Explain how an invasive species can cause the competitive exclusion of a native species, and suggest one long-term consequence for ecosystem function.

IB DP Biology Higher 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).

Invasive species are non-native organisms introduced to a new environment, often in the absence of the predators and parasites that regulated their populations in their native range.

Model answer (4 marks)

An invasive species can occupy a niche that overlaps substantially with that of a native species, so they compete for the same limiting resources such as food, shelter or nesting sites. The invader usually has a competitive advantage – for example it lacks natural predators, tolerates a wider range of conditions or reproduces more quickly – which lets it capture a larger share of the resources. According to the competitive exclusion principle, the inferior competitor (the native species) is eventually driven locally to extinction or a very low population size. A long‑term consequence is the loss of the native species’ ecological role; for instance, if it was a pollinator or a keystone predator, its disappearance can destabilise food webs, reduce biodiversity or alter nutrient cycling.

Examiner tips

  • Use the word ‘competitive exclusion principle’ – it shows you know the theory. Show the chain: niche overlap → resource competition → advantage of invader → native species decline. Mention a specific ecological role for the long‑term consequence. Keep the answer to 4 clear points, each worth one mark.

Common mistakes

  • Mixing up ‘invasive species’ with ‘native species’ in the explanation. Failing to mention the competitive advantage of the invader. Giving a consequence that is not linked to the loss of the native species’ role.

Mark scheme (4 marks)

  1. The invasive species occupies a niche that overlaps substantially with that of the native species, leading to direct competition for the same limiting resources (e.g. food, shelter, nesting sites).
  2. The invasive species has a competitive advantage (e.g. absence of natural enemies, broader tolerance, faster reproduction), allowing it to acquire a greater share of resources, reducing the fitness and population size of the native species.
  3. According to the competitive exclusion principle, two species cannot indefinitely occupy the same niche; the inferior competitor (native species) is eventually eliminated locally or driven to extinction.
  4. Loss of the native species can disrupt ecosystem function; for example, if the native species was a pollinator, seed disperser, or keystone predator, its absence can destabilise food webs, reduce biodiversity, or alter nutrient cycling.

Key terms in this question

invasive species · ecosystem function

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