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IB Biology HL D4.2.5 Keystone Species Questions

Analyse disproportionate keystone effects, top-down cascades and biodiversity changes in ecological communities.

Syllabus
First assessment 2025
Course
Biology HL
Level
HL

Exam points

  • Define or identify a keystone species by its disproportionate effect on community structure relative to abundance.
  • Explain top-down effects and trophic cascades when a keystone species is present, removed or protected, including changes in algae, populations and biodiversity.

IB Biology HL D4.2.5 Keystone Species Questions question 1

[Maximum number: 2]

The gut of a healthy human contains more than 1000 species of bacteria. Some of these species are shown in the cladogram.

Cladogram with species X branching beside a group containing M. luminyensis and paired M. smithii/M. stadmanae. A separate main branch contains E. faecalis and paired B. fragilis/B. stercosis.

Scientists have found that B. fragilis and B. stercosis are keystone species in the human gut ecosystem.

Define keystone species.

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