Why energy transfer is inefficientSpec 19.2.17, 19.2.18, 19.2.19
In short
Energy transfer between trophic levels is inefficient because not all of an organism is eaten, some food is lost as faeces, energy is lost in excretion, and respiration releases much energy as heat. So food chains usually have fewer than five trophic levels. Eating crop plants is more energy efficient than eating livestock, because there are fewer trophic levels.
Only a small fraction of the energy at one trophic level is transferred to the next. The transfer of energy from one trophic level to another is often not efficient, because energy is lost in these ways:
- Not all of an organism is eaten. Some parts, such as bones, roots and fur, are left uneaten.
- Some of what is eaten is not digested and is lost as faeces (egestion).
- Energy is lost in excretion, for example in urine.
- Energy is released by respiration to power life processes such as movement and, in mammals and birds, keeping warm. Much of this is lost as heat to the environment.
Efficiency of energy transfer
The grass in a field stores 20 000 kJ of energy per m² per year. The cattle that eat it gain 1 600 kJ per m² per year. Calculate the efficiency of energy transfer from the grass to the cattle.
- Efficiency = energy transferred ÷ energy available × 100
- = 1 600 ÷ 20 000 × 100
- = 8%
Answer: 8%
Food chains are short
Energy is lost at each trophic level, so less energy is available at each step. By the fourth or fifth trophic level there is too little energy left to support another population. This is why food chains usually have fewer than five trophic levels.
Eating plants or eating livestock
Humans can eat crop plants directly, or feed the crops to livestock and eat the livestock. It is more energy efficient to eat crop plants, because:
- there are fewer trophic levels in the food chain, so less energy is lost to the environment before it reaches humans;
- much of the energy in the crop fed to livestock is lost, for example in respiration, movement and keeping warm, and in faeces and urine, and is not stored in the meat or milk;
- so more people can be fed from the same area of land growing crops than from the same area used to grow food for livestock.
Do not say energy is 'lost' or 'used up' without saying how. Name the losses: heat from respiration, faeces, urine and parts not eaten.
Quick check
Define a trophic level.
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The position of an organism in a food chain, food web or ecological pyramid.
Which trophic level do tertiary consumers occupy?
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Trophic level 4.
State one advantage of a pyramid of biomass over a pyramid of numbers.
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It takes the size of the organisms into account, and is usually pyramid-shaped.
Supplement State one advantage of a pyramid of energy over a pyramid of biomass.
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It shows the rate of energy flow over time, and is always pyramid-shaped.
Supplement Why do food chains usually have fewer than five trophic levels?
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Energy is lost at each level, so too little remains to support more.
Written and checked against the Cambridge IGCSE Biology (0610) specification · Updated October 2026