1.10 Energy Flow and the 10% Rule

3 min readdecember 26, 2022



AP Environmental Science ♻️

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Solar energy provides every ecosystem with its energy supply, and as it moves through the trophic pyramid from producer to herbivore to carnivore and then to decomposers, this solar energy becomes chemical. Photosynthesis occurs when light is taken in by plants and converted into oxygen and food. Energy is stored in the glucose molecule which, through the chemical reactions of cellular respiration, is transformed again into thermal energy and a new form of chemical energy (ATP) used by the cells. 
Nothing in the universe is perfect and energy transferrence is not entirely efficient. Since energy, like matter, cannot be created or destroyed, the energy is not “lost,” but instead changes into a less organized form. This means that ecosystems do not directly use all the energy provided, but rather lose it to dysfunction or assorted environmental processes. These laws are important in ecology because they explain the distribution and variety of biotic and abiotic factors in ecosystems.

The First Law of Thermodynamics

This is the law of conservation of energy. The law states that, in a closed system, energy cannot be created or destroyed, it can only be transformed from one form to another. Like the image below, 100kcal of ice cream should get you 100kcal of kinetic energy. So why doesn’t it work that way?

The Second Law of Thermodynamics

The second law of thermodynamics states that entropy increases every time energy is forced to reroute or change forms. Entropy is the amount of disorganization present in a system. Thermal energy has the most disorder, or the most entropy. What you need to know is that energy transformations always result in losses to waste heat. In the case of organisms eating organisms, these losses result in only 10% of the energy (on average being passed onto the next trophic level. The entropy of trophic levels, as a result, is quite high, due to the low rate of energy transfer per tier. It also explains why it is impossible to use an energy resource that is 100% efficient, which is something you will review in unit 6.

The 10% Rule

The 10% rule states that only 10% of energy from one trophic level is able to move up to the next. So, if producers have 10,000 J of energy stored through photosynthesis, then only 1000 J is passed on to primary consumers. You can calculate the amount of energy up and down the pyramid by moving the decimal place to the left as you move up one trophic level, and to the right as you move back down. As a result, 90% of energy per tier is lost to waste heat. It is important to note that photosynthesis, on average, only captures 1% of the sun’s energy, while roughly 99% is lost to reflection or other miscellaneous systems. And not all ecosystems have a 10% efficiency, it depends on the type of biome. On average, though, 10% is an easy threshold to help understand why an individual jaguar needs at least one square mile to support enough vegetation to feed its prey
🎥 To learn more about trophic levels and the flow of energy, watch this stream.
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