Explain why only around 10% of the energy available at one trophic level is transferred to the next trophic level in a food chain.

Pearson Edexcel International GCSE Biology (4BI1) — 4.2 Feeding relationships · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

In a grassland ecosystem, grass is eaten by rabbits, which are in turn eaten by foxes. At each stage of this food chain, a large proportion of the energy is lost and is not available to the next consumer.

Model answer (4 marks)

Energy is lost through respiration and other metabolic processes, producing heat that leaves the organism. Energy is also lost in excretion and waste products such as urine and faeces. Some of the energy is used for movement and other physical activity. Consequently only the energy stored in the organism’s biomass is available to the next trophic level.

Examiner tips

Use the word ‘lost’ and mention respiration, excretion and movement
Show the 10% rule as a consequence of the losses
Keep the answer concise and use the exact terminology

Common mistakes

Confusing energy transfer with energy consumption
Failing to mention heat loss
Using vague terms like ‘energy waste’ without specifying respiration, excretion or movement

Mark scheme (4 marks)

Energy is lost through respiration / used for metabolic processes / heat loss to the environment
Energy is lost in excretion / waste products such as urine or faeces
Energy is used for movement / locomotion / physical activity
Therefore only the energy stored in the biomass of the organism is available to the next trophic level

Key terms in this question

trophic level · food chain