Explain how a keystone species influences the ecological niches available to other species within a community.

IB DP Biology Standard Level (2023 syllabus) — B4.2 Ecological niches · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

The sea otter (Enhydra lutris) feeds heavily on sea urchins in kelp forest ecosystems. In areas where sea otters have been removed, sea urchin populations increase dramatically and overgraze kelp, destroying the kelp forest habitat.

Model answer (4 marks)

A keystone species has a disproportionately large effect on community structure relative to its abundance.

By controlling the population of another species (e.g. sea otters keep sea urchins in check), it prevents competitive exclusion and allows a greater diversity of species to occupy niches.

The keystone species can create or maintain habitat or structural complexity, generating new or more diverse niches for other organisms (e.g. otters maintain kelp forests that provide habitats for many species).

When the keystone species is removed, dominant species expand, outcompete others and niche diversity decreases, reducing the number of realised niches in the community.

Examiner tips

Use the term "keystone species" and explain its disproportionate effect; link control of another species to prevention of competitive exclusion; mention habitat creation/maintenance; describe loss of niches when removed.
Include specific examples (sea otters, kelp forest) to show understanding of ecological concepts.

Common mistakes

Confusing a keystone species with a keystone predator; not mentioning the prevention of competitive exclusion; failing to explain how habitat complexity creates niches; omitting the effect of removal on niche diversity.

Mark scheme (4 marks)

A keystone species has a disproportionately large effect on community structure relative to its abundance / biomass.
By controlling populations of another species (e.g. sea otters controlling sea urchins), the keystone species prevents competitive exclusion / allows a greater diversity of species to occupy niches.
The keystone species can create or maintain habitat / structural complexity, thereby generating new or more diverse niches for other species (e.g. sea otters maintaining kelp forests which provide niches for many other organisms).
When the keystone species is removed, niche diversity decreases as dominant species expand and outcompete others, reducing the number of realised niches available in the community.

Key terms in this question

keystone species · ecological niche