STRESS AND ECOSYSTEMS 65 



rapid rise in protein turnover occurs above 50° C. The cost of 

 high-temperature survival in terms of protein synthesis is staggering, 

 how^ever, and only a few species w^ith very low biomass and a fast 

 turnover of this mass can survive such conditions. If the pH of the 

 water changes, then an aquatic organism's tolerance to high tempera- 

 ture decreases (Brock, 1970), presumably because of a synergistic 

 effect of one stressor on another (Vernberg and Vernberg, 1974). 



Thus it appears that we need to distinguish between the stressors 

 that are part of an ecosystem's "normal" everyday environment and 

 those added by man or by acute events with infrequent recurrence. 

 Both types cause energy drains, but the effects of natural stressors 

 are not as immediately obvious to an observer as are those of 

 additional energy drains imposed allogenically on a system. 



ENERGY COST OF STRESS 



The energy cost of stress to a system is probably a function of 

 the intensity of the stressor (i.e., how much energy it drains per unit 

 area and unit time), the multiplicative or additive effect of this 

 energy drain on the overall function and homeostasis of the system, 

 the frequency of its occurrence, the type of ecosystem being 

 stressed, the condition of the system at the time of impact, the 

 intensity of other stressors at the time of impact, the residual effects 

 of other stressors on the system, and the frequency of return of these 

 particular stressors. Chronic stressors, which operate for longer time 

 periods than acute stressors, usually drain more energy from a 

 system. Acute stressors may be very damaging, however, depending on 

 the intensity of the stress, the time interval at which it reoccurs, and 

 the adaptability of the stressed system. Also, the multiplicative 

 impact of energy drains on the overall function and homeostasis of 

 the system deserves careful consideration since a small energy drain 

 in a component with large amplification value on the function of the 

 ecosystem may be more damaging than a large energy drain in one 

 with less influence on other properties of the system. An example 

 would be the relative impact of a poison vs. that of a fire on the 

 growth of an ecosystem. 



PREDICTABLE AND UNPREDICTABLE ENVIRONMENTS 



Some investigators have differentiated environments on the basis 

 of predictability and used this difference as a measure of stress 

 (Slobodkin and Sanders, 1969; Sanders, 1969; Colwell, 1974). They 



