Ecology, Energy Flow

Organisms are complex biochemical machines that require a constant consumption of energy to grow, reproduce, and maintain their biological integrity. The use of energy must obey physical principles: the laws of thermodynamics. Constraints imposed by these principles have profound influence in the flow and conservation of energy and therefore the structure of an ecological community.

Energy from the sun powers the world's ecological communities. Solar energy is channeled into an ecological community by way of photosynthesis in green plants and many other photosynthetic microorganisms. Energy harvested by photosynthesis is used to produce plant tissue where light energy is saved as chemical energy. This chemical energy is transferred when plants are eaten by herbivores (plant-eating animals). Energy stored in herbivores can further be transferred to carnivores (animal-eating animals). This sequence of energy transfer from plants to herbivores and then carnivores is called a food chain. Along the food chain, the number of transfers for the solar energy to reach an organism defines its trophic level. Plants therefore occupy the first trophic level, herbivores the second trophic level, and herbivore-eating carnivores the third trophic level. A species population can occupy more than one trophic level depending on the source of energy actually assimilated.

Organisms can be classified into autotrophs and heterotrophs depending on the nature of energy and nutrients they use. Autotrophs, which include all the higher plants and algae, use light as their energy source and they depend completely on inorganic nutrients for their growth. Heterotrophs, which include all the animals, protocists, fungi, and many bacteria, use chemical energy for their needs and require organic compounds for their growth. Heterotrophs acquire both energy and organic carbon from their food. In an ecological community, autotrophs also are called producers for their roles in the harness of solar energy to convert inorganic nutrients into energy rich organic material. Heterotrophs are called consumers, for their dependence on autotrophs for energy and nutrients.

Laws of Thermodynamics

The laws of thermodynamics set stringent constraints on the use of energy by every organism. It is important to know what these constraints are and their ecological implications. The first law of thermodynamics states that energy is conserved and can neither be created nor destroyed. During photosynthesis, the energy in light is used to convert carbon dioxide and water into glucose and oxygen. Part of the light energy is harvested by the plant and stored in glucose with the rest of the energy dissipated . The amount of energy involved in photosynthesis remains the same before and after the process. The amount of energy that can be conserved by the process, the chemical energy stored in glucose, however, is constrained by the second law of thermodynamics.

Any natural process that involves the use, transformation, and conservation of energy is constrained by the second law of thermodynamics. The law requires that any irreversible process will result in the degradation of the energy involved. In other words, there is an energetic cost associated with every irreversible process. Each organism is a complex biochemical machine that is made up of a network of metabolic pathways. Every metabolic pathway amounts to a nonequilibrium chemical reaction and therefore is an irreversible process. Using photosynthesis as an example, for every one hundred calories of light energy absorbed by a plant, the amount of energy that can be harvested and stored in glucose will have to be less than one hundred calories. The second law, however, does not provide guidance on how much of the energy will have to be degraded during each irreversible process. Direct measurement is needed to determine the actual efficiency.

The Structure of an Ecological Community

Energy flow in an ecological community must obey the laws of thermodynamics. These constraints affect the flow of energy and therefore the structure of an ecological community. Using the grazing food chain as an example, let's see how these laws affect the flow of energy at each trophic level. For the harvest of solar energy by plants in the production of plant tissues, the first law of thermodynamics requires that the amount of solar energy captured by the plants remain the same before and after the transformation; the energy involved cannot be created nor destroyed. For every thousand calories of solar energy captured and transformed by plant, there remain a thousand calories afterward. The second law of thermodynamics, however, requires that the harvesting of solar energy cannot be 100 percent efficient; only a portion of the solar energy transformed by the plant can be conserved in the production of plant tissue. Measurements on various plant communities show that the actual efficiency is below 10 percent. Most of the light energy, over 90 percent, is degraded by respiration into nonusable form. The rate of production of plant tissue, a reflection of the net harvesting of solar energy, is defined as the net primary productivity.

The transformation of energy at the second trophic level, or any other trophic levels, follows the same pattern. As a rule of thumb, 90 percent of the energy involved is degraded at each trophic transfer and only 10 percent of the energy is conserved in the organism's tissue. With 1,000 calories of solar energy captured by the plant, 100 calories of plant tissue can be produced, which in turn can be used to produce 10 calories of herbivore tissue, and in turn 1 calorie of carnivore tissue. The amount of energy potentially available to a species population is greatly influenced by its position on the food chain; the lower its position the more its available energy. This energetic constraint is widely reflected in many ecological communities, as herbivores, whether they are zebras or deer, usually outnumber their predators, lions or wolves. Because of this rapid decrease in the amount of usable energy, the length of the food chain is usually limited to a maximum of four to five levels.

see also Ecology; Ecology, History of; Odum, Eugene.

Charles J. Gwo

Bibliography

Lewis, Gilbert Newton, and Merle Randall. Thermodynamics, 2nd ed., rev. by Kenneth Sanborn Pitzer and Leo Brewer. New York: McGraw-Hill, 1961.

Odum, Eugene P. Fundamentals of Ecology, 3rd ed. Philadelphia: Saunders, 1971.

Schrödinger, Erwin. What Is Life?: The Physical Aspect of the Living Cell. Cambridge, UK: Cambridge University Press, 1944.