A Framework for Evaluating Existing and Emerging Indicators of Biological Condition in Freshwater Ecosystems

By Michelle F. Bowman, Yong Cao and Charles P. Hawkins

Western Center for Monitoring and Assessment of Freshwater Ecosystems, Department of Watershed Sciences, Utah State University, Logan, Utah.

The characteristics of an effective indicator of biological condition include a link with valued ecosystem attribute(s) and an interpretable and predictable response to the stressor(s) of interest. Many existing indicators can be used to detect change in freshwater communities. However, there is no consensus on the indicator(s) that are most appropriate for an overall assessment of biological condition and for effective impact diagnosis. We summarized the current views on valued ecosystem attributes and their relationships with traditional and emerging benthic invertebrate indicators of biological condition in stream ecosystems. The indicators we evaluated included observed vs. expected taxa richness, various traditional metrics (e.g., percent chironomids), taxonomic distinctness, functional traits, number of rare species, and ordination summaries. We characterized the patterns these indicators exhibited along agricultural, water diversion, grazing, logging, and urbanization gradients. In addition, we adjusted a number of indicators to correct for variation associated with the natural gradients that co-varied with the stressor gradients. We considered relationships with valued ecosystem attributes, predictability and variability in response to stressors, and ability to discriminate among stressors in the evaluation of existing and emerging indicators of biological condition. This framework illustrates the need for a clearer definition of valued ecosystem attributes, better understanding of the relationships between these attributes and indicators of biological condition, well defined set of stressor gradients, consideration of natural co-variables, and evaluation of the utility of indicators for impact diagnosis.

Keywords: biological indicators, bioassessment, ecosystem services, stressor gradients, and impact diagnosis