STRESS AND ECOSYSTEMS 



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Fig. 5 Simplified model of an ecosystem influenced by a number of 

 stressors. (For identification of symbols, see Fig. 2, and, for a 

 discussion of the model, see the text.) Each stressor, by draining 

 energy from a different sector of the ecosystem, affects the response 

 in a unique way. Stressorr 4A, for example, has a stronger effect on 

 the system than stressors 4B or 4D because it directly affects the 

 photosynthetic process and, thus, decreases the ability of the system 

 to restore its energy stores. Stressors 1 to 3 are equally damaging to 

 ecosystems relative to the effects of stressors 4B to 4D and 5. 



The point we need to emphasize is that, although all stressors 

 divert potential energy away from the system, not all stressors 

 handicap the system to the same degree, even though they drain 

 equal amounts of energy. For example, a stressor that removes 

 structure but does not alter the primary productivity of the system 

 may not have as much impact as one that reduces the ability of the 

 system to photosynthesize and, thus, to replenish its energy stores. 

 In the first case the system will recover rapidly, but in the second 

 recovery may be slow. This aspect of the problem has important 

 implications for rates of recovery from stress and for management of 

 natural ecosystems. Table 4 lists a number of naturally stressed 

 ecosystems and classifies their respective natural stressors by type. I 

 have tried to illustrate the rate at which the systems respond to stress 



