Explain how the presence of a keystone species maintains biodiversity within a community.

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).

Model answer (4 marks)

A keystone species has a disproportionately large effect on community structure relative to its biomass or abundance.
It typically controls the population size of a dominant competitor or prey species, preventing competitive exclusion.
This control allows multiple species that would otherwise be outcompeted to persist, increasing species richness and diversity.
If the keystone species is removed, a trophic cascade or community collapse occurs, the dominant species increases and biodiversity declines sharply.

Examiner tips

Use the term ‘keystone species’ first. Show the causal chain: control → prevents exclusion → more species. Mention the trophic cascade on removal. Keep each point concise and linked to the mark scheme.

Common mistakes

Confusing a keystone species with a keystone predator only. Failing to explain the role of competitive exclusion. Omitting the effect of removal (trophic cascade).

Mark scheme (4 marks)

A keystone species has a disproportionately large effect on community structure relative to its biomass or abundance.
The keystone species typically controls the population size of a dominant competitor or prey species, preventing competitive exclusion.
This allows multiple species that would otherwise be outcompeted to persist, increasing species richness/diversity.
Removal of the keystone species causes a trophic cascade or community collapse, with the dominant species increasing and biodiversity declining sharply.

Key terms in this question

keystone species · biodiversity