Each trophic level gets increasingly less efficient at getting energy from the lower levels: each level higher than the next tends to hold 10% of the energy that the lower level could hold. For example, producers hold 1000 units of energy in a hypothetical environment. Primary consumers would then hold 100 units, secondary would hold 10, tertiary would hold only 1, and so on. Eventually, there is simply not enough energy to sustain higher levels. Therefore, food chains tend to be relatively short.
