70 LUGO 



ENERGY BASIS OF STRESS 



H. T. Odum's (1967) definition of stress as a drain of potential 

 energy has an advantage over all other definitions because it provides 

 a common basis (energy) on which to evaluate all types of stress in 

 all kinds of systems. More significant, however, is that the energy 

 required for all natural processes behaves according to well-defined 

 laws that provide a quantitative basis for evaluating ecosystem 

 function and response to stressors. The capacity of a system to 

 overcome stress, then, depends on the balance between the rate of 

 recovery of its energy stores and the magnitude of the energy drain 

 caused by the stressor. 



We could argue against Odum's definition of stress by pointing 

 out that many drains of potential energy are not normally called 

 stress but would be by his definition, e.g., the energy drain of 

 nighttime respiration in plants. We would not normally identify 

 nighttime darkness as a stressor, but shading of a plant or an 

 ecosystem for a prolonged time period certainly would be considered 

 one. Rogers (1977), for example, after shading a coral reef for 35 

 days, found significant rates of mortality and slow rates of recovery. 



The health of ecosystems that are normally considered highly 

 stressed could also be used to illustrate the futility of identifying 

 stressors and defining what they do to a system. Salt springs, thermal 

 springs, or deserts actually become stressed if the "stressful condi- 

 tions" to which they are adapted are changed. The usefulness of the 

 concept of stress is obviously limited if we cannot deal with the 

 apparent subjectivity and relativity of the term. 



Selye (1956) suggested that most environmental situations can 

 become stressful to given individuals and that stress is a normal 

 condition of anyone's environment. The realization that stressors and 

 energy drains are part of any natural environment is an important 

 step in generalizing about stressors and their impact on ecosystems. 

 This is the basis for my earlier suggestion that we differentiate 

 between background or normal stress and additional stress caused by 

 allogenic forces. In fact, as discussed in the following section, 

 stressors may become subsidies to certain ecosystems, and their 

 productivity and growth may decrease if the stressors are removed 

 (Fig. 1). 



Push-Pull Model of Stress 



Figure 2 is a model of the push— pull (positive— negative) effect 

 of ordering and disordering factors in the environment (in this case, 

 temperature) proposed by H. T. Odum (1974). The model depicts 



