# 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

> 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.

## 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

- [interspecific competition](https://www.gradenine.co.uk/glossary/interspecific-competition)
- [competitive exclusion](https://www.gradenine.co.uk/glossary/competitive-exclusion)
- [niche differentiation](https://www.gradenine.co.uk/glossary/niche-differentiation)

## Related

- [Revision notes for IB DP Biology Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-interspecific-competition-between-two-c9aa06ce) · Published by Druglandscape Ltd.