The use of the terms 'stress' and 'stressor' is sometimes inconsistent (e.g., Pickering, 1981). The term 'stressor' should be used to describe environmental or other factor intensities severe enough to require a compensatory response at any level of biological organization. A stressor is normally extrinsic. The term 'stress' indicates the organismic response initiated by the stressor, also at any level of biological organization. Thus, the original concept of Selye (1950) that stress is 'the sum of all the physiological responses by which an animal tries to maintain or re-establish a normal metabolism in the face of a physical or chemical force' has evolved into the concept that stress is the biological effect of any force that challenges homeostatic or stabilizing processes and extends them beyond their normal limits, at any level of biological organization - individual, population, or ecosystem (Esch and Hazen, 1978; Bayne, 1980).