84 LUGO 



through successional recovery. Note that systems stressed by removal 

 of structure or by acceleration of respiration can still function and 

 will recover rapidly because there has been no direct impact on 

 factors regulating primary productivity. Systems submitted to 

 stressors that do alter their productive capacity, however, have little 

 ability to recover or to negotiate further stress. If this is true, 

 stressors affecting high-quality energy stores appear to have a lesser 

 impact on a system than those affecting low energy-quality sectors of 

 the system. 



Change of Energy Signature 



Examples of changes in the energy signature of an ecosystem are 

 slow changes in climate (see Singer, 1970); changes caused in 

 downstream sites by construction of a dam, e.g., the change in 

 estuarine circulation patterns caused by the Aswan Dam (Sharaf El 

 Din, 1977); and regional changes caused by alteration of water 

 tables, e.g., in south Florida estuaries (Carter et al., 1973). 



Cooper and Copeland (1973), experimenting with microcosms, 

 simulated regional drainage changes for Trinity Bay in Texas. They 

 found more changes in the respiration and productivity of the system 

 than in species composition. Later, as stressors became more intense 

 (simulated by lack of freshwater nmoff or large organic-pollutant 

 inputs), diversity index changed significantly and so did species 

 composition. Alterations in energy flow rates before any significant 

 changes in species composition were documented in the field by 

 Carter et al. (1973). Apparently the substitution of species results 

 from altered energy flows through the system. New ecosystem 

 boundaries develop slowly in response to a different energy signature 

 impinging on the ecosystem. 



Under some circumstances a change in an energy signature could 

 result in a new ecosystem, with higher complexity and productivity 

 than the original system it replaced, e.g., the drainage of certain (but 

 not all) wetlands, which creates mesic ecosystems. In this case the 

 overall carrying capacity of the environment increases; the original 

 ecosystem is selected against by the stressor; and the new system is 

 more vigorous because it is better adapted to the new energy 

 signature. J. Ewel (1978) documented such a succession in the 

 Everglades of Florida. 



Figure 6 shows as an example the energy signature of a marsh 

 ecosystem subjected to thermal stress. It is obvious that, when 

 energy quality is taken into consideration, tidal energy becomes the 

 main energy source to a marsh, and fossil-fuel stress was not as 

 significant as it appeared to be initially from its heat equivalence. 



