338 ESCH AND HAZEN 



This concept of stress was developed for vertebrate animals with 

 autonomic nervous systems and endocrine glands capable of produc- 

 ing epinephrine and adrenocorticosteroids. Obviously many organ- 

 isms, both plant and animal, have neither. The question that then 

 occurs is, Can these organisms be considered within the framework 

 of Fig. 3? The answer to the question is a tentative yes. We believe 

 the schematic is general enough in scope to permit us to view the 

 response of any organism, plant or animal, vertebrate or invertebrate, 

 to the force of stressor input. 



Model for Response to Stressor Input at the Ecosystem Level 



Figure 3 represents what may occur when an individual organism 

 is subjected to stressor input. But what happens when an ecosystem 

 is perturbed? Clearly an ecosystem response to perturbation is far 

 too complex to model or represent by a series of solid or dashed 

 lines, especially if the perturbation is subtle. If, however, as 

 previously noted, the forces operating to maintain homeostatic 

 steady state in an individual are analogous to those maintaining 

 equilibrium at the ecosystem level, models for stress at these two 

 levels should be somewhat similar and, for the sake of this discussion, 

 simple. 



Before dealing with a conceptualization of stress at the ecosys- 

 tem level, we should emphasize that ecosystem stability and 

 complexity do not necessarily go hand-in-hand. Thus increased 

 stability does not necessarily follow from increased complexity 

 (May, 1976). Stability at the ecosystem level, as used here, refers 

 only to the ability of an ecosystem to return to equilibrium 

 following perturbation. 



We have assumed that there is a clear parallel between stress at 

 individual and ecosystem levels (Fig. 4). Initially, we perceive an 

 equilibrium, or steady state, operating at more or less a constant 

 level. If the ecosystem is perturbed, a new steady state, or 

 equilibrium, will be established. If the stressor input is withdrawn, 

 then either the original or the new equilibrium will be established, 

 depending on an array of factors, including the nature, magnitude, 

 and duration of the perturbing force and the initial fragility of the 

 system. These factors will also be of importance in determining the 

 length of time it would take the ecosystem to reach a stage of 

 exhaustion. Cairns (1976) referred to the capacity of an ecosystem 

 to resist insult in terms of inertia. 



Exhaustion at the individual level is manifested as a permanent 

 change in either mortality or natality probability. At the ecosystem 

 level exhaustion leads to death (rarely) or an irreversible change in a 



