STRESS AND ECOSYSTEMS 



71 



POSITIVE RESPONSE 



NEGATIVE 

 STEADY STATE RESPONSE AND EXTINCTION 



INTENSITY OF STRESSOR 



. -^" 



Fig. 1 Response of living systems to stressors. Similar curves of 

 response were proposed by E. P. Odum (1971), Gibbons (1976), and 

 Odum and Kroodsma (1976). Examples of positive response with 

 increasing stress intensity were reported by Yentsch et al. (1974) for 

 algal populations under temperature stress and by Carpelan (1964) 

 for algae under salinity stress. Conditions of steady-state response 

 are common in most stress experiments [e.g., Woodwell and Rebuck 

 (1967) for radiation and Erdman (1966) for DDT and Xrays]. 

 Rapid declines and extinction are equally common (Woodwell and 

 Marples, 1968; Coutant and Cox, 1976). Few studies cover the 

 whole spectrum of response for a given environmental factor. 



the well-known facts that all structures have a natural tendency to 

 deteriorate and that Ufe as we recognize it represents an improbable 

 state of organization whose maintenance requires continuous energy 

 flows. The implication is that certain drains of potential energy that 

 we call stresses when they irreversibly alter a system are, in fact, 

 necessary and constantly present components of all life processes. 

 H. T. Odum (1976) suggested that disordering stimulates ordering 

 and, thus, questioned the tendency to regard stress as bad and 

 disorder as evil. 



The push— pull model of stress proposes that stressors, such as 

 temperature, may initially accelerate and then decelerate processes. 



