The ability of a stressor to evoke a response in an individual, community, or population is often easy to observe. It is substantially more difficult, however, to demonstrate that the stressor caused the response. Philosophers have been interested in the subject of causation since the time of Aristotle; however, scientific interest has been more recent. Ecotoxicological associations in the natural environment are rarely unambiguous and scientists often depend on weight of evidence approaches such as ecological risk assessments to resolve the ambiguities. These weight of evidence approaches often depend on determinations that exposure of an organism to a chemical results in a toxico-logical response. To cite just one possible example, we consider the scenario where there is an observed reduced species diversity or abundance in a population of fish in a stream flowing through an urban area compared to a control population. Meaningful mitigation of this situation requires that the cause or causes of the biological conditions associated with diversity and abundance be identified. These causes may be toxicological, related to nutrients, habitat, predator-prey relationships, competition, and other factors. Application of causation criteria such as Bradford Hill's postulates may assist the investigator in identifying unambiguous causal elements and developing a mitigation strategy.

This article opens with a discussion of the historical evolution and theory of causation criteria focusing on Bradford Hill's postulates and related systems. This is followed by a discussion of applications of causation criteria to ecotoxicological problem solving.

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