At each transfer from one trophic level to another in a food chain or web, work is done, low- quality heat is given off to the environment, and the availability of high-quality energy to organisms at the next trophic level is reduced. This reduction in high-quality energy available at each trophic level is the result of the inevitable energy quality tax imposed by the second law of energy.

The percentage of available high-quality energy transferred from one trophic level to another varies from 2% to 30%, depending on the types of species involved and the ecosystem in which the transfer takes place.

In the wild, ecologists estimate that an average of about 10% of the high-quality chemical energy available at one trophic level is transferred and stored in usable form as chemical energy in the bodies of the organisms at the next level.

The rest of the energy is used to keep the organisms alive, and most is eventually degraded and lost to the environment as low-quality heat in compliance with the second law of energy. Some of it is transferred to decomposers, which use a small amount to stay alive and degrade the rest to low-quality heat.

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There is a (in figure) loss of usable-quality energy at each step in a simple food chain. The pyramids of energy How and energy loss in this diagram show that the greater the number of trophic levels or steps in a food chain or web, the greater the cumulative loss of usable high-quality energy.

The energy flow pyramid explains why a larger population of people can be supported if people shorten the food chain by eating grains directly (for example, rice -> human) rather than eating animals that feed on grains (grain steer -» human). To prevent protein malnutrition, a vegetarian diet must include a variety of plants that provide enough of the 10 nitrogen-containing amino acid molecules used to make proteins that our bodies cannot synthesize. Poor people surviving on a plant diet often don’t have enough money to grow or purchase the variety of plants needed to avoid protein malnutrition.