Getting more out of bioassessment data: Causal inferences from taxon tolerances for individual stressors

Data collected during biossessment are usually interpreted by comparing indices with reference data. While this practice demonstrates biological departure from reference conditions, it provides no insight into cause-effect relationships and therefore is of limited value for informing management. However, if the tolerances of individual taxa for particular proximate stressors are known, it is possible to use data on assemblage composition at a site to place upper or lower limits on values of those stressors. These limits can be contrasted with values expected in the absence of the specific anthropogenic phenomena, providing insights into whether such phenomena are likely ultimate causes of biological responses. For example, data on biological assemblage composition and temperature tolerances of individual taxa can be used to place limits on upper and lower temperatures in a stream. Biota-implied temperatures can be compared with modelled values in the absence of an anthropogenic phenomenon such as the operation of a dam with bottom release, a discrepancy between biota-implied and without-dam temperature ranges suggesting a causal pathway from dam operation to stream temperature to assemblage composition. Even if tolerance data are not available for all candidate stressors, tolerance-based inferences can rule out particular pathways as likely causes of biological impact.