Explain how interspecific competition between two species occupying similar niches can affect the population sizes of both species over time.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Interspecific competition occurs when two species use the same limited resource(s) such as food, space or light.
The species that has a competitive advantage uses the resource more efficiently, so it can survive and reproduce at a higher rate.
The less competitive species has less access to the resource, so its population size falls.
If the competition is strong, the less competitive species may be excluded from the habitat entirely (competitive exclusion) while the dominant species’ population stabilises or increases.
The species that has a competitive advantage uses the resource more efficiently, so it can survive and reproduce at a higher rate.
The less competitive species has less access to the resource, so its population size falls.
If the competition is strong, the less competitive species may be excluded from the habitat entirely (competitive exclusion) while the dominant species’ population stabilises or increases.
Examiner tips
- Use the term "interspecific competition" first. Explain the resource limitation. Show the effect on each species. Mention competitive exclusion as an extreme outcome.
Common mistakes
- Confusing intraspecific with interspecific competition. Failing to state that the dominant species’ population can stabilise or increase. Omitting the resource type (food, space, light).
Mark scheme (4 marks)
- Interspecific competition occurs when two species compete for the same limited resource(s), such as food, space, or light.
- The species with a competitive advantage uses resources more efficiently, allowing it to survive and reproduce at a higher rate.
- The less competitive species experiences reduced resource availability, leading to a decline in its population size.
- In extreme cases, the less competitive species may be excluded from the habitat entirely (competitive exclusion), whereas the dominant species' population may stabilise or increase.
Key terms in this question
interspecific competition · niche · population size
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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