Explain how the edge effect, resulting from habitat fragmentation, can reduce the biodiversity of a remaining habitat patch.

IB DP Biology Standard Level (2023 syllabus) — A4.2 Conservation of biodiversity · Explain · 4 marks · View as Markdown

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

Habitat fragmentation occurs when large, continuous areas of natural habitat are divided into smaller, isolated patches, typically due to human activities such as agriculture or urban development. The boundary between a habitat patch and the surrounding disturbed land is known as the edge.

Model answer (4 marks)

The edge of a habitat patch has different abiotic conditions from the interior, such as increased light penetration, higher temperatures, lower humidity, or greater wind exposure.
Species adapted to interior/core habitat conditions cannot survive in edge zones, so interior‑specialist species are lost or their populations decline.
In small, fragmented patches the edge‑to‑interior ratio is high, meaning a large proportion of the patch is edge habitat, reducing the effective area of suitable interior habitat.
Increased exposure to predators, invasive species, or human disturbance from the surrounding matrix penetrates deeper into the patch via the edge, further reducing species richness.

Examiner tips

Use the exact terms ‘edge effect’, ‘interior‑specialist species’, ‘edge‑to‑interior ratio’ and ‘matrix’ as in the mark scheme.
Show the causal chain: edge abiotic change → loss of interior species → reduced interior area → more disturbance penetration.
Keep each point brief but complete – 4 marks, one point each.

Common mistakes

Mixing up edge and interior conditions, or saying the edge is the same as the interior.
Failing to mention the increased edge‑to‑interior ratio in small patches.
Using vague language such as ‘more light’ without specifying the effect on species or area.

Mark scheme (4 marks)

The edge of a habitat patch has different abiotic conditions from the interior, such as increased light penetration, higher temperatures, lower humidity, or greater wind exposure.
Species adapted to interior/core habitat conditions cannot survive in edge zones, so interior-specialist species are lost or their populations decline.
In small, fragmented patches, the edge-to-interior ratio is high, meaning a large proportion of the patch is edge habitat, reducing the effective area of suitable interior habitat.
Increased exposure to predators, invasive species, or human disturbance from the surrounding matrix penetrates deeper into the patch via the edge, further reducing species richness.

Key terms in this question

edge effect · habitat fragmentation