Explain why a woodland ecosystem with high biodiversity is likely to be more stable than a woodland ecosystem with low biodiversity, following the removal of a herbivore species.

OCR A-Level Biology A (H420) — 6.5 Ecosystems · Explain · 4 marks · View as Markdown

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

Two separate woodland ecosystems exist in the same region. Woodland A contains 47 plant species, 23 herbivore species, 12 predator species and numerous decomposer species. Woodland B contains 6 plant species, 4 herbivore species, 3 predator species and few decomposer species. In both woodlands, a single herbivore species is completely removed due to a novel disease.

Model answer (4 marks)

In the high‑biodiversity woodland, many herbivore species have similar feeding niches, so when one is removed the others can increase in number and continue to graze the plants, keeping the plant community in check. The predators in this woodland have many alternative prey species, so their populations are not forced to crash and the predator–prey dynamics remain stable. Because the plant community is diverse, no single plant species can dominate; the loss of one herbivore does not allow any plant to grow unchecked, so the overall plant biomass stays balanced. In contrast, the low‑biodiversity woodland has a very simple food web – only a few herbivores and predators – so the removal of one herbivore removes a key energy source for predators and allows the few remaining plant species to grow unchecked, causing large disturbances in population sizes and energy flow.

Examiner tips

  • Use the word ‘alternative’ to show species can replace the lost one
  • Show that predators have many prey options in the high‑biodiversity case
  • Explain that plant diversity prevents unchecked growth
  • Contrast the simple food web of the low‑biodiversity woodland

Common mistakes

  • Confusing biodiversity with species richness only
  • Failing to mention predator alternative prey
  • Using vague terms like ‘more stable’ without explanation

Mark scheme (4 marks)

  1. In the high-biodiversity woodland, other herbivore species can fulfil a similar ecological role to the removed species
  2. Predator species in the high-biodiversity woodland have alternative prey available, so predator populations are maintained
  3. Plant species in the high-biodiversity woodland are less likely to undergo unchecked population growth following removal of the herbivore
  4. In the low-biodiversity woodland the food web is simpler, so the loss of one species causes greater disruption to energy flow / population sizes throughout the ecosystem

Key terms in this question

biodiversity · herbivore · ecosystem

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