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Trophic Levels and Food Chains

The biotic elements that comprise an ecosystem fall into one of several trophic levels. The trophic level of an organism is its position in a food chain, the sequence of consumption and energy transfer through the environment. For example, a simple grazing food chain is comprised of

Plant -> herbivore -> carnivore

At the base of the food chain lies the primary producers. Primary producers are principally green plants and certain bacteria. They convert solar energy into organic energy. Above the primary producers are the consumers who ingest live plants or the prey of others. Decomposers, such as, bacteria, molds, and fungi make use of energy stored in already dead plant and animal tissues.

Two laws of physics are important in the study of energy flow through ecosystems. The first law of thermodynamics states that energy cannot be created or destroyed; it can only be changed from one form to another. Energy for the functioning of an ecosystem comes from the Sun. Solar energy is absorbed by plants where in it is converted to stored chemical energy. The second law of thermodynamics states that whenever energy is transformed, there is a loss energy through the release of heat. This occurs when energy is transferred between trophic levels as illustrated in a food web. When one animal feeds off another, there is a loss of heat (energy) in the process. Additional loss of energy occurs during respiration and movement. Hence, more and more energy is lost as one moves up through trophic levels. This fact lends more credence to the advantages of a vegetarian diet. For example, 1350 kilograms of corn and soybeans is capable of supporting one person if converted to beef. However, 1350 kilograms of soybeans and corn utilized directly without converting to beef will support 22 people!

Free Energy in the Food Chain

If we consider energy in a very broad sense, we recall that the first and second laws of thermodynamics state that "the total energy content in the universe is constant, and the total entropy is continually increasing."¹ The idea that there is a set and finite amount of energy in the universe is something we may comprehend, but the fact that this energy continually moves to a less usable state is more difficult to understand.

Moreover, we find that the more advanced the species, the more free energy is required for its survival. Because 80-90% of stored energy is typically lost in the translation from prey to predator, the food that reaches our table comes at an exorbitant cost. Chemist G. Tyler Miller describes a sample food chain in this way: "Three hundred trout are required to support one man for a year. The trout, in turn, must consume 90,000 frogs, which must consume 27 million grasshoppers, which live off of 1,000 tons of grass."<sup>2

1</sup>Asimov, Isaac. "In the Game of Energy and Thermodynamics You Can't Even Break Even." Smithsonian. August 1970. p. 9
²Miller, G. Tyler, Jr. Energetics, Kinetics and Life. Belmont, California: Wadsworth, 1971. p. 46