When energy moves from one level of a food chain to the next, only about 10 percent carries forward as usable biomass. The other 90 percent is split among three main exits: most of it is burned as heat through cellular respiration, a portion leaves as waste the organism can’t digest, and some biomass simply never gets eaten at all. This isn’t a flaw in nature. It’s a consequence of basic physics that shapes every ecosystem on Earth.
Most of It Becomes Heat
The single biggest destination for that 90 percent is heat, released during cellular respiration. Every organism constantly breaks down sugars and other molecules to fuel its daily operations: maintaining body temperature, building proteins, moving, reproducing, and simply keeping cells alive. That chemical process converts the energy stored in food into usable fuel for the body, but a large share escapes as heat in the process. A rabbit, for example, uses a significant fraction of the energy it absorbs from plants just “being a rabbit,” hopping around, staying warm, and running its metabolism. That energy radiates out of its body and dissipates into the environment. It doesn’t vanish, but it’s no longer available to the fox that eats the rabbit.
This is dictated by the second law of thermodynamics: no energy transfer in the universe is completely efficient. Every time energy changes form, some portion becomes unusable heat, increasing the overall disorder (entropy) of the system. Even at the level of individual cells, growth efficiency seldom exceeds 40 percent, and that’s before you account for all the other ways energy leaks out of an organism. Under natural conditions, the ratio of new biomass produced to total food ingested rarely exceeds 30 to 35 percent, and it’s often considerably lower.
Waste and Indigestible Material
Not everything an animal eats can actually be broken down. Food that passes through the gut without being absorbed exits as feces, taking its stored energy with it. This is especially significant for herbivores. Plant cell walls contain cellulose and lignin, tough structural compounds that most animals can’t fully digest. Lignin in particular has been recognized for over 50 years as a major barrier to digestion. It’s chemically bonded to other cell wall components, and the plant tissues with the highest lignin concentrations are consistently the least digestible. Grasses, wood, bark, and leaves all carry substantial energy that simply passes through the animal untouched.
Beyond feces, organisms also lose energy through urine and other excretions. Nitrogen-containing waste products still hold chemical energy, but the organism expels them rather than using them. Together, egestion and excretion represent a meaningful chunk of the energy budget at every trophic level. This excreted material doesn’t disappear from the ecosystem entirely. It enters the detritus chain, where decomposers like bacteria and fungi break it down, but that energy is no longer flowing upward through the food chain to larger predators.
Biomass That Never Gets Eaten
There’s a category of energy loss that has nothing to do with metabolism or digestion: organisms that simply die without being consumed. A plant that falls and rots, an insect that drowns, a fish that dies of disease before a predator finds it. All of that biomass represents stored energy that exits the living food chain. It gets channeled into the decomposer pathway instead, feeding fungi, bacteria, and detritivores rather than the next trophic level up.
This non-grazing mortality is a significant factor. No predator population is efficient enough to consume every individual at the level below it. Leaves fall, carcasses decay, and organic matter washes downstream or settles into sediment. Each of these represents energy that was available in theory but never made the transfer in practice.
Why the Number Varies
The “10 percent rule” is a useful average, but real transfer efficiency depends heavily on the ecosystem. Aquatic food webs tend to be more efficient than terrestrial ones. Grazing zooplankton in lakes consume a far greater fraction of available plant material than land herbivores do. Field measurements show that herbivores in water consume three to four times more living plant biomass than their counterparts on land. Forests sit at the low end of the efficiency spectrum, grasslands fall in the middle, and lake plankton systems are the most efficient.
The reasons are partly structural. Terrestrial plants invest heavily in woody stems, bark, and roots loaded with lignin and cellulose, all of which resist digestion. Phytoplankton, by contrast, are single-celled organisms with no woody tissue, making them far easier for zooplankton to consume and convert into biomass. Warm-blooded animals also lose more energy to heat than cold-blooded ones, since maintaining a constant body temperature is metabolically expensive.
Why This Limits Food Chains
The 90 percent loss at each step is the reason food chains rarely exceed four or five levels. If a field of plants captures 10,000 units of energy from the sun, herbivores convert roughly 1,000 of those into their own biomass. Small predators that eat the herbivores retain about 100 units. Larger predators that eat them keep around 10. By the time you reach a fifth level, there’s only about 1 unit of energy left, which simply isn’t enough to sustain a viable population of apex predators.
This is why top predators like eagles, sharks, and wolves are always far less abundant than the organisms they feed on. It’s also why large carnivores need enormous territories. The energy pyramid narrows so sharply that each successive level can support roughly one-tenth the biomass of the level below it. A proposal for a sixth or seventh trophic level isn’t theoretically impossible, but in practice there just isn’t enough energy left to keep those animals alive and reproducing. The 90 percent loss, compounding at every step, is the fundamental constraint on how complex a food web can become.