Ecological risk assessment is the probability of an effect occurring to an ecological system. Important components of a risk assessment are the estimations of hazard and exposure due to a stressor. A stressor is a substance, circumstance or energy field that causes impacts, either positive or negative, upon a biological system. Hazard is the potential of a stressor to cause alterations upon a biological system. The determination of an LD50 or the mutagenicity of a material are attempts to estimate the hazard posed by a chemical stressor.
Exposure is a measure of the concentrations or persistence of a stressor within the defined system.
Exposure for a chemical substance can be expressed as a dose, but in environmental toxicology it is often possible to measure environmental concentration. Other measures of exposure might include percentage of riparian cover, temperature days, or other measures of stressors within an ecological system. The real purpose of ecological risk assessment is to ultimately provide relevant information for decision making. This is a very applied science, tied to societal questions and with important ramifications for society. Questions about pesticide use, the introduction of genetically modified organisms, how clean is clean, and what are the impacts of urbanization of a watershed can all be addressed using ecological risk assessment as a tool.