Energy transfer in ecosystems — Edexcel GCSE Biology
How energy is lost at each trophic level, how that limits food chains and shapes pyramids of biomass, and how to calculate efficiency.
Revision notes
2 short notes, in the order of the specification. Each one in short:
At each trophic level, energy is lost because not all of an organism is eaten or digested, energy is released by respiration and transferred to the surroundings as heat, and some is lost in waste such as urine. So there are fewer organisms at each level, food chains are short, and pyramids of biomass narrow upwards.
Efficiency of transfer is the percentage of the energy or biomass at one trophic level that passes to the next. Efficiency (%) = energy or biomass transferred to the next level ÷ energy or biomass available at the previous level × 100. For example, 4 000 kJ ÷ 40 000 kJ × 100 = 10 %.
Exam questions
4 exam-style questions (8 marks), each with its mark scheme.
Answer the questionsFlashcards
6 cards: flip them, mark what you knew, and practise the rest.
Practise the cardsMind map
The whole of ecosystems and material cycles on one page, so you can see where this subtopic fits.
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Quick check questions
Give two ways energy is lost between trophic levels.
Show answer
Any two of: respiration (heat), movement, waste (faeces, urine), parts not eaten.
Why are food chains short?
Show answer
Energy is lost at each level, so too little remains to support more trophic levels.
What shape is a pyramid of biomass?
Show answer
Wide at the bottom, narrower at each higher trophic level.
State the equation for efficiency of transfer.
Show answer
Energy transferred to next level ÷ energy available at previous level × 100.
Frequently asked questions
Why is energy lost between trophic levels?
Energy is lost between trophic levels because not all of an organism is eaten, such as bones or roots, 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.
Why are food chains limited in length?
Food chains are limited in length because energy is lost at every trophic level, as heat from respiration, in movement, in waste and in uneaten or undigested parts. Less energy is available to each successive level, so there are fewer organisms. After a few levels there is too little energy left to support another trophic level.
What is a pyramid of biomass?
A pyramid of biomass is a diagram in which each bar shows the dry mass of living material at one trophic level, with the producers at the base. It is wide at the bottom and gets narrower at each level, because less energy and biomass is available to support each successive trophic level.
How do you calculate the efficiency of energy transfer?
Divide the energy or biomass transferred to the next trophic level by the energy or biomass available at the previous level, then multiply by 100. For example, if grass contains 40 000 kJ and cattle gain 4 000 kJ, the efficiency is 10 %. Always divide by the amount at the lower trophic level.
Written and checked against the Edexcel GCSE Biology (1BI0) specification · Updated October 2026