86 LUGO 



The change in the magnitude of an energy source when energy 

 quality is taken into consideration raises an important question. Do 

 energy-quality differences in stressors affect their impact on the 

 ecosystem? For example, do poisons owe their effectiveness as 

 stressors to their higher energy quality? The data in Table 3 do not 

 allow for any valid generalizations, but the hypothesis could remain 

 that stressors with high energy quality may be more effective than 

 those with lower energy quality. Since I have already postulated the 

 opposite effect when the quality of the stressed component is 

 considered, it is possible that maximum stress results when a stressor 

 with high-quality energy delivery affects a sector of the system with 

 energy flow of low quality. 



PATTERNS OF ECOSYSTEM RESPONSE TO STRESSORS 



The response of cells, organisms, populations, and ecosystems to 

 a wide variety of stressors may follow a common pattern (Fig. 1). 

 This has been pointed out by a number of investigators (Selye, 1956; 

 Piatt, 1965; H. T. Odum, 1967; Woodwell, 1970), who also 

 identified patterns of response. Usually, when a system is stressed, it 

 overcomes the initial low intensity of the stressor by activating 

 internal homeostatic mechanisms. These were described for cells, 

 organisms, and populations by Selye (1956) and for ecosystems by 

 Piatt (1965) and Woodwell (1970). Many times low intensities of 

 stress actually stimulate a system to performance above that of 

 controls, probably because of higher internal physiological activity 

 induced by the higher energy input (Fig. 1). As the intensity of the 

 stressor increases, there is a period of steady state, when homeostatic 

 mechanisms compensate for the energy drain, then a period of 

 decline, and finally, if the stressor continues to intensify, a point of 

 irreversible change. The point of collapse may be when energy 

 reserves are exhausted or when the system reaches the limits of its 

 adaptability. Figure 1 suggests that different stress response patterns 

 reported in the literature may represent different sections of one 

 single response curve that extends for the whole range of intensity of 

 a given stressor. 



Ewel (1971a), describing the pattern of ecosystem response to an 

 intermittent stressor (Fig. 7), showed that, if the stress condition is 

 repeated before full recovery, the system gradually loses its capacity 

 to recover and slowly degenerates into a low-biomass steady state. 

 This same pattern of response has been reported in salt marshes 

 receiving chronic exposure to oil pollution (Baker, 1971) and in 

 floodplains exposed to chronic flooding (Gardner et al., 1972). 



