Explain how the introduction of an invasive species can lead to a reduction in the biodiversity of a native ecosystem.

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).

Model answer (4 marks)

Invasive species often lack natural predators, parasites or diseases in the new environment, so their populations grow unchecked.
They compete with native species for food, space or light, reducing the fitness and population size of the natives.
They may also prey directly on native species that have no anti‑predator defences, causing local extinctions.
The loss of native species reduces species richness and destabilises ecosystem functioning, triggering further losses through disrupted food webs or trophic cascades.

Examiner tips

Use the exact terminology from the mark scheme – e.g. ‘species richness’, ‘trophic cascades’.
Show the causal chain: lack of predators → population growth → competition/prey → native decline → loss of biodiversity.
Keep each point brief and to the point – 1 mark per point.
Use bullet or numbered points if allowed to show structure.

Common mistakes

Mixing up ‘invasive species’ with ‘native species’ in the explanation.
Failing to mention the lack of natural enemies or the direct predation aspect.
Using vague terms like ‘more animals’ instead of ‘population growth’ or ‘species richness’.

Mark scheme (4 marks)

Invasive species often lack natural predators, parasites, or diseases in the new environment, allowing their populations to grow unchecked.
The invasive species competes with native species for resources such as food, space, or light, reducing the fitness and population size of native species.
Invasive species may prey directly on native species that have not evolved anti-predator defences against them, causing local extinctions.
Loss of native species reduces species richness/diversity, which can destabilise ecosystem functioning and trigger further losses through disrupted food webs or trophic cascades.

Key terms in this question

invasive species · biodiversity