The whole ecosystem studies of the 1950s raised a new problem. Measuring the flows of energy through an ecosystem could describe its metabolism, but understanding its organization required a way to represent the relationships among the processes producing those flows. Odum increasingly turned to diagrams and electrical analogs as means of representing and investigating these relationships.
In a 1960 paper, Ecological Potential and Analog Circuits for the Ecosystem, Odum constructed an electrical circuit representing flows and transformations within an ecosystem. Batteries represented concentrated energy sources, electrical currents represented flows, and resistances represented processes associated with producing and consuming populations. Variable resistances and switches allowed different relationships to be explored. Data from Silver Springs were used to adjust the circuit so that electrical currents corresponded to measured ecosystem flows, allowing the analog to function as a computational representation of an actual ecosystem. Odum noted that more complex circuits incorporating feedback, oscillations, and transient behavior remained to be developed.
The work was therefore already moving beyond representation toward using models to generate and explore hypotheses about ecosystem behavior. By 1962 Odum was describing an energy simulator as a means of developing ecosystem theory. Electrical analogs provided a way to ask how relationships among components might produce observed system behavior. During the 1960s these methods were still rudimentary, but they established an important connection among systems diagrams, quantitative relationships, models, and theory.
These developing methods became important in the study of the tropical rainforest in Puerto Rico. From 1963 to 1966, while Odum was Chief Scientist at the Puerto Rico Nuclear Center, the Luquillo rainforest became the setting for a large interdisciplinary investigation involving energy flow, nutrient cycling, vegetation, animals, soils, hydrology, atmospheric processes, and other components of the forest system. Experimental irradiation of a section of forest provided an unusual opportunity to observe the response of an entire ecosystem to a major disturbance and to follow processes associated with its subsequent recovery. The work was brought together in the 1970 synthesis A Tropical Rain Forest.
The rainforest project illustrates an important feature of the emerging systems approach. Research was not simply an application of an already completed theory. Diagramming the system was often an early step. Constructing the diagram required relationships to be made explicit, raised questions about how the system worked, and helped identify what needed to be measured. Measurements and experiments then provided information with which those relationships could be evaluated and revised. Models required assumptions about how components interacted, while observations of the system before and after disturbance provided opportunities to compare those assumptions with the behavior of an actual ecosystem. Theory, measurement, diagramming, and modeling developed together.
Across these studies, the objective was becoming broader than the construction of ecosystem energy budgets. Energy remained a common basis for examining flows and transformations, but increasing attention was directed toward the interactions and controls producing the behavior of the whole system. Feedback provided a means of understanding how the products of one process could influence others, while diagrams and models provided ways of representing these relationships and exploring their consequences.
By the end of the 1960s, these developments were converging into an increasingly general approach to ecological systems. Whole system measurement, energy flow and transformation, feedback and control, experimentation, diagramming, and modeling were becoming parts of an integrated methodology. The emphasis was shifting from describing the energetics of particular ecosystems toward understanding the principles governing the organization and behavior of systems as wholes.
Milestones and Key References
1960 — Electrical analog of an ecosystem
Odum constructed an electrical analog in which ecosystem flows and transformations could be represented quantitatively. Using Silver Springs data, the circuit served not merely as a diagram but as a computational device for investigating relationships within an ecosystem.
Ecological Potential and Analog Circuits for the Ecosystem — H.T. Odum (1960)
1962 — Modeling as a means of developing ecosystem theory
Odum used a network energy simulator not simply to represent an ecosystem, but to synthesize relationships among its components and explore analogous principles of system organization. This marked an important step from describing ecosystem energetics toward using models to develop systems theory.
1963–1970 — Puerto Rico Rain Forest Project
The Luquillo research brought together interdisciplinary measurement, whole-system experimentation, diagramming, and modeling. Gamma irradiation provided an unusual whole-system disturbance experiment, and the work culminated in the two-volume A Tropical Rain Forest in 1970.