ECOSYSTEM RESPONSES TO STRESS 



103 



is the response of the organism or system which is used as a criterion 

 for identifying a specific force as a stress. Certain characteristic 

 parameters must display abnormal behavior before a system can be 

 considered stressed. At an organismal level, epinephrine, glucocorti- 

 coids, and blood glucose are such parameters. To identify stress at an 

 ecosystem level, we must monitor appropriate ecosystem-level 

 parameters. Ecosystem stress studies should consider some character- 

 ization of the total community metabohsm, the nutrient-cycling 

 mechanisms, and the structural properties of the system. Deviations 

 of such parameters from some normal condition can be used to 

 identify exogenous inputs as stresses and to describe ecosystem 

 responses to those stresses. 



THE CONCEPT OF ECOLOGICAL STRESS 



As used here, stress is defined as amy deviation in input variables 

 which produces an output different from the system's normal 

 response. Obviously this definition depends on the description of the 

 normal, unstressed state. The region of state space within which an 

 ecosystem normally functions is called its normal operating range. 

 The dashed lines in Fig. 1 delineate such a region for the very simple 



Period of 

 perturbation 



l-cH 



TIME 



Fig. 1 Diagram of five measures of relative stability. Dashed lines 

 are boundaries of the normal operating range. Letters refer to 

 quantitative stability measures: A, constancy; B, resistance; C, 

 response time; D, resilience; and E, total relative stability. 



