extrapolate further to community- and ecosystem-level consequences of toxicity. Suter et al. (1985, p. 400) describe the series of extrapolations involved in trying to predict community- and ecosystem-level consequences based on measures of an effluent's toxicity to individuals.
The LC50 must be extrapolated from the test species to the species of interest, to life-cycle toxicity, to long-term toxicity in the field, to changes in population size due to direct toxic effects and, finally, to the combined direct and indirect toxic effects. Similarly, the emission rate must be converted into an effective environmental concentration in an imperfectly known hydrologic, chemical, physical, and biological system.
Ecology has not reached the point where such projections can be made with confidence. As a result, we are generally not able to make comprehensive quantitative predictions about the community- and ecosystem-level consequences of changes in, for example, the feeding rate of one member of the biotic community (Golley, 1994). In fact, many would argue that we are not even able to make confident extrapolations from bioassay data to consequences for field populations of the test organisms themselves.
A 1981 report from the National Research Council (NRC), Testing for Effects of Chemicals on Ecosystems, suggested that more-effective tests might be possible if they incorporated
. . . a significant number of species representing the degree of diversity found in the ecosystem, detailed observations on physiological and behavioral responses for individual species, [and] a time period similar to the duration of expected chemical exposure in the ecosystem. (p. 7)
However, such a "multispecies microcosm" approach has its own problems, both because the test conditions are oversimplified relative to real ecosystems and because the test conditions are more complex than those of single-species toxicity tests. Because microcosms are, of necessity, simplifications of actual ecosystems, they do not allow for all of the potential pathways of toxic effects that could occur in actual ecosystems. For example, few microcosm designs are large enough to include the largest organisms that occur in the natural ecosystems that the microcosms are intended to represent. Therefore, such tests can neither detect effects on those missing organisms, nor can they detect indirect effects that involve those missing organisms. However, because the tests involve more variables than do single-species bioassays, the mechanisms of any observed effects are more difficult to determine in multispecies microcosms than in single-species toxicity tests.
All of the above criticisms of effluent bioassays were identified by the authors of that same NRC review, which concluded that