Feeding relationships — Edexcel International GCSE Science (Double Award)
Trophic levels, food chains and webs, pyramids of number, biomass and energy, and why only about 10% of energy passes to the next level.
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. Producers, such as plants and algae, are level 1, primary consumers (herbivores) are level 2, secondary consumers are level 3 and tertiary consumers are level 4. Decomposers, which are bacteria and fungi, secrete enzymes onto dead matter and waste, and the small soluble molecules diffuse in.
A food chain shows the feeding relationships in a community. It always begins with a producer, and the arrows show the direction food and energy pass, from the food to the organism that eats it. A food web shows how the food chains in a habitat are linked, so one organism can feed at more than one trophic level.
Ecological pyramids use bars to compare the trophic levels in a food chain, with producers at the bottom. A pyramid of number counts organisms and can be inverted, for example one oak tree supporting many caterpillars. A pyramid of biomass shows the mass of living material and almost always narrows, while a pyramid of energy transfer always narrows upwards.
Only about 10% of the energy at one trophic level is transferred to the next. The rest is lost: not all of the organism is eaten, some food is not digested and is lost in faeces, energy released by respiration is transferred to the surroundings as heat, and some is lost in urine. So food chains are limited in length.
Exam questions
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Quick check questions
What number trophic level are producers?
Show answer
Level 1.
Name the organisms that break down dead material.
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Decomposers.
What do the arrows in a food chain show?
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The direction food and energy pass, from the food to the organism that eats it.
What percentage of energy is transferred to the next trophic level?
Show answer
About 10%.
Why are food chains limited in length?
Show answer
Too little energy is left to support more trophic levels.
Frequently asked questions
Why is energy lost between trophic levels?
Energy is lost between trophic levels because not all of an organism is eaten, and some of what is eaten is not digested and is lost in faeces. Energy is also released by respiration and transferred to the surroundings as heat, used for movement and keeping warm, and lost in waste such as urine.
What is a pyramid of biomass?
A pyramid of biomass is a diagram whose bars show the mass of living material at each trophic level, with producers at the bottom. It almost always narrows at each higher level, because energy and biomass are lost at every step. Unlike a pyramid of number, it accounts for size, so one large oak tree gives a wide base.
Why are food chains short?
Food chains are short, usually four or five trophic levels, because only about 10% of the energy is transferred from one level to the next. The rest is lost as heat from respiration, in faeces and in waste. After a few levels, too little energy is left to support another population of organisms.
Why can a pyramid of number be inverted?
A pyramid of number counts the organisms at each trophic level but ignores their size. When one large producer supports many small consumers, such as one oak tree with thousands of caterpillars, the bottom bar is narrower than the one above. Parasites, like hundreds of fleas on one fox, can invert it too.
How do you calculate the percentage of energy transferred in a food chain?
Divide the energy transferred to the next trophic level by the energy available at the previous level, then multiply by 100. For example, if grass stores 20 000 kJ and the rabbits that eat it gain 1800 kJ, the percentage transferred is 1800 ÷ 20 000 × 100 = 9%.
Written and checked against the Edexcel IGCSE Science Double Award (4SD0) specification · Updated October 2026