Ecotoxicology involves the study of the propagation of toxins or other harmful substances through an ecosystem. This applies to both natural and man-made toxic substances.
It is sometimes regarded as a sub-field of ecological risk assessment (ERA) since real-world applications frequently involve predicting which particular species are affected by environmental disasters (usually toxic spills) and whether these toxins will travel up the food chain and pose a risk to humans.
Specific applications of ecotoxicology include the study of the "red tide" phenomenon (due to toxins in dinoflagellates or other algal blooms) that can render seafood (mostly molluscs and other shellfish) unfit for human consumption, as well as studies on mercury poisoning from eating fish in contaminated areas.
The ECOTOXICOLOGY-L mailing list archives may be found at http://listserv.vt.edu/archives/ecotoxicology-l.html.