Explain why the removal of a top predator from an ecosystem can lead to a reduction in the overall biodiversity of that ecosystem.

OCR A-Level Biology A (H420) — 6.6 Populations and sustainability · Explain · 4 marks · View as Markdown

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

In some coastal ecosystems, sea otters are the only significant predator of sea urchins. When sea otter populations declined sharply due to hunting, sea urchin populations increased dramatically. Sea urchins graze heavily on kelp, a large brown alga that forms complex underwater forests providing habitat for hundreds of species.

Model answer (4 marks)

Removal of the top predator allows its prey population to increase without regulation.
The unregulated prey over‑exploits a key resource or habitat‑forming species.
Loss of the habitat‑forming species reduces the number of ecological niches available.
Fewer niches and resources mean fewer species can survive, so species richness and biodiversity decrease.

Examiner tips

Use the exact sequence of points from the mark scheme; each point counts one mark.
Show cause‑effect links clearly; avoid vague wording.
Mention the specific example (sea otter–sea urchin–kelp) to demonstrate understanding.

Common mistakes

Failing to explain that the prey population grows unchecked; only stating that predators are removed.
Confusing biodiversity with population size; not linking to loss of niches.
Using generic terms like ‘more animals’ without specifying the prey species or the habitat impact.

Mark scheme (4 marks)

Removal of the top predator allows its prey population to increase without regulation
The unregulated prey population over-exploits a key resource / habitat-forming species
Loss of the habitat-forming species reduces the number of ecological niches available
Fewer niches / resources leads to fewer species being able to survive, so species richness / biodiversity decreases

Key terms in this question

top predator · biodiversity · ecosystem