Should aquatic ecosystem condition be based on taxonomy or ecology or both? A comparison of approaches

The use of freshwater invertebrates, especially insects, to describe the condition of freshwater streams, rivers, ponds, lakes, and wetlands originated in Europe in the late 1800s, and has been always premised on an assumption that taxonomic diversity is positively related to the health or condition of a freshwater system. However, most assessment protocols employ a mixture of levels of taxonomic resolution: some groups to class or order, some to family, with an occasional one to species. Measurement of species diversity is usually more accurately described as a measurement of taxa diversity. An often erroneous conclusion of a taxonomically-based assessment of condition is that the better (more highly resolved) the taxonomy, the better an ecosystem will be judged to be. Assessment of condition based on functional group structure of invertebrate assemblages circumvents this problem. Functional groups distinguish invertebrates by the ecological roles played in a system, and employ only the level of taxonomy required to establish the functional roles of invertebrates present. Other advantages and limitations to the employment of functional groups for assessment of system condition are described.