important methodological questions: What is the electrical significance of a function observed in nature? Given an electrical unit in a circuit, what is it in the ecological system? For example, what is a diode in nature? One needs a diode to allow biomass to accumulate after the voltage of the sun has gone down, or else the circuit reverses. Higher organisms like fish are diodes. The Silver Springs study Silver Springs is a common type of spring-fed stream in Florida with a constant temperature and chemical composition. The study Odum conducted here was the first complete analysis of a natural ecosystem. Odum started with an overall model and in his early work used a diagramming methodology very similar to the Sankey diagrams used in chemical process engineering. Starting from that overall model, Odum "mapped in detail all the flow routes to and from the stream. He measured the energy input of sun and rain, and of all organic matter - even those of the bread the tourists threw to the ducks and fish - and then measured the energy that gradually left the spring. In this way he was able to establish the stream's energy budget." Ecological and biological energetics Around 1955 Odum directed studies into radioecology, which included the effects of radiation on the tropical rainforest in El Verde, Puerto Rico (Odum and Pidgeon), and the coral reefs and ocean ecology at Eniwetok atoll. The Odum brothers were approached by the Atomic Energy Commission to undertake a detailed study of the atoll after nuclear testing; the atoll was sufficiently radioactive that upon their arrival the Odums were able to produce an autoradiographic image of a coral head by placing it on photographic paper. These studies were early applications of energy concepts to ecological systems, and explored the implications of the laws of thermodynamics when used in these new settings. From this view, biogeochemical cycles are driven by radiant energy. Odum expressed the balance between energy input and output as the ratio of production (P) to respiration (R): P-R. He classified water bodies based on their P-R ratios, which separated autotrophic from heterotrophic ecosystems: "[Odum's] measurements of flowing water metabolism were measurements of whole systems. Odum was measuring the community as a system, not adding up the metabolism of the components as Lindeman and many others had done". This reasoning appears to have followed that of Odum's doctoral supervisor, G.