Nothing like 100% of a trophic level’s energy reaches the level above it. There are four separate leaks, they happen in a fixed order, and if you can name all four in that order you can answer almost any question on this topic.
📘 What you need to know
The organic matter transferred between trophic levels is never 100%.
Not all the food available is harvested.
Of what is harvested, not all is consumed.
Of what is consumed, not all is absorbed.
Of what is absorbed, not all is stored.
The biggest single loss is heat from respiration.
The four leaks, in order
Follow a caterpillar eating the leaves of an oak tree and each leak becomes obvious.
🧩 From leaf to caterpillar biomass
Not all the food is harvested. The caterpillars do not reach every leaf — there may be too many leaves, too few caterpillars, or leaves in places they cannot get to.
Not all of what is harvested is eaten. They may eat only the softer, more nutritious parts and leave tough sections or parts containing toxins.
Not all of what is eaten is absorbed. Some of the leaf is indigestible, so it passes through and is egested as faeces.
Not all of what is absorbed is stored. Much of the energy is released in respiration and lost as heat rather than being built into new tissue.
Each red arrow is energy leaving the food chain. Only the green box on the right is available to the next trophic level.
A quick memory hook: reached, eaten, absorbed, stored. Four words, four losses, in the order the food actually travels.
Which loss is biggest?
Respiration. Every organism has to keep itself alive — moving, breathing, transporting substances, staying warm if it is a bird or mammal. All of that costs energy that is released as heat and never becomes body tissue.
That is why warm-blooded animals are especially inefficient. Maintaining a constant body temperature is expensive, so a smaller fraction of what a mammal eats ends up as new biomass compared with, say, an insect.
Egestion or excretion?Egestion is undigested material leaving as faeces — it was never absorbed. Excretion is waste from metabolism, such as urea in urine — it was absorbed and then used. Examiners notice when students swap these.
Why it limits food chain length
Because roughly nine tenths leaks away at each step, the energy remaining shrinks fast. After four or five transfers there is simply not enough left to support another population of predators, which is why most terrestrial ecosystems have no more than five trophic levels.
WORKED EXAMPLE
Rabbits in a field have access to 46 000 kJ m−2 yr−1 of grass. They eat 11 000, egest 3 400 in faeces, and lose 6 800 as heat from respiration. Calculate the energy stored as rabbit biomass.
Step 1: Start from what was eaten, not what was available11 000 kJ enters the rabbitsStep 2: Add up the losses after eating3 400 + 6 800 = 10 200 kJStep 3: Subtract11 000 − 10 200 = 800800 kJ m−² yr−¹ storedThe 46 000 figure is a distractor – most of it was never eaten
WORKED EXAMPLE
Explain why a food chain based on cattle supports fewer people than one based on wheat, using the same area of land.
Step 1: Count the trophic levelswheat → human is one transferwheat → cattle → human is two transfersStep 2: Apply the lossesEach extra transfer loses roughly 90% as heat, faeces and uneaten materialStep 3: State the outcomeFar less energy reaches people through the longer chainThis is the energy argument for plant-based diets
💡 Exam tip
Give all four reasons when a question asks why transfer is inefficient. One reason rarely earns full marks.
Put heat from respiration first — it is the largest loss and the one examiners look for.
Read the numbers carefully. “Available” and “eaten” are different starting points.
Use egestion for faeces and excretion for urine and urea.
Link back to food chain length if the question asks about consequences.
⚠ Common mix-up
Saying energy is “destroyed”. It leaves the chain as heat — the first law still holds.
Only mentioning respiration. Uneaten and undigested material matter too.
Starting the calculation from food available. If the question gives what was eaten, use that.
Confusing egestion with excretion. One never entered the body’s tissues; the other did.
Treating 10% as a law. It is a rough average and varies a lot between ecosystems.
Up next: Biomass and Productivity — how we actually measure the energy stored at each level, and what gross and net productivity mean.
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