Explain how the introduction of an invasive species into a new community can affect the population size of a native species occupying a similar niche.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The invasive species competes with the native species for the same or overlapping resources such as food, shelter or nesting sites because they occupy similar ecological niches.
The invasive species is usually free from the natural predators, parasites or pathogens that keep the native species in check, giving it a competitive advantage.
This increased competition reduces the availability of resources for the native species, which lowers its birth rate and/or raises its death rate.
Consequently the native species’ population size falls, and if competitive exclusion is complete it may be locally eliminated or driven to extinction in that community.
The invasive species is usually free from the natural predators, parasites or pathogens that keep the native species in check, giving it a competitive advantage.
This increased competition reduces the availability of resources for the native species, which lowers its birth rate and/or raises its death rate.
Consequently the native species’ population size falls, and if competitive exclusion is complete it may be locally eliminated or driven to extinction in that community.
Examiner tips
- Use the word ‘compete’ and mention resources; link to lack of predators; explain birth/death rate effects; finish with population decline or extinction.
- Show clear causal chain: niche overlap → competition → resource limitation → demographic change → population decline.
Common mistakes
- Confusing ‘predators’ with ‘competitors’; not linking resource limitation to birth/death rates; omitting the competitive advantage of the invader; using vague terms like ‘more animals’ instead of specific demographic effects.
Mark scheme (4 marks)
- The invasive and native species compete for the same or overlapping resources (e.g. food, shelter, nesting sites) because they occupy similar niches.
- The invasive species is not limited by the natural predators, parasites, or pathogens present in the native community, giving it a competitive advantage.
- Increased competition reduces the availability of resources for the native species, leading to a decrease in its birth rate and/or an increase in its death rate.
- As a result, the native species population size declines and, if the competitive exclusion is complete, the native species may be locally eliminated / driven to extinction in that community.
Key terms in this question
invasive species · niche · native species
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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