Simulating Biological Impairment to Evaluate the Accuracy of Ecological Indicators

Environmental management depends on accurate monitoring and assessment of ecological conditions. Bioassessments are generally based on the measurement of selected indicators. However, the accuracy of these indicators is difficult to evaluate because the true biological impairment of a system is almost always unknown. We describe a simulation procedure that allows objective comparisons of estimated indicator values against known impairment. We modeled how densities changed with increasing stress as a function of tolerance values. We applied this procedure to data from five reference sites and compared non-metric multidimensional scaling (NMDS) and observed taxa richness for detecting known impairments. NMDS showed that the trajectories of impairment caused by stress were all evident at 100, 300, or 600 counts. However, the discrimination of stress levels within ordination space greatly improved with increasing count. Estimates of richness based on 100- or 300-count samples often severely underestimated true taxa loss and frequently indicated taxa gain. Estimates of taxa loss based on 600 counts also underestimated true taxa loss, but these estimates were strongly correlated with true taxa loss. This simulation procedure should be applicable to the evaluation of how well a variety of biotic indicators measure biological impairment.