STRESS AND ECOSYSTEMS 89 



associated with normal respiration and all incoming energies that 

 contribute to ecosystem growth and complexity minus all incoming 

 energies that tax the energy budget of the system. Slobodkin and 

 Sanders (1969) asked the question, Why don't more species adapt to 

 stress? The answer must be related to the high energy cost of certain 

 adaptations vs. the energy limitations to which ecosystems and 

 populations are exposed. In highly stressed environments we would 

 expect strong selective pressure for the most energy-efficient solution 

 to the stress problem; this adaptation would then win over 

 less-efficient competitors. High efficiency, however, is not conducive 

 to the fast rates of energy flow which are associated with lower 

 efficiency systems. 



Figure 8 presents a model that summarizes the proposed 

 interactions and assumptions involved in maintaining ecosystem 

 complexity. The first assumption is that the development and 

 maintenance of ecosystem complexity involves energy expenditure. 

 H. T. Odum (1970) proposed a formula to illustrate the rapid rise in 

 the cost of ecosystem organization with increasing species diversity: 



N^ - N _ fEaA 

 2 K 



where N = number of species 



f = fraction of the energy budget allocated to organization 

 Ea = energy input per unit area 

 A = total area of the system 

 K = daily cost of maintaining species interactions 



The terms on the left side of the equation represent the number 

 of possible interactions between species if organization is complete, 

 and the terms on the right represent the amount of energy available 

 for organization. Since the input of energy to a system is limited on a 

 unit-area basis, the strategy of energy -resource allocation and the 

 amount of work derived from energy transformations become 

 significant factors in determining the competitive survival of a 

 system. 



H. T. Odum (1970) stated that, when a system has a low number 

 of species, only a small amount of energy is allocated to organization 

 and a larger fraction of its energy resource goes to other work 

 functions; thus more energy is needed to eliminate a species and 

 more energy is available to compensate for stress. Hickman (1975), 

 for example, showed that energy allocation for reproduction 

 increased with stress in Polygonum plants; he reported that as much 

 as 71% of the energy stores goes into this function. 



