With these concepts established, a system may be defined as an interacting, 

 interdependent group of components functioning as a whole. External driving 

 inputs are received, outputs are exported to other systems, and transformations 

 of energy and storage of material or energy occur within the system. By knowing 

 details of how each component operates and relations between components, the 

 characteristics of the whole system may be described. 



Ecosystems are complex. The large number of links and the different regu- 

 lating factors make it difficult and often cumbersome to explain an ecosystem 

 simply by a narrative description. Quantitative models are available for some 

 systems or components thereof. These are excellent predictors of dynamic sys- 

 tems. However, it is difficult to visualize the systems that these models 

 describe from a group of equations alone. Often, the model 's greatest use in 

 describing a system is through graphic presentation. 



H.T. Odum (1967, 1971, 1972a_, 1972b^) suggested an energy circuit diagram 

 language to allow the visualization of ecosystems. This tool shows graphically 

 the same sorts of relationships among ecosystem components as are seen among 

 the elements of electrical circuit diagrams or computer program flowcharts. 

 While not a perfect representation of the system, the circuit diagram does allow 

 conceptualization of system operation. Odum has extended this concept to 

 simulation models on analog computers. The use of the energy circuit diagram 

 ; inguage here, however, is not quantitative and will be limited to identifica- 

 tion of important components and characterization of possible impacts through 

 changes in energy, material, and information flow. By tracing through the 

 pathways, storages, energy transformations, and workgates, possible results of 

 system alternations, based on a logical sequence of events, often can be de- 

 scribed. 



The ecosystems diagram does not claim to be a definitive or perfect treat- 

 ment of ecosystems in general, or of any ecosystem in particular. The diagram 

 does not include all the detail, subtleties, complexities, or eventualities. 

 Only the important aspects necessary to characterize the functioning of eco- 

 systems have been included - other details have purposely been omitted. Some 

 aspects of ecosystems are very difficult to show without making the diagrams 

 too complicated to use. Time relationships are difficult to show graphically, 

 particularly if some of the processes modeled take place on different time 

 scales. For example, temperature phenomena are measured in terms of days or 

 months (seasons). Subsidence, however, occurs on a time scale of decades or 

 centuries. Yet both must appear in the same model for appreciation of some 

 systems. 



Patterns of variation within the same time frame are also difficult to 

 show. Without considerably complicating the diagram, the consequences of 

 flooding in spring are indistinguishable from those in fall. The use of the 

 diagrams, therefore, requires knowledge concerning quantities, durations, 

 variability, and patterns of variation. The ecosystems diagram is a genera- 

 lized description of the system, and can be a useful tool in thinking about 

 how natural ecosystems operate. 



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