Explain how competitive exclusion can lead to character displacement in two closely related species occupying the same habitat.

IB DP Biology Higher Level (2023 syllabus) — B4.2 Ecological niches · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Competitive exclusion occurs when two species compete for the same limiting resource, with one species outcompeting the other. Where the niches of two species overlap significantly, individuals whose phenotype differs most from the competitor gain a selective advantage because they experience less competition. Over generations, natural selection drives the two populations to diverge in the traits associated with resource use (e.g. beak size, body size), a process called character displacement. Character displacement effectively partitions the resource/niche so that the two species can coexist, reducing the pressure of competitive exclusion.

Examiner tips

  • Use the term "competitive exclusion" first, then explain the selective advantage of differing phenotypes, mention natural selection and trait divergence, finish with coexistence.
  • Include specific examples of traits (beak size, body size) to show understanding of resource use.
  • Keep answer concise – 4 points, no extra explanation.

Common mistakes

  • Mixing up competitive exclusion with competitive advantage without stating the limiting resource.
  • Failing to mention natural selection or trait divergence as the mechanism.
  • Using vague terms like "differences" instead of specific traits such as beak size or body size.

Mark scheme (4 marks)

  1. Competitive exclusion occurs when two species compete for the same limiting resource, with one species outcompeting the other
  2. Where the niches of two species overlap significantly, individuals whose phenotype differs most from the competitor gain a selective advantage because they experience less competition
  3. Over generations, natural selection drives the two populations to diverge in the traits associated with resource use (e.g. beak size, body size), a process called character displacement
  4. Character displacement effectively partitions the resource/niche so that the two species can coexist, reducing the pressure of competitive exclusion

Key terms in this question

competitive exclusion · character displacement

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