Explain why invasive alien species are considered one of the greatest threats to biodiversity, with reference to the ecological mechanisms by which they reduce native species populations.

IB DP Biology Higher 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 alien species are a major threat to biodiversity because they can grow unchecked in the new environment, as they lack natural predators, parasites or pathogens that would normally control their numbers. This allows their populations to increase rapidly and dominate.

They also outcompete native species for limiting resources such as food, space or light. By monopolising these resources, invasive species reduce the reproductive success and survival of native species.

Furthermore, many invasive species act as novel predators or herbivores. Native species have not evolved defences against these new predators, leading to disproportionate mortality and population decline.

Finally, invasive species can introduce new diseases or parasites to which natives have no immunity, or they can hybridise with closely related natives. This reduces genetic integrity and can ultimately lead to extinction of the native species.

Examiner tips

  • Use the four key mechanisms in separate sentences or bullet points; keep each point concise and directly linked to biodiversity loss.
  • Show cause–effect links: lack of predators → unchecked growth → dominance; competition → resource deprivation; novel predation → mortality; disease/hybridisation → genetic loss.
  • Use exam‑specific terms: ‘limiting resources’, ‘novel predators’, ‘genetic integrity’.
  • Avoid unnecessary detail; focus on mechanisms that reduce native populations.

Common mistakes

  • Mixing up causes and effects, e.g. saying invasive species ‘cause’ lack of predators rather than lack of predators enabling invasion.
  • Failing to mention all four mechanisms or combining them into one long sentence, losing clarity.
  • Using vague language such as ‘they hurt native species’ without specifying the ecological processes.

Mark scheme (4 marks)

  1. Invasive species often lack natural predators, parasites or pathogens in the new environment, allowing unchecked population growth.
  2. Invasive species outcompete native species for limiting resources such as food, space or light, reducing native species' reproductive success or survival.
  3. Invasive species can act as novel predators or herbivores, causing disproportionate mortality in native populations that have not evolved defences against them.
  4. Invasive species can introduce novel diseases or parasites to which native species have no immunity, or hybridise with closely related natives, reducing genetic integrity and potentially leading to extinction.

Key terms in this question

invasive alien species

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