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D4.2.5—Keystone species

Keystone species maintain community structure disproportionately, so their loss can trigger trophic cascades, biodiversity decline, population imbalance, and reduced ecosystem stability.

Syllabus
First assessment 2025
Objective
D4.2.5
Level
HL

Exam analysis

Chance of appearing5%of analysed past papers
Latest appearanceNovember 2025
Most common paperPaper3
Typical marks2–3

Common command terms

  • Outline
  • Suggest
  • Explain
  • Define

Scoring notes

Common mistake
Equating keystone species with the most abundant species or only with top predators.

Recent exam appearances

November 2025Paper2 ["HL"] · TZ37(b)(i)[ 2 ]D4.2.5—Keystone species
November 2023Paper3 ["HL"] · TZ216(d)[ 2 ]D4.2.5—Keystone species
May 2019Paper3 ["HL"] · TZ118(b)[ 3 ]D4.2.5—Keystone species
November 2018Paper3 ["HL"] · TZ017[ 6 ]D4.2.5—Keystone species
May 2018Paper3 ["HL"] · TZ114(a)[ 1 ]D4.2.5—Keystone species
Practice this objective

Coverage 2017–2025 · Updated 16 Jul 2026

Concept essentials

  • Keystone species have effects on community structure that are disproportionate to their abundance.
  • Removal of a keystone species can alter population sizes across several trophic levels.
  • Protecting keystone species can preserve biodiversity because many other species depend on their role.
ConceptIB Biology HL