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I. An introduction to systems ecology and ecological modeling

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The living biological community and the nonliving physical environment work together in nature to function as an ecological system (ecosystem). Each type of system usually has a particular combination of environmental factors that allows it to be separated from other ecosystem types, such as forests, beaches, and cities. In every system the component parts work together or interact in the way that is necessary for the perpetuation of the system. Any ecosystem evaluation must incorporate means for recognizing the differences inherent in each system type, and at the same time it should pick out those threads that tie all those different systems together into a functioning unit of man and nature.

In evaluating ecosystems, we will consciously or subconsciously construct conceptual models (visual diagrams) to aid in placing the component parts into a functioning arrangement. The selection and arrangement of the component parts are often by intuitive reasoning. For example, in a freshwater-marsh system, water, light, and vegetation are obviously important parts. Intuitively we know that alteration of the flow of water or amount of light will affect the marsh vegetation although we do not know to what degree. Also, we know that the flow of water, for example, is affected by other factors such as man's diversion of water for public water supply. As more factors are included in the model and as quantitative data are incorporated, intuition falls short of effecting a solution. We must then make use of a more sophisticated treatment. One possibility is the use of a mathematical model. Another is to make use of a model that employs symbols rather than numerical parameters to represent the real-world system. Such a model, which uses pictures of familiar forms, objects, or figures, is easy to understand and simple to apply.

A simple example of how these symbols are used to represent a system is illustrated in figure 27 where a human dwelling is diagrammed in two different ways. Figure 27A shows a house with food, building material, and fuel oil being brought into the house for consumption. Figure 27B translates this picture language into an energy-flow diagram. We see that by using the energy-flow diagram we can equate dissimilar components of the system under study and provide a better understanding of the whole system.

diagrammatic illustrations of energy flow
FIGURE 27. Diagrammatic illustrations of energy flow. [larger image]

In south Florida one is confronted with a set of complex problems, all interrelated in some fashion. It became evident that systems analysis with a regional modeling approach would be valuable to illuminate the main environmental issues and to predict changes in the system under specific types of potential development. The modeling approach used by Lugo and others1 is well adapted for obtaining a broad overview as well as for predicting changes in south Florida because it incorporates the following features:

1. The common denominator used to compare system components is the energy value expressed in potential energy.

2. A symbolic systems language permits rapid visual identification and understanding of models both conceptually and mathematically. This language is analogous to analog computer diagrams and is easily used in simulation studies.

3. A broad view of a large regional system is possible in which the important major pathways vital to the region can be evaluated. The forcing functions (such as energy from the sun or from fossil fuels) outside the system under study can be seen as they enter, drive, and leave the system.

4. Smaller subsystems or ecosystems making up the larger systems may be identified and evaluated in greater detail if necessary.

5. Computer simulations of conceptual models can be checked for accuracy with data from the real world. After this validation the model can be used to predict future trends.

6. Predictions can be applied to resource management in an effort to aid decisionmakers in the planning process.

Using this approach, systems ecology incorporates the use of diagrams of systems in an "energy language" where the component parts of the system are connected by lines that represent the flow of potential energy from one energy storage compartment in the system to another. This conceptual model can be made large or small, depending upon the particular system, but always the view taken is large enough to include all important features of the system, including the driving functions from outside the system.2

As systems are studied, and as the energy sources and pathways are drawn into a model, the whole becomes greater than the sum of its parts. This is because the whole system includes not only the parts but the interaction between the parts. An analogy is a box of parts needed to assemble a tricycle. When the pedals are attached to the wheels, then both pedal and wheel bear a functional relationship that did not exist in the unconnected box of parts. The same is true for ecosystems. The parts are intricately related, and changes in one part frequently produce unsuspected changes in others. Systems ecology strives to identify these important interactions and allows an investigation into them by the use of the computer simulation.

By using systems ecology in south Florida, data needs have been illuminated, south Florida systems have been studied, and recommendations for future resource management have been generated based on the energy denominator.3


1 Lugo, A. E., Snedaker, S. C., Bayley, S., and Odum, H. T., 1971, Models for planning research for the South Florida Environmental Study: U.S. Natl. Park Service, available from U.S. Dept. of Commerce, Natl. Tech. Info. Service, Springfield, Va. 22151 as PB-231 938.
2 Lugo and others, op. cit.
3 Brown, Mark, and Genova Grant, 1974, Energy indices in the urban pattern: Gainesville, Fla.. Univ. of Florida Center for Wetlands.

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