What is the 10% rule used in an energy pyramid?
The 10% rule refers to the amount of energy available to the next trophic level.
When energy moves between trophic levels , 10% of the energy is made available for the next level. (The exception is the transition from the sun to producers, in which case only 1% of the energy is retained.)
When a consumer eats a plant, it gains energy from the plant. That energy is used for growth, reproduction, and other biological processes. Some of that energy is also lost through heat loss. Thus, when a predator eats that consumer, all of the energy the consumer gained from the plant is not available to the predator: it has been used and lost.
As we move up an energy pyramid or a trophic level, we can see that less and less of the original energy from the sun is available. Roughly ten percent of the previous trophic level's energy is available to the level immediately higher up. This is called the 10% Rule.