Biology · Introductory biology · Concept
Trophic levels and energy flow: the 10% rule
A trophic level is a feeding step in a food chain: producers, then primary consumers (herbivores), secondary consumers and so on. At each step most of the energy is used in respiration and lost as heat, or is never eaten or digested, so only a small fraction, on average about 10% and typically between 5% and 20%, becomes new biomass for the next level. That is why energy pyramids narrow upward and food chains rarely have more than four or five levels.
Producers, consumers and decomposers
Producers, mostly photosynthetic plants and algae, capture energy and build organic matter. Primary consumers eat producers, secondary consumers eat primary consumers, and so on up the chain. Decomposers and detritivores feed on dead matter from every level. Omnivores feed at more than one level, which is why a food web describes an ecosystem better than a single chain.
Trophic transfer efficiency
Compare the energy at one level, Eₙ₊₁, with the energy at the level below it, Eₙ. Measured values usually run from about 5% to 20%; 10% is a rough average, not a law.
Where the rest goes
Most of the energy a level takes in is spent on respiration and leaves as heat. Some of the level below is never eaten, and some of what is eaten is not digested and leaves as waste. Those leftovers feed decomposers; the heat leaves the ecosystem. Energy flows one way through an ecosystem, while matter cycles.
Pyramids of energy, biomass and numbers
An energy pyramid always narrows upward. Pyramids of biomass or numbers can be inverted: one tree supports thousands of insects, and in open water fast-reproducing phytoplankton can have less standing biomass than the zooplankton eating them.
Why food chains are short
At 10% per step the fourth trophic level receives about 0.1% of the producers’ energy. Too little is left over to support a further level of large predators.
Common mistakes
- Treating 10% as a fixed law: measured efficiencies range from about 5% to 20%.
- Drawing food-web arrows from eater to eaten: they point from food to consumer, the direction energy moves.
- Expecting biomass and number pyramids always to narrow upward, as energy pyramids do.
- Saying energy is recycled: it flows through and leaves as heat, while nutrients cycle.
Key terms
- Trophic level
- An organism’s feeding position in a food chain: producers first, then herbivores, then carnivores. Omnivores can feed at more than one level.
- Food web
- A network of who eats whom in an ecosystem. In these tools, arrows point from the food to the eater, the direction energy and matter move.
- Primary producer
- An organism that makes its own organic matter from carbon dioxide, usually by photosynthesis, such as plants and algae. Producers bring new energy into a food web.
- Consumer
- An organism that gets its energy and matter by eating other organisms or their remains. Herbivores eat plants, carnivores eat animals and omnivores eat both.
- Decomposer
- An organism, such as a fungus or bacterium, that breaks down dead material and returns its nutrients to the soil or water. The energy it uses comes from that dead material.
- Biomass
- The total mass of living things in an area or at a trophic level, such as kg per m². It is an amount present at one time, not a rate of production.
- Trophic transfer efficiency
- The share of energy at one trophic level that reaches the next, calculated from the data. It is often about 10%, but that isn’t a fixed rule.
- Ecological pyramid
- A diagram stacking trophic levels by energy flow, biomass or number of individuals, with producers at the base. Energy pyramids always narrow upward; biomass and number pyramids can be inverted.
Work through an example
Net production in a grassland is 25,000 kJ/m² a year for producers, 2,100 for primary consumers, 180 for secondary consumers and 12 for tertiary consumers. Find the transfer efficiency between each pair of levels, and from producers to tertiary consumers.
Calculate trophic transfer efficiency →Sources and scope
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