76 LUGO 



species diversity, structural organization, physiognomy, and pheno- 

 logical organization. 



The concept of stress should, then, be applied in a hierarchical 

 fashion relative to the various levels of biological organization. As 

 ecologists, we are interested in stressors as they relate to life zones, 

 individual ecosystems, and populations. Other scientists do their 

 analysis at the organismal, cellular, or molecular and atomic levels of 

 organization. To avoid the pitfalls of subjectivity, we need to develop 

 appropriate criteria for evaluating the relative effects of stressors on 

 the complexity and rate of recovery of systems at each of these levels 

 of organization. 



The Energy Signature of Ecosystems 



The push— pull model (Fig. 2) suggests that the amount of 

 structure and complexity a system is capable of building and 

 maintaining depends on the net effect of positive and negative forces 

 in the environment. 



The sum of all incoming energy flows to a system and the pattern 

 of their delivery expressed on an equal energy- quality basis is called 

 the "energy signature" of the system (H. T. Odum et al., 1977). The 

 quality of an energy source depends on its concentration (Odum 

 et al., 1977). High-quality sources have a high energy content per 

 unit volume, and low quality sources, like solar energy, are diluted 

 and, thus, perform less work per unit volume. Energy signatures must 

 be expressed on an equal energy-quality basis to show the relative 

 ability of the components to generate work and control the system. 

 The energy signature provides a way to categorize ecosystems 

 according to environmental carrying capacity or capacity to deal 

 with stress, e.g., as Odum, Copeland, and McMahan (1974) did for 

 coastal systems. As the energy signature changes, the loss or gain of 

 energy may become a source of stress or a subsidy to the system. 



Since the actual intensity of background or normal disordering 

 energies in a given environment may be impossible to measure, the 

 measure of stress at the level of the life zone must be made relative 

 to the most complex systems on earth, because they represent the 

 best that can be done with the energy flows and resources available 

 to all natural systems. Other life zones could then be arranged in a 

 hierarchical and relative scale. 



Perhaps the best system for classifying terrestrial and wetland 

 ecosystems is Holdridge's life-zone system (1967) (Fig. 3). This 

 system has worldwide application, is an objective determination of 

 life zones based on climatic data (which approximate the energy 



