Understanding the 10% Rule
The 10% rule is a principle in ecology stating that, on average, only about 10 percent of the energy stored in one trophic level (a step in a food chain) is converted into biomass at the next higher trophic level. The remaining 90% is used by the organisms for metabolic processes or is lost to the environment as heat.
Section 2: Why Is Energy Lost Between Trophic Levels?
The vast majority of energy is not transferred because organisms use it to live. Energy is expended on activities like respiration, movement, reproduction, and maintaining body temperature. Furthermore, not all parts of an organism are eaten or digestible by the next level, and this uneaten or undigested matter still contains energy that is not passed on.
Section 3: A Practical Example
Imagine a simple food chain. If the producers, like grass, contain 10,000 joules of energy, the primary consumers (herbivores like rabbits) that eat the grass will only incorporate about 1,000 joules into their bodies. When secondary consumers (carnivores like foxes) eat the rabbits, they will only gain about 100 joules of energy. A tertiary consumer, like an eagle eating the fox, would only get 10 joules.
Section 4: Importance of the 10% Rule
This rule explains why food chains are typically short, usually not exceeding four or five levels. There is simply not enough energy remaining at the top to support more levels. It also illustrates why the biomass of producers in an ecosystem must be far greater than the biomass of the top predators, forming a structure known as an energy pyramid.