64 LUGO 



homeostasis through ordinary, nonemergency adjustment processes." 

 Selye (1956) and Fitch and Johnson (1977) distinguished 

 between stress and stressors. A stressor is any condition or situation 

 that causes a system to mobihze its resources and increase its energy 

 expenditure. Stress is the response of the system to the stressor. 

 Responses to stressors may include adaptation or functional disorder. 

 Adaptations allow the system to overcome the stress or to avoid it. 

 Functional disorders can be ameliorated or can eventually lead to 

 exhaustion and death. H. T. Odum (1967) pointed out that, although 

 different stressors had different impacts on an ecosystem (e.g., 

 removal of organisms, higher respiration rates, or diversion of 

 incoming resources), they all diverted potential energy flows that 

 otherwise could do useful work in the system. He then defined stress 

 as a drain of calories of potential energy flow. According to his 

 definition, stress can be measured by changes in the flows of energy 

 in a system, disappearance of previously existing flows, or ac- 

 celeration of repair work. 



TYPES OF STRESSES AIMD STRESSORS 



Regardless of how stress is defined or of the stressor involved, the 

 concept of stress as normally used invokes an interference wdth the 

 normal function of a system; its effects are most dramatically 

 observed after certain thresholds of tolerance are exceeded; and it 

 appears that beyond these thresholds recovery is usually difficult. 

 When systems are stressed for short periods of time and have an 

 opportunity to recover during periods of low stress, the stressor is 

 said to be acute. Other systems are exposed to the effects of 

 continuous or chronic stressors. When organisms function despite the 

 presence of a chronic or acute stressor, the intensity and the type of 

 stressor are said to be adaptable because they allow the systems to 

 survive. Surviving systems are equipped to overcome the drain of 

 potential energy and still remain competitive. Adaptability does not 

 imply absence of an energy drain, however. All stressors are 

 analogous to energy barriers that organisms and ecosystems must 

 continuously overcome if they are to survive. For example, a 

 halophyte must constantly deal with the problem of salinity even if 

 it is adapted to salinity, just like wetlands and certain forests must 

 overcome flooding or fire even though they have adaptations to cope 

 with these factors. Morowitz (1968) said that the rate of protein 

 turnover in organisms follows an exponential function of tempera- 

 ture which rises rapidly above 40"^ C for most animals. Thermophilic 

 bacteria, however, can survive higher temperatures because their 



