A grassland ecosystem contains grass, grasshoppers, frogs and herons arranged in a food chain. A scientist measures the biomass at each trophic level and finds that the biomass of grass is very much greater than the biomass of herons. Explain why the biomass of herons is so much lower than the biomass of grass.

Pearson Edexcel A-Level Biology A (9BI0) — 3.2 Energy flow and ecosystems · Explain · 5 marks · View as Markdown

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

In a grassland food chain: grass → grasshoppers → frogs → herons. The total biomass of grass measured across the ecosystem is approximately 10 000 kg. The total biomass of herons in the same ecosystem is approximately 10 kg.

Model answer (5 marks)

Energy is lost as it moves up the food chain. At each trophic level organisms use some of the energy for respiration and the rest is lost as heat. The remainder is lost in waste products such as faeces, urine and undigested material. Consequently only a small proportion of the energy or biomass is transferred to the next level – roughly 10 %. Because this loss occurs at each of the four trophic levels, by the time the energy reaches the herons very little biomass remains, so the heron biomass is much lower than that of the grass.

Examiner tips

  • Use the 10 % transfer rule to explain the drop in biomass; mention respiration and waste loss; keep answer concise and structured; include the final point about cumulative loss across four levels.

Common mistakes

  • Failing to mention respiration or waste loss; confusing biomass with energy; giving a generic statement without the 10 % figure; not linking the loss at each level to the final low heron biomass.

Mark scheme (5 marks)

  1. Energy is lost at each trophic level / as you move along the food chain
  2. Energy is used in respiration by organisms at each trophic level
  3. Energy is lost in waste products such as faeces / urine / undigested material
  4. Only a small proportion of energy / biomass is passed on to the next trophic level (approximately 10%)
  5. Because energy is lost at each of the four trophic levels, by the time energy reaches the herons very little biomass / energy remains, so herons have a much lower biomass than grass

Key terms in this question

biomass · trophic level · food chain

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