(e). Food Chain as an Example of a System

A food chain models the movement of energy in an ecosystem (a form of environmental system). Figure 4e-1 below illustrates the movement of energy in a typical food chain. In this diagram, we begin the food chain with 100,000 units of light energy from the Sun. Note, the amount of energy available at each successive level (called trophic levels) of this system becomes progressively less. Only 10 units of energy are available at the last level (carnivores) of the food chain. A number of factors limit the assimilation of energy from one level to the next. We will examine these factors later in this textbook.

The Sun is the original source of energy, in the form of light, for the food chain. (100,000 Units of Energy)

Plants capture approximately 1% of the available light energy from the Sun for biomass production by way of photosynthesis. Photosynthesis can be described chemically as: Light Energy + 6CO2 + 6H2O ==> C6H12O6 + 6O2 (1,000 Units of Energy)

Herbivores consume approximately 10% of the plant biomass produced in a typical food chain. (100 Units of Energy)

Carnivores capture and consume about 10% of the energy stored by the herbivores. (10 Units of Energy)

Figure 4e-1: Model of the grazing food chain showing the movement of energy through an ecosystem.

 

Why is the above illustration an example of a system? The concept of what makes something a system was fully explained in section 4b. In this topic, it was suggested that all systems share the following seven common characteristics:

1. Systems have a structure that is defined by its parts and processes. In the above example, the structure consists of the system's three types of properties. This system has the following elements: the Sun, plants, herbivores and carnivores. Within this system the main characteristic, or attribute, of the elements being perceived is units of energy. The last component that makes up the structure of this system is the cause and effect relationships between the elements and attributes. For example, the Sun creates energy via nuclear fusion. This energy is radiated from the Sun's surface and received by the surface of the Earth. On the surface of the Earth plants capture some of this solar radiation in the chloroplasts that exist in their tissues. Through photosynthesis the plants convert the radiant energy into energy rich organic matter. Some of the energy fixed by the plants is passed on to herbivores through consumption. Finally, a portion of the energy assimilated by the herbivores is then passed on to carnivores through consumption.
2. Systems are generalizations of reality. The food chain process described above is a simple abstraction of what actually happens in a variety of different types of terrestrial ecosystems of much greater complexity.
3. Systems tend to function in the same way. All systems consist of groups of parts that interact with each other according to various cause and effect processes. In the food chain model, the parts are the Sun, plants, herbivores and carnivores. There are two main processes taking place in this system. The first involves the movement of energy, in the form of radiation, from the Sun to the plants. The second process involves the movement of energy, in the form of organic molecules, from plants to herbivores, and then finally to carnivores through biomass consumption.
4. The various parts of a system have functional as well as structural relationships between each other. The structure within the food chain is defined by the functional relationships between the elements and attributes of the system.
5. The fact that functional relationships exist between the parts suggests the flow and transfer of some type of energy or matter. Systems exchange energy and matter internally and with their surrounding environment through various processes of input and output. The main material being transferred into this system (input) is energy in the form of solar radiation. The solar radiation is then fixed into organic matter (output) by way of photosynthesis in the plants. Herbivores consume the constructed plant organic molecules for nutrition to run their metabolism. The herbivores then provide food for the carnivores.
6. Systems often exchange energy and/or matter beyond their defined boundary with the outside environment, and other systems, through various input and output processes. The organisms found in a food chains transfer organic matter into the detritus food chain when they shed tissues or die. This transfer represents a net output of matter out of the food chain. With decomposition, the organic matter is converted into inorganic nutrients which can be taken up by plants in the food chain to produce new organic matter. This transfer represents a net input of matter into the food chain system.
7. Functional relationships can only occur because of the presence of a driving force. The driving force in the food chain is the Sun.
8. The parts that make up a system show some degree of integration - in other words the parts work well together. Integration in the food chain comes primarily from the process of evolution. It was through evolution that plants, herbivores, and carnivores came about and developed ecological associations between each other.

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