A marine food chain contains the following organisms: phytoplankton → krill → mackerel → dolphin. Describe what is meant by a trophic level and explain why the dolphin, as a tertiary consumer, has the least biomass available to it compared with organisms at lower trophic levels.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
A trophic level is the position of an organism in a food chain.
The dolphin is a tertiary consumer and therefore occupies trophic level 4.
At each trophic level only a small proportion of the biomass is transferred to the next level; the rest is lost as heat, waste, respiration and movement.
Because the dolphin is three levels above the primary producers, most of the original biomass has been lost, so only a very small amount of biomass remains for the dolphin.
The dolphin is a tertiary consumer and therefore occupies trophic level 4.
At each trophic level only a small proportion of the biomass is transferred to the next level; the rest is lost as heat, waste, respiration and movement.
Because the dolphin is three levels above the primary producers, most of the original biomass has been lost, so only a very small amount of biomass remains for the dolphin.
Examiner tips
- Define trophic level first, then state the dolphin’s level. Explain energy loss at each level before linking to the dolphin’s low biomass. Use terms: primary producer, primary consumer, secondary consumer, tertiary consumer, biomass, energy transfer.
- common_mistakes
- :
- Confusing trophic level with food chain order. Failing to mention energy loss mechanisms. Giving the dolphin a lower trophic level or not explaining why biomass is low.
Mark scheme (4 marks)
- A trophic level is the position of an organism in a food chain
- Biomass is lost at each trophic level / not all biomass is transferred to the next level
- Biomass is used for respiration / movement / heat loss / metabolic processes by organisms at each trophic level
- The dolphin is at trophic level 4 so biomass has been lost across three previous trophic levels, meaning very little biomass remains available
Key terms in this question
trophic level · biomass · tertiary consumer
Related
- Revision notes: Trophic levels in an ecosystem (Bio only) →
- How to answer a "Describe and Explain" question →
- Decode the mark scheme abbreviations →
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