A food web in a grassland ecosystem contains the following organisms: grass, grasshoppers, frogs, snakes, and hawks. Describe how a sudden large decrease in the number of frogs would affect the populations of grasshoppers and snakes. Give reasons for your answer.

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)

The number of grasshoppers would increase because frogs are consumers of grasshoppers – fewer frogs means less predation of grasshoppers.
The number of snakes would decrease because frogs are a food source for snakes – fewer frogs means less food available for snakes.

Examiner tips

  • Use the word ‘increase’ or ‘decrease’ to show the change in population. Explain the cause in one sentence – link frogs to grasshoppers and frogs to snakes. Keep the answer concise – 4 marks only.
  • common_mistakes
  • :
  • Writing a long explanation or extra detail. Confusing the direction of the effect (e.g. saying snakes increase). Using vague terms like ‘more food’ without specifying the predator–prey link.

Mark scheme (4 marks)

  1. The number of grasshoppers would increase
  2. Because frogs are consumers of grasshoppers / frogs eat grasshoppers, so fewer frogs means less predation of grasshoppers
  3. The number of snakes would decrease
  4. Because frogs are a food source for snakes / snakes eat frogs, so fewer frogs means less food available for snakes

Key terms in this question

food web

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