STRESS AND ECOSYSTEMS 



79 



/ V 



Fig. 4 Simplified model of an ecosystem without stress. The large 

 bullet-shaped outline is the boundary of the system; P is photo- 

 synthesis; and R is respiration (for other symbols, see Fig. 2, and, for 

 a discussion of the model, see the text). All energy inputs are 

 eventually dissipated into degraded heat, according to the second 

 law of thermodynamics. 



variables and three ecosystem processes. The state variables are 

 nutrient concentrations and water availability (a measure of fertil- 

 ity), live biomass, dead organic matter, and a hypothetical variable 

 called organic complexity, which includes measures of species 

 diversity and three-dimensional diversity and complexity discussed 

 by Whittaker (1969). It is included here simply to point out that 

 the organization of the ecosystem is an energy-demanding process 

 and, thus, is subject to the impact of stressors. The cost of this 

 variable is borne by each of the living components of the system, but 

 this does not invalidate the fact that complexity of function and 

 organization is an ecosystem property that responds to its energy 

 signature. All state variables have a positive role in the system and 

 provide positive feedback to its productive sector. The processes 

 illustrated in the model are the photosynthetic conversion of solar 



