Ecological exposure to chemical stressors occurs when a chemical reaches an organism and is in a form that is bioavailable. In order to be bioavailable, a chemical must reach a location on or in an organism where it can cause an effect. The degree of bioavailability can be measured through various studies, depending upon the type of contaminant under consideration. The lower the bioavailability, the lower the risk that will be posed by the contaminant. In the absence of bioavailability, there is no exposure and the contaminant will essentially pose no risk to an organism. Keep in mind however, that the degree of bioavailability differs among species and can change under different environmental conditions.