72 



LUGO 



k,IJD 



kjlJD 



Fig. 2 Model of push— pull effects of thermal energy on ecosystems 

 proposed by H. T. Odum (1974). Note that ordering and disordering 

 processes depend on each other. Arrows show the direction of 

 energy flows; tanks are state variables; the large double arrowhead 

 represents multiplicative energy interactions; and the circle repre- 

 sents the energy source. 



Depending on the characteristics or state of the stressed system, the 

 energy influx of the stressor may or may not benefit the system. 

 H. T. Odum (1974) pointed out, for example, that high-temperature 

 stress may be useful in accelerating rates of respiration, thus causing 

 the release of minerals for photosynthesis. This function would be a 

 positive asset to an oligotrophic aquatic system but not to a 

 eutrophic system. Nixon (1969) and Odum, Nixon, and DiSalvo 

 (1970) postulated a similar role for photorespiration in ohgotrophic 

 hypersaline systems rich in organic matter. In this case, the energy 

 drain of photorespiration was shown to benefit mineral availability in 

 an otherwise nutrient-poor system. Disruptive stressors, such as fires, 

 tides, floods, water currents, volcanic eruptions, and hurricanes, also 

 have a positive role at the ecosystem level. Without a periodic 

 disruption ecosystem growth processes stagnate as resources are 

 immobilized by their structure. Bursts of growth and high net 

 productivity usually follow disturbances, and rejuvenated systems 

 replace senescent systems. To have a positive effect, the energy input 



