Only about a tenth of the energy at one trophic level ends up in the next. The interesting part is not the number — it is where the other nine tenths go. There are four separate leaks, and a full-mark answer names them in order.
📚 What you need to know
Energy transfer between trophic levels is never 100% efficient.
The four losses, in order: not all food is harvested; not all harvested food is consumed; not all consumed food is absorbed; not all absorbed energy is stored.
The largest single loss is heat from cellular respiration.
Other losses include egestion (undigested material) and excretion of waste products.
Roughly 10% of energy passes to the next level, though real values vary.
Efficiency = (energy in the next level ÷ energy in this level) × 100.
These losses are why food chains are short and top predators are rare.
The four leaks, in order
Picture caterpillars feeding on an oak tree. Follow the energy from the tree to the caterpillars and you can see each loss happen.
Only the last bar is available to the blue tit that eats the caterpillar. Everything to the right of each bar has left the food chain.
🧩 The four losses, with the caterpillar example
Not all food is harvested. The caterpillars cannot reach every leaf on the tree, and there are not enough of them to eat it all.
Not all harvested food is consumed. They eat the soft parts and leave tough stalks and leaves containing toxins.
Not all consumed food is absorbed. Indigestible material passes through the gut and is egested as faeces.
Not all absorbed energy is stored. Most is released by respiration for movement and growth, and leaves as heat. Waste products are excreted.
The energy budget of one animal
Take 100 units of energy eaten by a herbivore. A typical split looks like this — and the small green slice at the end is the only part that a predator can ever obtain.
Real figures vary a lot. Warm-blooded animals lose far more to heat than cold-blooded ones, which is why fish farming is more energy-efficient than cattle farming.
Energy transfer efficiency
efficiency (%) = (energy stored at next level ÷ energy stored at this level) × 100
Egested and excreted are not the same word. Egestion is undigested food leaving as faeces — it was never absorbed. Excretion is the removal of waste made by the body’s own reactions, such as urea. Examiners notice the difference.
Worked examples
WORKED EXAMPLE
Producers in a grassland store 8 600 kJ m² per year. The primary consumers store 1 200 kJ m² per year. Calculate the energy transfer efficiency.
Step 1: write the equation
efficiency = (energy at next level ÷ energy at this level) × 100
Step 2: substituteefficiency = (1 200 ÷ 8 600) × 100efficiency = 0.1395… × 100efficiency = 14.0%Step 3: comment
Slightly above the usual 10% figure, which is normal — 10% is a rough average, not a rule.
Keep the units identical on top and bottom. If they differ, convert before dividing.
WORKED EXAMPLE
Explain three reasons why less than 100% of the energy in grass reaches a cow’s biomass.
Reason 1: not all is eaten
The cow cannot graze every plant in the field, and it leaves roots and tough stems behind.
Reason 2: not all is absorbed
Some plant material, such as parts of the cellulose, is indigestible and is egested in faeces.
Reason 3: not all is stored
Most of the absorbed energy is released by respiration for movement, digestion and keeping warm, and is lost as heat.
Only what is left builds new body tissueGive a different loss each time. Three versions of “it is lost as heat” scores one mark, not three.
WORKED EXAMPLE
Explain why more people can be fed from a hectare of wheat than from a hectare used to raise cattle.
Step 1: count the transfers
Eating wheat means one transfer, from producer to human. Eating beef means two, producer to cattle to human.
Step 2: apply the loss
Around 90% of the energy is lost at each transfer, mainly as heat from respiration.
Step 3: conclude
The extra trophic level wastes most of the energy that the wheat captured.
Shorter food chains deliver more energy to peopleThis is a favourite question because it applies the second law to a real decision.
💡 Exam tip
Name losses specifically: uneaten material, egestion, excretion, heat from respiration. Vague “energy is lost” earns little.
State that respiration is the biggest loss if the question asks for the main reason.
Treat 10% as an approximation. Real data will give something else and that is fine.
Show the substitution line in efficiency calculations so method marks survive an arithmetic slip.
Watch the units: kJ m-2 yr-1 is an amount of energy per area per year, and it must stay consistent.
⚠ Common mix-up
Egestion and excretion confused. Egestion is undigested food; excretion is metabolic waste.
Thinking 10% is exact. It is a rough average across many ecosystems.
Saying energy is “used up”. It is transformed into heat and leaves the food chain.
Forgetting uneaten material. A lot of energy never enters the next level at all because nothing eats it.
Dividing the wrong way round. The higher level goes on top of the fraction.
Ignoring the decomposers. Faeces and dead bodies are not wasted for the ecosystem — they feed decomposers — but that energy has still left the food chain.
Up next: Biomass and Productivity — gross and net productivity, how biomass is actually measured, and what sustainable yield really means.
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