Explain how the competitive exclusion principle relates to the concept of ecological niches.

IB DP Biology Standard 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).

Two species of flour beetle, Tribolium castaneum and Tribolium confusum, were cultured together in the same container with identical food and environmental conditions. Over time, one species was always eliminated from the culture.

Model answer (4 marks)

The competitive exclusion principle states that two species cannot occupy the same ecological niche indefinitely in the same habitat.

If two species have identical or nearly identical niches they compete for exactly the same resources.

One species will be a more efficient competitor – it will have a higher reproductive or survival rate – giving it a competitive advantage.

The inferior competitor is excluded (its population declines to zero), so in nature co‑existing species must occupy different niches or have niche differentiation.

Examiner tips

Use the phrase "competitive exclusion principle" first. Show the cause–effect chain: identical niches → competition → competitive advantage → exclusion. Mention that the surviving species has a higher reproductive or survival rate. End with the requirement for niche differentiation for coexistence.

Common mistakes

Mixing up the principle with the competitive release principle. Forgetting to state that the excluded species is locally extinct. Using vague terms like "different habitats" instead of "different niches".

Mark scheme (4 marks)

The competitive exclusion principle states that two species cannot occupy the same ecological niche indefinitely in the same habitat.
When two species have identical (or near-identical) niches, they compete for exactly the same resources.
One species will be a more efficient competitor and will have a higher reproductive rate / survival rate, giving it a competitive advantage.
The inferior competitor is excluded (locally extinct / population declines to zero), so in nature coexisting species must occupy different niches / have niche differentiation.

Key terms in this question

competitive exclusion principle · ecological niche