Explain why energy transfer between trophic levels in an ecosystem is never 100% efficient.

OCR A-Level Biology A (H420) — 6.5 Ecosystems · Explain · 4 marks · View as Markdown

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

In a temperate grassland ecosystem, ecologists measured the energy content of producers and primary consumers. They found that only approximately 10% of the energy stored in the producers was incorporated into the biomass of the primary consumers.

Model answer (4 marks)

1. Energy is lost as heat during respiration by organisms at each trophic level.
2. Not all ingested material is assimilated; some is excreted as faeces or egested.
3. Energy is also lost through excretion of nitrogenous waste such as urea.
4. Not all biomass is consumed; some organisms die and are decomposed by decomposers rather than being eaten by the next trophic level.

Examiner tips

Use the exact wording from the mark scheme (e.g. "heat released during respiration", "faeces / egested", "excretion of nitrogenous waste", "decomposers")
Keep each point brief and separate with a number or bullet for clarity

Common mistakes

Mixing up energy transfer efficiency with trophic level number; forgetting to mention heat loss
Over‑generalising and saying all energy is lost, ignoring the specific mechanisms listed

Mark scheme (4 marks)

Some energy is lost through heat released during respiration by organisms at each trophic level
Not all ingested material is assimilated — some is lost in faeces / egested
Some energy is lost via excretion of nitrogenous waste products such as urea
Not all biomass at a trophic level is consumed / some organisms die and are broken down by decomposers rather than being eaten by the next trophic level

Key terms in this question

trophic levels · ecosystem