A woodland ecosystem contains oak trees, caterpillars, blue tits and sparrowhawks. Explain how interdependence means that a sudden increase in the number of sparrowhawks could affect the populations of both blue tits and oak trees in this woodland ecosystem.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Sparrowhawks are predators that feed on blue tits. Blue tits feed almost entirely on caterpillars during spring and summer. Caterpillars feed on the leaves of oak trees.
Model answer (4 marks)
More sparrowhawks increase predation on blue tits, reducing the blue‑tit population.
Fewer blue tits mean less predation on caterpillars, so the caterpillar population rises.
An increase in caterpillars causes greater leaf consumption, damaging oak trees and reducing oak leaf biomass.
This chain shows interdependence: a change in one species’ numbers affects the others and the ecosystem’s balance.
Fewer blue tits mean less predation on caterpillars, so the caterpillar population rises.
An increase in caterpillars causes greater leaf consumption, damaging oak trees and reducing oak leaf biomass.
This chain shows interdependence: a change in one species’ numbers affects the others and the ecosystem’s balance.
Examiner tips
- Use the exact sequence of cause and effect; include all four points. Show the link between predator, prey and plant. Use terms like ‘predation’, ‘population’, ‘leaf biomass’. Keep sentences short and to the point.
Common mistakes
- Omitting the step that fewer blue tits lead to more caterpillars. Using vague wording such as ‘more birds’ instead of ‘more sparrowhawks’.
Mark scheme (4 marks)
- More sparrowhawks means more predation of blue tits, so the blue tit population decreases
- Fewer blue tits means fewer caterpillars are eaten, so the caterpillar population increases
- More caterpillars eat more oak leaves, so the oak trees are damaged / oak leaf biomass decreases
- This shows interdependence because a change in one species' population affects other species / the organisms in the ecosystem depend on each other to survive
Key terms in this question
Related
- Revision notes: Ecosystems →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Ecosystems questions
- Explain why decomposition is important for the recycling of materials in an ecos…
- Explain how both biotic and abiotic factors can affect the size of a population …
- Describe how the carbon cycle returns carbon dioxide to the atmosphere, referrin…
- A coral reef is an ecosystem where clownfish live among the tentacles of sea ane…