Trophic levels and ecological pyramids — Cambridge IGCSE Biology
Trophic levels, pyramids of numbers, biomass and energy, and why energy transfer is inefficient so that food chains are short.
Revision notes
4 short notes, in the order of the specification. Each one in short:
A trophic level is the position of an organism in a food chain, food web or ecological pyramid. Producers are trophic level 1, primary consumers level 2, secondary consumers level 3, tertiary consumers level 4 and quaternary consumers level 5. An organism in a food web can be at more than one trophic level, depending on what it eats.
A pyramid of numbers shows the number of organisms at each trophic level, and a pyramid of biomass shows the mass of living material at each level. A pyramid of numbers can be inverted, because one large tree can support many caterpillars. A pyramid of biomass is usually better, because it takes the size of the organisms into account.
Pyramids of energy
SupplementA pyramid of energy shows the amount of energy that flows through each trophic level in a given area over a period of time, for example in kJ per m² per year. It is always pyramid-shaped, because energy is lost at each trophic level. It shows the rate of energy flow and allows transfer efficiency to be compared.
Why energy transfer is inefficient
SupplementEnergy 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.
Exam questions
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Quick check questions
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.
Frequently asked questions
Why is energy lost between trophic levels?
Supplement Energy is lost between trophic levels because not all of an organism is eaten, such as bones and roots, and some of what is eaten is not digested and is lost in faeces. Energy is also lost in excretion, such as urine. Respiration releases energy for movement and keeping warm, and much is lost as heat.
What is a pyramid of biomass?
A pyramid of biomass shows the mass of living material at each trophic level, for example in g per m², with the producers at the bottom and bars drawn to scale. It is usually pyramid-shaped, because there is less biomass at each higher level, and it is better than a pyramid of numbers because it accounts for organism size.
Why are food chains short?
Supplement Food chains are short because energy is lost at each trophic level, so less energy is available at each step. Energy is lost as heat from respiration, in faeces and urine, and in parts that are not eaten. By the fourth or fifth trophic level, there is too little energy left to support another population.
Why can a pyramid of numbers be inverted?
A pyramid of numbers can be inverted because it ignores the size of the organisms. One large oak tree can support hundreds of caterpillars, so the bar for the producer is shorter than the bar above it. A pyramid of biomass takes size into account, so it is usually pyramid-shaped.
Why is it more efficient to eat plants than meat?
Supplement Eating crop plants is more energy efficient because there are fewer trophic levels, so less energy is lost before it reaches humans. Much of the energy in crops fed to livestock is lost in respiration, movement, keeping warm, faeces and urine, so it is not stored in meat or milk. More people can be fed from the same land.
Written and checked against the Cambridge IGCSE Biology (0610) specification · Updated October 2026