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Energy decreases at each trophic level due to metabolic costs, growth, and movement. Approximately 10% of energy transfers from one trophic level to the next (ranging from 5-20% depending on ecosystem). This creates pyramids of energy and biomass, with progressively fewer organisms at higher trophic levels.
From trophic levels and food webs, you know that energy enters ecosystems through producers (plants, algae, chemotrophs) and flows upward through herbivores, predators, and top predators. From ecosystem productivity, you know the difference between gross primary productivity (GPP) and net primary productivity (NPP) — the total energy fixed by photosynthesis minus what plants use for their own respiration. Energy pyramids visualize what happens to that energy as it passes through the food web, and the picture is dramatic: at every step, most of the energy disappears.
The 10% rule is a rough but useful approximation: only about 10% of the energy available at one trophic level is converted into biomass at the next level. Where does the other 90% go? Most is lost as metabolic heat through cellular respiration — organisms burn calories to maintain body temperature, move, grow, reproduce, and repair tissues. Some energy is never consumed at all: leaves fall and decompose, prey animals escape predation, and inedible structures like bones and shells pass through the food web without being assimilated. Of the food that is consumed, a portion passes through the digestive system undigested and enters the detrital pathway. Only the fraction that is both consumed and assimilated — then allocated to growth and reproduction rather than respiration — becomes available to the next trophic level.
This relentless energy loss explains why food chains are typically short — usually only 4 or 5 links. Consider a concrete example: if a grassland fixes 10,000 kcal/m²/year of net primary productivity, herbivores (grasshoppers, cattle) capture roughly 1,000 kcal. Primary carnivores (frogs, small birds) get about 100 kcal. Secondary carnivores (hawks, snakes) get roughly 10 kcal. By the time you reach a top predator, there simply is not enough energy to support a viable population. This is why apex predators are rare — not because they are inefficient hunters, but because the thermodynamic tax on energy transfer leaves very little for the top of the pyramid.
The actual trophic transfer efficiency varies considerably across ecosystems and organism types. Ectotherms (cold-blooded animals like insects and fish) tend to have higher efficiencies (around 10–15%) because they spend less energy on maintaining body temperature. Endotherms (birds and mammals) are less efficient (often 1–5%) because they burn enormous amounts of energy generating heat. Aquatic ecosystems often show higher transfer efficiencies than terrestrial ones because phytoplankton are small, fast-growing, and almost entirely edible, whereas terrestrial plants invest heavily in inedible structural tissue like wood and bark. These differences have practical consequences: aquaculture of herbivorous fish is far more energy-efficient than cattle ranching, and ecosystems dominated by ectothermic food webs can support relatively more biomass at higher trophic levels. The energy pyramid is not just an ecological diagram — it is a fundamental constraint shaped by thermodynamics that determines the structure, length, and productivity of every food web on Earth.