According to Paracelsus, ''The right dose differentiates a poison and a remedy.'' In toxicology, however, dose has always been defined rather loosely. Strictly, dose applies to the administered levels of a drug or chemical to a test organism under controlled environmental conditions. In ecological context, a dose will be referred to as a quantity of stressor received by an entity or the quantity of exposure to a stressor - hence stressor dose. Since a stressor is any physical, chemical, or biological agent or condition that can induce adverse response in any entity of interest, a stressor dose therefore may include the amount of an environmentally stable chemical such as PCB or a physical agent such as radiation or a biological agent such as bacteria that produces a response from the entity receiving it. The entity could be an individual species, population of species, community, or an ecosystem. Dose - chemical, physical, or biological - can sometimes be differentiated on the basis of the amount encountered in the environment (exposure dose), or the actual amount of the exposed dose that enters the organism's body (absorbed dose), or the quantity administered to an organism under controlled situations (administered dose), and the combination of all types (total dose). Note however that it may be practically difficult to administer or measure progressive doses of a stressor on ecosystems, unlike the animal models used in human health risk assessment under controlled laboratory situations.
In addition to differentiating the doses encountered by an organism(s), the frequency of exposure to the stressor is also an important consideration in ecological risk assessment. This becomes even more relevant if the stressor is seasonal or permanent in an ecosystem. Organisms, for example, a fish species exposed to frequent sublethal doses of a xenobiotic may respond differently than those that receive one high but infrequent lethal dose of the same stressor.
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