A pond ecosystem contains the following organisms: phytoplankton, water fleas, small fish, and pike. State the trophic level of each organism and describe what would happen to the population of small fish if pike were completely removed from the pond by over-harvesting.

Cambridge International IGCSE Biology (0610) — 19.2 Food chains and food webs · Describe · 4 marks · View as Markdown

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

Model answer (4 marks)

Phytoplankton are producers – trophic level 1.
Water fleas are primary consumers – trophic level 2.
Small fish are secondary consumers – trophic level 3.
Pike are tertiary consumers – trophic level 4.
If pike are removed, the population of small fish would increase because the predation pressure on them is removed, so fewer small fish are killed.

Examiner tips

  • State each trophic level clearly; use the exact terms 'producers', 'primary consumer', etc.
  • Explain the cause–effect link: removal of predator → less predation → population growth.
  • Use concise sentences to fit the 4‑mark limit.

Common mistakes

  • Mixing up trophic levels (e.g. calling water fleas producers).
  • Failing to explain why the small fish population rises (missing predation link).

Mark scheme (4 marks)

  1. Phytoplankton are producers / at trophic level one
  2. Water fleas are primary consumers / trophic level two; small fish are secondary consumers / trophic level three; pike are tertiary consumers / trophic level four
  3. The population of small fish would increase (after pike are removed)
  4. Because pike are no longer predating / eating the small fish, so fewer small fish are killed / there is less predation pressure on small fish

Key terms in this question

trophic level · over-harvesting

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