Revised 24 May 2019
The following energetic symbols and diagrams used by Odum (1976) help us to visualise the laws and flows of energy within ecosystems by introducing the idea of an ecosystem as a combination of interacting parts. The symbols might be familiar to those who have studied electronics or dynamic systems analysis.
The Energy Circuit symbol shows the direction and flow of energy from a Source of energy or a unit such as a Storage Tank.
Figure 1: Energy Circuit Symbol
The Source symbol is used for sources of energy that are external to the system under examination. The flow of energy from an energy source might be steady and constant such as that from a hydroelectric dam or intermittent such as that from wind turbines or photovoltaic panels.
Figure 2: Source Symbol
The Storage Tank symbol represents a stock of energy such as fossil fuels in the ground, petrol in the tank of a car, the embodied energy stored in a building, the repository of information, water, and minerals in the ground.
Figure 3: Storage Tank Symbol
The Heat Sink symbol, an arrow pointing down into the ground, represents the loss of degraded energy from a system and the depreciation and degradation of that system as the material of the system becomes dispersed. All Storage Tanks have a Heat Sink as a result of the Second Law of Thermodynamics.
Figure 4: Heat Sink Symbol
The Interaction symbol: All processes involve the interaction of two or more types of energy and matter. All Interactions also have a Heat Sink as a result of the Second Law of Thermodynamics.
Figure 5: Interaction Symbol
The Money Transaction symbol represents the flow of money in the opposite direction to pay for the flow of embodied energy and materials contained in goods and services. Price is shown as an external source.
Figure 6: Money Transaction Symbol
The Producer symbol represents the processes, interactions, and storages that collects and transform low-quality energy into high-quality energy flows. An example is the process of photosynthesis where biomass is produced from diffuse solar energy.
Figure 7: Producer Symbol
The Self-Interaction symbol represents processes where self-interaction results in a faster action that would otherwise take place. An example is where the interaction of a group of people building a house results in a quicker build than when each person works simultaneously alone without any interaction.
Figure 8: Self-interaction symbol
The Consumer symbol represents a system that maintains itself by using Storage Tanks and Interactions to make use of inflowing energy and materials. Examples include an organism, or a town and a city.
Figure 9: Consumer Symbol
The Miscellaneous symbol is used for subsystems as labelled.
Figure 10: Miscellaneous Symbol for sub-systems
Figure 11 shows the flow of energy embodied in goods and services from a town to a farm which produces food for the town using energy from the sun.
Figure 11: Example of flow of energy from farm to town (Odum, 1976, p50)
Figure 12 shows that energy is used to run processes and maintain order on a farm. Heat energy is dispersed whenever work is done on the farm. Farm buildings and machinery undergo depreciation and require maintenance. The fertility of the soil declines unless replenished with natural or artificial fertilisers.
Figure 12: Example of the order – disorder cycle (Odum, 1976, p40)