336 ESCH AND HAZEN 



an attempt to obviate the objections, suggested that stress be defined 

 as "the effect of any force which tends to extend any homeostatic or 

 stabihzing process beyond its normal limit, at any level of biological 

 organization" such that the result will be either an enhancement or a 

 diminishment in the probability of mortality, natality, or permanent 

 change. This definition is broad enough in scope to permit 

 application at individual, population, or ecosystem levels of organiza- 

 tion but is explicit enough that the possibilities for quantification of 

 stress are still maintained. 



Model for Response to Stressor Input at the Individual Level 



Possible response of an individual organism to stressor input is 

 shown in Fig. 3. A resting or undisturbed individual operates within 

 the constraints of a normal homeostatic range. In addition, there are 

 wider limits within which the organism may continue to function, 

 although not optimally. The dimension of these ranges varies in both 

 sDace and time among individuals within a single population and 

 among individuals in different populations. 



If the organism is subjected to a stressor input of the proper kind 

 and with an appropriate magnitude, the alarm stage of the GAS will 

 be initiated. Generally, the duration of the alarm stage is brief, and 

 the direction of the response is dictated by the nature of the specific 

 biochemical or physiological process being measured. If the stressor 

 input is withdrawn, the original steady state of the organism will 

 return. 



If the stressor input persists in time or grows in magnitude, then 

 the respondent organism enters the resistance stage. The length of 

 time an individual is able to resist stressor input and maintain itself 

 within maximum homeostatic limits is variable. It depends to some 

 extent, of course, on the inherent (or genotypic) potential of the 

 individual, in combination with such intrinsic factors as its state of 

 well-being, age, etc. Other extrinsic factors that modify the length of 

 time the organism is able to operate within homeostatic limits 

 include the season of the year and the nature, magnitude, and 

 duration of the stressor input. As the model suggests, if the stressor is 

 withdrawn during the resistance stage, then the original steady state 

 returns. 



If the stressor input continues long enough or if the magnitude of 

 input increases beyond a critical point, the individual organism will 

 no longer be able to cope homeostatically. The final stage, 

 exhaustion, then ensues. If this end point is reached, the probability 

 of an individual organism's surviving, reproducing, or returning to its 

 original steady state is permanently altered. 



