Explain how interspecific competition between two species occupying similar niches can lead to competitive exclusion or niche differentiation.

IB DP Biology Higher Level (2023 syllabus) — C4.1 Populations and communities · Explain · 4 marks · View as Markdown

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

Two species of barnacle, Chthamalus stellatus and Semibalanus balanoides, occupy rocky intertidal zones. When grown separately, each species can survive across the full range of tidal heights. When grown together, Semibalanus displaces Chthamalus to the upper intertidal zone only.

Model answer (4 marks)

Interspecific competition occurs when two species use the same limiting resource, such as space, food or light, within a shared habitat.

The competitively superior species obtains more of the resource, reproduces more successfully and its population grows, while the inferior species is reduced in abundance and may be driven to local extinction – this is competitive exclusion (Gause’s principle).

Alternatively, natural selection can favour individuals that exploit slightly different resources or micro‑habitats. The realised niche of each species becomes narrower than its fundamental niche – this is niche differentiation or character displacement.

In the barnacle example, Semibalanus outcompetes Chthamalus in the lower intertidal zone, forcing Chthamalus to the upper zone. In the upper zone Chthamalus has a competitive advantage because it tolerates desiccation better, so the two species coexist through niche differentiation.

Examiner tips

  • Use the exact phrases "interspecific competition", "competitive exclusion" and "niche differentiation". Show the cause–effect chain: resource competition → superior species gains resources → inferior species declines. Include the specific barnacle example to demonstrate application.
  • common_mistakes
  • :
  • Confusing competitive exclusion with competitive advantage without mentioning local extinction. Using vague terms like "competition" without specifying limiting resources. Failing to link the barnacle example back to the general concepts.

Mark scheme (4 marks)

  1. Interspecific competition occurs when two species compete for the same limiting resources (e.g. food, space, light) within a shared habitat.
  2. The competitively superior species gains more resources, reproduces more successfully, and its population grows at the expense of the inferior species, which may be driven to local extinction — this is competitive exclusion (Gause's principle).
  3. Alternatively, natural selection may favour individuals of each species that exploit slightly different resources or microhabitats, causing the realised niche of each species to become narrower than its fundamental niche — this is niche differentiation (character displacement).
  4. In the barnacle example, Semibalanus outcompetes Chthamalus in the lower intertidal zone (competitive exclusion from that zone), while Chthamalus persists in the upper zone where desiccation stress gives it a competitive advantage — demonstrating niche differentiation leading to coexistence.

Key terms in this question

interspecific competition · competitive exclusion · niche differentiation

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