80 LUGO 



energy into chemical energy (P), the respiratory process (R), and the 

 many cycles and feedbacks of the system which contribute to its 

 homeostasis. 



In a system like this, there is a high cost of depreciation to 

 support live biomass and organization. In steady-state systems the 

 quality of energy feedbacks among ecosystem components increases 

 relative to those of successional systems. Higher quality energy flows 

 contribute to a higher rate of energy flow through the system as a 

 whole. The respiration of the system increases on a unit-area basis 

 with age because of the increasing cost of upgrading energy qualities 

 in the system (i.e., the cost of converting low-quality forms to higher 

 quality states). 



In Fig. 5 stressors are added to the model. The energy signature 

 of the system is subdivided into five potential sources of stress. 

 Source 1 delivers the primary energy of the system, which in this 

 example is solar energy for photosynthesis; source 2 diverts solar 

 energy before it is transformed to chemical energy by plants (e.g., by 

 shading); source 3 diverts energy after it is transformed into chemical 

 energy but before it is incorporated into structure (e.g., by the 

 harvest of labile sugars by a consumer, such as man); source 4 

 removes storages (e.g., by harvests or by drainage of water); and 

 source 5 accelerates the respiration of the system (e.g., by changes in 

 temperature). Each of these external sources impinges on the 

 ecosystem at a certain rate and with a certain periodicity of delivery 

 which depends on the type of life zone. When the energy signature 

 changes, the change may become stressful. 



The model of stress in Fig. 5 suggests that different stressors 

 affect different functional sectors of the ecosystem and that perhaps 

 the response of the system to these different types of drains also 

 changes according to the type of stressor. Causal relationships must 

 then be analyzed relative to the type of stressor, its pattern of 

 delivery, its intensity, and the kinds of changes that it causes in the 

 system. After a certain threshold is exceeded, the stressor causes 

 energy flows to be diverted away from the system and diminishes the 

 ability of the system to continue to upgrade internal energy stores. 

 Thus the overall organization and homeostasis of the system is 

 altered. There is flexibility in each system, however, vdth regard to 

 its response to and the ultimate impact of the stressor. According to 

 Fig. 5, more energy may be diverted into living mass, less into 

 complexity, or more into respiration, or there may be an increase in 

 the amount of dead organic material. The ultimate strategy probably 

 depends on the type of stressor and where in the system it operates. 



