The Need for Multiple Lines of Evidence for Predicting Site-Specific Ecological Effects

Abstract

The goal of this paper is to illustrate the value and importance of the “weight of evidence” approach (use of multiple lines of evidence from field and laboratory data) to assess the occurrence or absence of ecological impairment in the aquatic environment. Single species toxicity tests, microcosms, and community metric approaches such as the Index of Biotic Integrity (IBI) are discussed. Single species toxicity tests or other single lines of evidence are valuable first tier assessments that should be used as screening tools to identify potentially toxic conditions in a effluent or the ambient environment but these tests should not be used as the final quantitative indicator of absolute ecological impairment that may result in regulatory action. Both false positive and false negative predictions of ecological effects can occur due to the inherent variability of measurement endpoints such as survival, growth and reproduction used in single species toxicity tests. A comparison of single species ambient toxicity test results with field data showed that false positives are common and likely related to experimental variability or toxicity to selected test species without measureable effects on the ecosystem. Results from microcosm studies have consistently demonstrated that chemical exposures exceeding the acute or chronic toxicity concentrations for highly sensitive species may cause little or no ecologically significant damage to an aquatic ecosystem. Sources of uncertainty identified when extrapolating from single species tests to ecological effects were: variability in individual response to pesticide exposure; variation among species in sensitivity to pesticides; effects of time varying and repeated exposures; and extrapolation from individual to population-level endpoints. Data sets from the Chesapeake Bay area (Maryland) were used to show the importance of using “multiple lines of evidence” when assessing biological impact due to conflicting results reported from ambient water column and sediment toxicity tests and biological indices (benthic and fish IBIs). Results from water column and sediment toxicity tests with multiple species in tidal areas showed that no single species was consistently the most sensitive. There was also a high degree of disagreement between benthic and fish IBI data for the various stations. The lack of agreement for these biological community indices is not surprising due to the differences in exposure among habitats occupied by these different taxonomic assemblages. Data from a fish IBI, benthic IBI and Maryland Physical Habitat Index (MPHI) were compared for approximately 1100 first through third-order Maryland non-tidal streams to show the complexity of data interpretation and the incidence of conflicting lines of evidence. A key finding from this non-tidal data set was the need for using more than one biological indicator to increase the discriminatory power of identifying impaired streams and reduce the possibility of “false negative results”. Based on historical data, temporal variability associated with an IBI in undisturbed areas was reported to be lower than the variability associated with single species toxicity tests.