Explain how keystone species influence the structure of an ecological community, including the effect of their removal on species diversity.
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 abundance or biomass.
It regulates populations of other species (for example, by preying on a dominant competitor or controlling a prey population), thereby maintaining niche diversity and preventing competitive exclusion.
If the keystone species is removed, one or a few species become dominant, causing a trophic cascade that reduces species diversity and collapses the community structure.
Thus the keystone species defines the range of realised niches available to other species, and its presence maintains niche heterogeneity.
It regulates populations of other species (for example, by preying on a dominant competitor or controlling a prey population), thereby maintaining niche diversity and preventing competitive exclusion.
If the keystone species is removed, one or a few species become dominant, causing a trophic cascade that reduces species diversity and collapses the community structure.
Thus the keystone species defines the range of realised niches available to other species, and its presence maintains niche heterogeneity.
Examiner tips
- Use the phrase "disproportionately large effect" to show understanding of the definition.
- Explain the regulatory role (prey/competitor control) and link it to niche diversity.
- Describe the outcome of removal (dominance, trophic cascade, diversity loss).
- Mention that the keystone species maintains realised niche heterogeneity.
Common mistakes
- Confusing a keystone species with a keystone predator only; ignore other roles.
- Failing to link removal to a trophic cascade or diversity collapse.
- Using vague terms like "important" without specifying disproportionate effect.
Mark scheme (4 marks)
- A keystone species has a disproportionately large effect on community structure relative to its abundance or biomass.
- Keystone species regulate populations of other species (e.g. prey or competitors), thereby maintaining niche diversity and preventing competitive exclusion.
- Removal of the keystone species leads to dominance by one or a few species, reducing species diversity (trophic cascade / diversity collapse).
- The keystone species therefore defines the range of realised niches available to other species in the community; its presence maintains niche heterogeneity.
Key terms in this question
keystone species · species diversity
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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