A farmer decides to grow wheat and sell it directly for human consumption rather than using it to feed cattle that are then eaten by humans. Explain why this change means the wheat can support a greater number of people.

Cambridge International IGCSE Biology (0610) — 19.1 Energy flow · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Energy is lost at each trophic level, so when wheat is eaten directly by humans it bypasses the cattle trophic level. This means there are fewer trophic steps and less energy is lost to respiration, movement, heat and excretion of the cattle. Consequently a larger proportion of the energy and biomass in the wheat is available to humans, allowing the farmer to support more people.

Examiner tips

  • Use the phrase ‘energy is lost at each trophic level’ to show understanding of the 10‑% rule.
  • Explain that removing the cattle removes one trophic level, so less energy is wasted.
  • Show that the energy that would have been lost in cattle respiration, movement, heat and excretion is now available to humans.
  • Keep the answer concise and use the exact terminology from the mark scheme.”]

Mark scheme (4 marks)

  1. Energy is lost at each trophic level as it passes along a food chain
  2. Eating wheat directly means fewer trophic levels / one fewer trophic level are involved
  3. More energy / biomass from the wheat is available to humans when cattle are removed from the food chain
  4. Energy is lost from cattle through processes such as respiration / movement / heat production / excretion, so this energy is no longer lost when cattle are removed

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