The movement of energy is the single most defining process in an ecosystem, connecting all living things in a continuous flow. This energy begins with producers, such as plants and algae, which capture light energy to create chemical energy in the form of organic compounds. Energy then moves up through the feeding levels, known as trophic levels, starting with primary consumers (herbivores) and progressing to secondary and tertiary consumers (carnivores and omnivores). As energy passes from one level to the next, organisms use the captured chemical energy to fuel their own life processes. The fundamental question in ecology is how much of that energy successfully makes the jump to the next level, and how much is lost in the transfer.
The Quantitative Rule of Energy Transfer
Ecological efficiency, which measures the success of energy transfer between two successive trophic levels, is remarkably low. This transfer is often summarized by the “10% rule,” a foundational concept in ecology. This rule states that, on average, only about ten percent of the total energy available at one trophic level is incorporated into the biomass of the next. For example, if 1,000 units of energy are stored in plant tissue, only about 100 units are fixed into the body mass of the consumer. The remaining ninety percent is lost to the environment through various processes.
This ten percent figure is an average, and the actual Trophic Level Transfer Efficiency (TLTE) can vary significantly between different organisms and environments. Efficiency can range from as low as five percent to closer to twenty percent, particularly in marine ecosystems. Despite these variations, the vast majority of energy is lost at each step, creating a massive energy deficit up the food chain. This consistent loss fundamentally shapes the structure of life on Earth.
Biological Reasons for Energy Loss
The primary reason for the large energy drop is the unavoidable consequence of the laws of thermodynamics. Organisms constantly perform cellular respiration to generate power, and this energy is ultimately dissipated as heat. This heat loss is unusable energy that cannot be passed on to the next trophic level. Maintaining basic life functions, such as breathing and circulation, consumes a substantial portion of assimilated energy, especially for warm-blooded animals (endotherms) which require constant internal temperature regulation.
Not all consumed organic matter is usable by the predator, leading to another form of energy loss. Energy remains locked in waste products, such as feces, which consist of undigested food material never absorbed into the consumer’s tissue. This undigested matter, like cellulose or bones, still contains chemical energy but is egested from the food chain as waste. Furthermore, consumers rarely eat every part of the organism they capture, leaving behind energy-containing scraps like roots or skeletal remains.
A substantial portion of energy is lost when organisms die naturally without being consumed by the next trophic level. The energy in these unconsumed dead organisms becomes detritus, processed by decomposers and detritivores, such as fungi and bacteria. This energy is rerouted into the decomposer food web and is not transferred up the main consumer food chain. This combined energy usage—for metabolism, waste products, and unconsumed biomass—accounts for the ninety percent of energy that fails to transfer.
How Energy Loss Structures Ecosystems
The steep reduction in available energy dictates the physical structure of ecosystems, often visualized through ecological pyramids. An energy pyramid graphically represents the energy content at each trophic level, with the base (producers) being the widest and the top (apex predators) being the narrowest. The pyramid shape is a direct result of the ten percent transfer rule, showing the dramatic decrease in energy. This structure confirms that the total energy available to support life shrinks rapidly at successively higher levels.
This energy limitation explains why food chains rarely extend beyond four or five trophic levels in most natural environments. After four to five energy transfers, the remaining energy is insufficient to support a viable population at a higher level. For instance, if producers start with 10,000 joules of energy, a quaternary consumer would only receive about 10 joules. The scarcity of energy at the highest levels explains why top predators, such as large sharks or eagles, are always fewer in number than the herbivores they feed upon.
The massive energy loss also correlates with a decrease in the total mass of living organisms, or biomass, supported at each level. Since less energy is available to be fixed into new organic matter, each successive trophic level sustains a smaller total biomass than the level below it. This principle ensures that the ecosystem’s foundation of producers must always contain a significantly greater biomass than all the consumers it supports. The low ecological efficiency fundamentally constrains the number of organisms, their total mass, and the length of the food chain.