STRESS AND ECOSYSTEMS 91 



The third assumption is the existence of some kind of priority 

 development of structure in an ecosystem. Initially a minimal 

 amount of structure necessary for survival develops. Later in 

 successional time, if there is more net energy, complexity and 

 diversity develop if the cost of development and maintenance do not 

 exceed the gains that result from their feedback work. Ewel (1971b) 

 provided evidence for the sequential development of biomass 

 compartments in the succession of tropical ecosystems. He found 

 that leaf biomass was the first compartment to reach steady-state 

 values, then stems, and finally roots. The result of sequential 

 development of compartments is a steady increase in the system's 

 ability to use all the resources available in its surroundings. Another 

 implication of this model is that ecosystems appear to reach 

 functional and structural steady states early in succession, and later, 

 with considerable lag, floristic complexity reaches its steady-state 

 value. 



In discussing ecosystem stability, we must realize that complex 

 systems can survive only in locations with high environmental energy 

 subsidies and low intensities of stress. An environmental change that 

 reduces total energy flow results in a rapid decrease in structural 

 complexity, particularly if the system loses its main power source. A 

 decrease in complexity is interpreted as ecosystem instability. 

 Complex cities that lose monetary subsidies, coral reefs that lose 

 solar input energies, or complex forests growing on leached soils that 

 lose their nutrients rapidly, lose their complexity, as expected from 

 the behavior of the model in Fig. 8. Systems with high organic 

 productivity but low species diversity and complexity usually export 

 their production to other systems and, in so doing, lose the capacity 

 to diversify. This is certainly true for mangrove forests, marshes, 

 freshwater wetlands, some lakes, rivers, and systems stressed with 

 high organic-matter loads. All these systems either store organics 

 without using their potential energy or constantly export organics 

 and receive inorganic nutrients to subsidize their primary produc- 

 tivity. The apparent stability of these systems is caused by the 

 uninterrupted input of certain energy sources that represent a 

 significant fraction of their total energy signature. Figure 6 shows 

 evidence for this idea in relation to the role of tidal energy in 

 maintaining stability in marshes. As long as tidal circulation 

 continues, the marsh is capable of resisting other types of stress and 

 recovers quickly after being subjected to a harvest or to other acute 

 stressors. Stability is a function of stable energy input, and in most 

 systems stability disappears when the main energy source is diverted 

 (e.g., stress of type 1 in Fig. 5). 



