STRESS AND ECOSYSTEMS 93 



Table 5 summarizes some attempts at quantifying a number of 

 stresses in natural systems and some important threshold values for 

 specific stressors. Data of this kind, although important, are very 

 meager. Biologists are good at counting dead or sick organisms, but 

 little effort is made to calculate impacts per unit area or to correlate 

 mortality with given intensities of stressors under natural conditions. 

 Some calculations in Table 5 are based on the amount of energy 

 actually drained from the system, but others also consider the energy 

 the system loses when one of its components is not functioning. 

 These unrealized energy flows are usually higher than the actual 

 energy lost after the stressor's action. In few studies were measure- 

 ments of stressor intensity and ecosystem response made (all in the 

 same comparable units). Until researchers make these measurements, 

 I cannot see how they will be in a position to develop a predictive 

 capacity. For example, What is the maximum amount of energy that 

 can be drained from an ecosystem without damaging its ability to 

 recover? If we can learn to categorize stressors by the quality and 

 intensity of their energy and by their point of attack in a given 

 ecosystem and to categorize ecosystems by their energy signatures 

 and their capacity to bounce back, then I believe we can begin to 

 answer some of the questions about management and pollution that 

 are being asked by those who manage natural ecosystems. 



ACKNOWLEDGMENTS 



I am grateful to the following colleagues for their contributions, 

 through informal discussions, to the development of this manuscript: 

 H. T. Odum, J. J. Ewel, S. L. Brown, C. S. Rogers, F. F. Benedict, 

 C. A. S. Hall, S. C. Snedaker, and R. R. Twilley. None of them, 

 however, agrees with most of this manuscript. Alma L. Lugo did 

 most of the art work. 



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