide more effective and focused diagnostic capability to aid in defining causes of degradation. For biological assessment, most applications of purely chemical data, of tolerance indexes that measure organisms' tolerance of one or a few chemical pollutants, and of multivariate statistical models that yield ratios of observed to expected number of species assume, but do not demonstrate, such diagnostic power.
The metrics in a multimetric index are selected to evaluate a diverse range of biological attributes, such as species richness; indicator taxa (stress intolerant and stress tolerant); relative abundances of trophic guilds and other species groups; presence of nonindigenous species; and the incidence of hybridization, disease, and anomalies such as lesions, tumors, or fin erosion (in fish) or head capsule abnormality (in stream insects). The diversity of biological signals incorporated into a multimetric index ensures that the wider consequences ofhuman activity for living systems will be detected and understood.
In addition to being scientifically rigorous, multimetric biological indexes are also policy relevant. They are, for example, sensitive enough to provide reliable assessments of both existing and emerging problems and to evaluate the effectiveness of environmental policies and programs. Integrative approaches to biological monitoring directly support efforts to attain the integrity called for in national and international policy initiatives.
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