Explain how two species occupying the same habitat can coexist without competitive exclusion occurring.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Two warbler species, species A and species B, live in the same coniferous forest. Species A forages predominantly at the tips of upper branches, whilst species B forages mainly in the lower interior branches of the same trees.
Model answer (4 marks)
1. Each species occupies a distinct ecological niche, so direct competition for identical resources is avoided.
2. Resource partitioning means the two species use different parts of the resource – for example, foraging at different heights in the tree – reducing overlap in food or space.
3. The competitive exclusion principle states that two species cannot indefinitely occupy exactly the same niche; niche differentiation allows stable coexistence by preventing one species from outcompeting the other.
4. Niche differentiation can involve any combination of abiotic or biotic factors such as food type, feeding time, microhabitat use, or predator‑avoidance behaviour, allowing populations of both species to be maintained.
2. Resource partitioning means the two species use different parts of the resource – for example, foraging at different heights in the tree – reducing overlap in food or space.
3. The competitive exclusion principle states that two species cannot indefinitely occupy exactly the same niche; niche differentiation allows stable coexistence by preventing one species from outcompeting the other.
4. Niche differentiation can involve any combination of abiotic or biotic factors such as food type, feeding time, microhabitat use, or predator‑avoidance behaviour, allowing populations of both species to be maintained.
Examiner tips
- Use the word ‘explain’ to describe mechanisms, not just state facts. Mention both the niche concept and the competitive exclusion principle. Show how resource partitioning reduces competition. Include examples of abiotic/biotic factors that can differ.
Common mistakes
- Confusing ‘habitat’ with ‘niche’ – students say the species share the same habitat but ignore niche differences. Failing to mention the competitive exclusion principle. Giving only one factor (e.g., food) and ignoring other niche dimensions.
Mark scheme (4 marks)
- Each species occupies a distinct ecological niche, so direct competition for identical resources is avoided
- Resource partitioning means the two species use different parts of the resource (e.g. foraging at different heights / zones in the tree), reducing overlap in food or space
- The competitive exclusion principle states that two species cannot indefinitely occupy exactly the same niche; niche differentiation allows stable coexistence by preventing one species from outcompeting the other
- Niche differentiation can involve any combination of abiotic or biotic factors such as food type, feeding time, microhabitat use, or predator avoidance behaviour, allowing populations of both species to be maintained
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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