The big idea: Energy enters most ecosystems as sunlight, which producers trap by photosynthesis.
As that energy passes along a food chain, only about 10% reaches the next trophic level — roughly 90% is lost at each step.
Because so much energy disappears at every level, there is less and less available the higher up you go. A pyramid of energy shows this: each bar is far smaller than the one below it.
A pyramid of energy: only about 10% of the energy passes to each higher trophic level, so each bar is roughly a tenth of the one below; the other ~90% is lost as heat (respiration) and in waste.
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- Trophic level
- A feeding level in a food chain — for example producers, primary consumers, secondary consumers.
- Energy transfer efficiency
- The percentage of energy at one trophic level that is passed on to the next. In most food chains it is only about 10%.
- Pyramid of energy
- A diagram in which each bar represents the energy (per unit area, per year) at one trophic level. The bars get smaller up the levels because energy is lost at each step.
- Biomass
- The mass of living material (the energy stored in an organism's body). Only the energy built into biomass can be passed to the next level by being eaten.
Energy flows, it does not cycle: Unlike nutrients, energy is not recycled.
It enters as sunlight, is passed along the chain, and is steadily lost as heat until none is left — so ecosystems need a constant supply of sunlight to keep going.
If only about 10% of the energy moves up to the next level, the obvious question is: where does the other ~90% go?
It is lost in three main ways at every trophic level.
The three big energy losses
- Respiration — most of the energy is released to power the organism and escapes as heat, which cannot be eaten
- Undigested waste (faeces) — some food is never absorbed and is egested, passing to decomposers instead of up the chain
- Uneaten or dead material — not every part of an organism is eaten (bones, roots, animals that die of other causes), so that energy also goes to decomposers
| Where the energy goes | What happens | Counts as 'lost' to the next level? |
|---|---|---|
| Cell respiration | Energy is released to power the organism and is given off as heat | Yes — heat cannot be passed on as food energy |
| Undigested waste (faeces) | Some food is never absorbed and leaves the body in faeces | Yes — it passes to decomposers, not up the chain |
| Uneaten / dead material | Not all of an organism is eaten by the next level (bones, roots, deaths) | Yes — it passes to decomposers, not up the chain |
| Built into body (biomass) | The small remainder is stored as new growth | No — only THIS part can be eaten by the next level |
Heat is the big one: The single largest loss is heat from respiration.
Every organism respires to release energy for movement, growth and keeping warm, and that heat leaves the ecosystem — it can never be passed on as food.
This is why energy transfer is so inefficient, and why each trophic level holds only about a tenth of the energy of the one below.
| Trophic level | Energy available (example) | Roughly what fraction of the level below |
|---|---|---|
| Producers | 10 000 kJ | — |
| Primary consumers | 1 000 kJ | about 10% |
| Secondary consumers | 100 kJ | about 10% |
| Tertiary consumers | 10 kJ | about 10% |
Why food chains are short: Each level passes on only ~10% of its energy, so the numbers fall fast: 10 000 → 1 000 → 100 → 10 kJ.
After only four or five levels there is too little energy left to support another trophic level — there simply is not enough food energy for a fifth or sixth consumer.
That is why food chains rarely have more than four or five links, and why top predators are rare.
A real-world consequence: It takes far more plant energy to feed a meat-eater than a plant-eater, because energy is lost at the extra trophic level.
That is why beef (cow eats plants, human eats cow — two transfers) is less energy-efficient than eating the plants directly, and why raising chickens or insects (which convert feed to body mass more efficiently) wastes less energy than raising cattle.
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How this is tested: On Paper 2 a 4-mark Explain or Describe question often asks why energy decreases as it passes up a food chain — give separate scoring points (only ~10% passes on; heat lost in respiration; energy in faeces / undigested waste; uneaten or dead material).
On Paper 1 a 1-mark question asks you to interpret a pyramid of energy — the bars shrink up the levels because each level holds less energy than the one below.
Watch for data questions: you may be given transfer-efficiency or feed-conversion figures and asked to deduce why beef is less efficient than poultry, or why predators reach lower densities than the prey beneath them.
IB-style question — explain why energy decreases up a food chain
Explain why the amount of energy available decreases at each successive trophic level in a food chain. [4]
How to score all four marks
- State the efficiency. Only about 10% of the energy at one trophic level is passed on to the next, so most is lost at each step.
- Heat from respiration. Most of the energy is released by respiration and lost from the ecosystem as heat, which cannot be passed on as food.
- Undigested waste. Some of the food eaten is not absorbed and is lost in faeces (undigested waste), passing to decomposers rather than up the chain.
- Not all eaten. Not all of an organism is eaten by the next level (uneaten or dead material), so that energy is also lost from the chain. (Award 1 mark for each distinct point, up to 4.)
Final answer
Only ~10% passes on; most energy is lost as heat from respiration; some is lost in undigested waste (faeces); and not all of an organism is eaten — so less energy is available at each higher level.
✓ Why this scores full marks: Each sentence is a separate, distinct reason — ~10% passed on, heat from respiration, faeces, uneaten material.
A 4-mark 'explain' needs four scoring points, not the single idea 'energy is lost' written four ways.