The diagram approach is open ended. The subtleties of biota and environ- 

 mental interaction as well as the potential impacts on them can be integrated 

 into the diagram format as new knowledge is acquired. 



The system diagram approach does, however, require specific information 

 for its construction and interpretation. H.T. Odum (1971, 1972a^) introduced a 

 symbolic language which combined energy laws, principles of kinetics, and some 

 philosophical tenets of electrical systems. The language consists of several 

 basic modules shown in Figure 3-1. A circular symbol (Figure 3-la) represents 

 a source of materials or energy from outside the system such as the sun, tidal 

 forces, suspended sediment, or rainfall. Also referred to as "forcing func- 

 tions," these entities are described as outside programs affecting temporal 

 patterns inside the system. In a few cases, these inputs are man-induced or 

 controlled: for example, ditching, channelization, and impounding. The circu- 

 lar symbols also show losses or outputs leaving system boundaries and entering 

 adjacent ecosysteTis. 



The large bullet-shaped symbol (Figure 3-lb) represents primary pro- 

 ducers, which receive energy from the sun. They transform radiant energy into 

 plant material and are affected by numerous components within the ecosystem. 

 Often several producer symbols are used to represent major functional plant 

 groups in the ecosystem. These have been included to show differential effects 

 of various ecosystem components and relationships between plant groups. 



The tank-shaped symbol with the slanting roof (Figure 3-lc) represents a 

 storage site of materials, which receives inputs from other sources and trans- 

 fers material, energy, or information to other storage sites and organisms or 

 out of the system. 



The hexagons (Figure 3-ld) represent consumer organism groups (herbivores, 

 detritivores, omnivores, and carnivores). Consumers utilize energy produced 

 in the system and pass on energy to higher trophic levels. Perhaps the most 

 important aspect of consumers is their regulatory function, selectively prey- 

 ing on or consuming certain species and, as a result, maintaining a particular 

 equilibrium of producers and consumers. The trophic status of the consumer 

 groups may be noted by the position relative to the flows and other consumer 

 groups on the diagram. 



A special case of consumers appears on diagrams where a storage and hexa- 

 gon have been combined (Figure 3-le). This is the detritus-microbe complex. 

 Since detritus and micro-organisms are intimately associated, what affects one 

 affects the other. Therefore they are combined. 



The control (regulating) mechanism, or workgate, is shown by an arrow- 

 shaped symbol (Figure 3- If). Energy or material flows in the concave end and 

 out the pointed end. The flow rate is controlled by the pathway leading in 

 from the side. The symbol defines some sort of regulating process and may be 

 considered analogous to a valve or faucet. Inside the workgate is another 

 symbol, either "x" or "-," which show the direction of control of the 



17 



