The transfer of energy within an ecosystem from one feeding level to the next is a fundamentally inefficient process. This principle is known as the 10% rule, stating that only about one-tenth of the energy from one trophic level is stored in the biomass of the next. This low rate of transfer, which typically ranges between 5% and 20% but is commonly approximated as 10%, dictates the structure and dynamics of life on Earth. Understanding this significant energy loss reveals how the laws of physics and the biology of organisms govern the flow of energy in nature.
The Physical Law Governing Energy Flow
The inefficiency of energy transfer is a direct consequence of the laws of physics. The First Law of Thermodynamics states that energy cannot be created or destroyed, only converted from one form to another. This means the total amount of energy remains constant as it flows through an ecosystem, such as when solar energy captured by plants is converted into chemical energy.
The underlying reason for the dramatic 90% loss is the Second Law of Thermodynamics, which introduces the concept of entropy, or increasing disorder. This law dictates that every energy conversion increases the amount of unusable energy in the system, typically dissipated as heat. Whenever energy changes form—from light to chemical, or from chemical to mechanical—some of it is inevitably lost as thermal energy. This thermal energy is no longer available to do biological work or be passed up the food chain to the next consumer.
Since living organisms are highly ordered systems, they constantly require energy to maintain their structure and function. This continuous need to perform work necessitates the release of heat, making a 100% efficient transfer of energy impossible. The energy is converted into a less concentrated, less usable form at each successive trophic level. This unavoidable physical constraint establishes the upper limit for energy transfer efficiency in any ecosystem.
Where the Missing 90 Percent Goes
The 90% of energy that fails to transfer is accounted for by three primary biological processes within the organism itself. The largest contributor to this loss is the energy an organism uses simply to stay alive, known as metabolic heat loss. Organisms constantly perform cellular respiration to fuel basic functions like muscle contraction and nerve signaling. These internal processes convert chemical energy into heat, which radiates away into the environment and cannot be consumed.
A significant portion of energy is also lost through the incomplete consumption and assimilation of food. When an animal eats, not all parts of the prey or plant are digestible, such as bones, hair, cellulose, or lignin. This indigestible material is excreted as waste, carrying with it the chemical energy it still contains. This lost energy is then shunted toward the decomposer trophic level, rather than moving up the primary food chain to the next consumer.
Finally, some energy remains tied up in unconsumed biomass, representing the energy of organisms that die without being eaten by the next trophic level. A plant’s roots might die and decompose, or an animal might die of disease, with only a fraction of its body being consumed by scavengers. This unused organic matter is eventually broken down by detritivores and decomposers, diverting its energy away from the main consumer pathway.
How Inefficient Transfer Shapes Ecosystems
The profound inefficiency of energy transfer has a direct and visible impact on the structure of all ecosystems. This fundamental limitation is graphically represented by the ecological pyramid, which illustrates the dramatic decrease in energy, biomass, and population numbers as one moves up the trophic levels. The wide base of the pyramid, representing producers like plants, supports a much smaller level of primary consumers, which in turn supports an even smaller level of secondary consumers.
Because only a fraction of energy is retained at each step, food chains are inherently limited in length. Ecosystems rarely support more than four or five trophic levels, simply because the energy available at the top becomes too dilute to sustain a viable population. If a primary producer level starts with 10,000 units of energy, the tertiary consumer level can only access approximately 10 units. This constraint explains why top predators, such as eagles or large sharks, are always far less numerous than the herbivores and plants they rely upon.