Energy Transfer

Energy transfers within food webs are governed by the first and second laws of thermodynamics. The first law relates to quantities of energy. It states that energy can be transformed from one form to another, but it cannot be created or destroyed. This law suggests that all energy transfers, gains, and losses within a food web can be accounted for in an energy budget.

The second law relates to the quality of energy. This law states that whenever energy is transformed, some of must be degraded into a less useful form. In ecosystems, the biggest losses occur as respiration. The second law explains why energy transfers are never 100% efficient. In fact, ecological efficiency, which is the amount of energy transferred from one trophic level to the next, ranges from 5-30%. On average, ecological efficiency is only about 10%.

Because ecological efficiency is so low, each trophic level has a successively smaller energy pool from which it can withdraw energy. This is why food webs have no more than four to five trophic levels. Beyond that, there is not enough energy to sustain higher-order predators.