Establishing causality between environmental stressors and effects on aquatic ecosystems

Establishing causal relationships between environmental stressors and observed effects in natural systems is difficult due to the many intrinsic environmental factors that can hinder this process and because there are no widely accepted and proven approaches for determining such relationships. Several types of approaches or combinations of approaches, each with their own sets of advantages and limitations, have been applied in a variety of ecological systems to investigate possible causal relationships between stressors and effects. These include controlled laboratory studies (including acute and chronic bioassays), experimental field manipulations, field studies based on synoptic field surveys, mathematical simulation modeling, statistical associations, various combinations of laboratory, experimental, and field studies, and the ecoepiderniological (weight or evidence) approach. The use of ecoepidemiological ("forensic toxicology") principles is becoming increasingly attractive as a method to help establish causality because it does not involve the same limitations of other approaches and it can also be used to integrate disparate information within a logical framework so that scientifically and defensible regulatory decisions can be made. The objective of this Commentary series of papers on the issue on causality is to demonstrate the application of the ecoepidemiology approach, using a variety of case history studies, for establishing causal relationships between specific stressors and biological effects. For each case history provided in the following series of papers, the authors describe their study situation, summarize the results supporting a causal relationship, and then compare their study results against seven standard causal criteria.