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United StatesEnvironmental ScienceSyllabus dot point

Why is only about ten percent of energy passed from one trophic level to the next?

Topic 1.10 Energy Flow and the 10% Rule: explain how energy is lost between trophic levels, apply the 10% rule, and calculate energy transfer and ecological efficiency.

A focused answer to APES Topic 1.10, covering the one-way flow of energy, the 10% rule, why energy is lost as heat and through respiration, ecological efficiency, and energy pyramids, with full worked multi-level energy calculations.

Generated by Claude Opus 4.810 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

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  1. What this topic is asking
  2. Energy flows one way
  3. The 10% rule
  4. Where the lost energy goes
  5. The energy pyramid and food-chain length
  6. Try this

What this topic is asking

The College Board (Topic 1.10) wants you to explain why energy decreases up the trophic levels and to apply the 10% rule quantitatively. You must know where the lost energy goes, calculate energy transfer between levels, and explain how the loss limits food-chain length and shapes the energy pyramid. This is a quantitative topic.

Energy flows one way

The 10% rule

So if producers hold 10,000 units of energy, primary consumers receive about 1,000, secondary consumers about 100, and tertiary consumers about 10. To find the energy at a level, multiply by 0.10 for each step up.

Where the lost energy goes

The 90% that does not pass up is lost in several ways:

  • Cellular respiration: organisms use most of their energy to power life processes, releasing it as heat (the largest loss).
  • Not consumed: much biomass is never eaten and instead dies and goes to decomposers.
  • Not digested (egested): some eaten material passes through undigested as waste.
  • Movement and maintenance: energy spent on activity is ultimately lost as heat.

The energy pyramid and food-chain length

Because each level holds only about a tenth of the energy of the one below, a graph of energy by trophic level forms an energy pyramid that is always widest at the producers and narrows sharply upward. This explains why food chains are short: after four or five transfers, so little energy remains that it cannot support another level. It also explains why top predators are rare and need large ranges, and why eating lower on the food chain (more plants, fewer animals) feeds more people per unit of land, because less energy is lost. This quantitative loss is the single most important idea linking the ecosystem topics: primary productivity (Topic 1.8) sets the starting energy, the trophic levels (Topic 1.9) are the steps, and the 10% rule is the toll paid at each step.

Try this

Q1. Identify the main fate of the energy that does not pass to the next trophic level. [1 point]

  • Cue. It is lost as heat during cellular respiration.

Q2. Calculate the energy available to primary consumers if producers store 80,000 kcal/m^2/year. [1 point]

  • Cue. 80,000×0.10=8,00080{,}000 \times 0.10 = 8{,}000 kcal/m^2/year.

Exam-style practice questions

Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

AP 2021 (style)4 marksSection II (FRQ, quantitative). A grassland's producers store 100,000 kcal/m^2/year. Assume the 10% rule applies between trophic levels. (a) Calculate the energy available to primary consumers. (b) Calculate the energy available to secondary consumers. (c) Explain where the lost energy goes between trophic levels. (d) Explain why this energy loss limits the number of trophic levels in an ecosystem.
Show worked answer →

A 4-point quantitative FRQ on energy flow.

(a) Calculate (1 point): primary consumers receive 10% of 100,000 = 100,000×0.10=10,000100{,}000 \times 0.10 = 10{,}000 kcal/m^2/year.
(b) Calculate (1 point): secondary consumers receive 10% of 10,000 = 10,000×0.10=1,00010{,}000 \times 0.10 = 1{,}000 kcal/m^2/year.
(c) Explain (1 point): most energy is lost as heat during cellular respiration; some is never consumed or not digested (egested), and some remains in uneaten or dead biomass passed to decomposers.
(d) Explain (1 point): because only about 10% transfers each step, energy runs too low after a few levels to support another, so food chains rarely exceed four or five levels.

Markers reward both correct calculations (multiplying by 0.10 each step), naming respiration heat loss as the main fate, and connecting energy loss to the limited number of levels.

AP 2019 (style)1 marksSection I (multiple choice). According to the 10% rule, if producers contain 50,000 kcal of energy, approximately how much is available to secondary consumers? (A) 5,000 kcal (B) 500 kcal (C) 50 kcal (D) 25,000 kcal. Justify your choice.
Show worked answer →

A 1-point quantitative MCQ. The answer is (B).

Apply the 10% rule twice: producers to primary consumers gives 50,000×0.10=5,00050{,}000 \times 0.10 = 5{,}000 kcal; primary to secondary consumers gives 5,000×0.10=5005{,}000 \times 0.10 = 500 kcal. Option (A) applies the rule only once; (C) applies it three times; (D) halves instead of taking 10%. The trap is forgetting that secondary consumers are two transfers up.

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