B4.2.13—Competitive exclusion
Competitive exclusion occurs when species with strongly overlapping niches compete for limiting resources until one is excluded locally from the niche.
- Syllabus
- First assessment 2025
- Objective
- B4.2.13
- Level
- SL
Exam analysis
Chance of appearing5%of analysed past papers
Latest appearanceMay 2025
Most common paperPaper3
Typical marks2–3
Common command terms
- Explain
- Outline
- Describe
Scoring notes
- Common mistake
- Saying competitive exclusion always means worldwide extinction rather than local exclusion from a niche.
Recent exam appearances
May 2025Paper1A ["SL"] · TZ227[ 1 ]B4.2.13—Competitive exclusion
November 2020Paper3 ["SL"] · TZ012(b)[ 2 ]B4.2.13—Competitive exclusion
May 2019Paper3 ["SL"] · TZ114[ 4 ]B4.2.13—Competitive exclusion
May 2018Paper3 ["SL"] · TZ113(b)[ 2 ]B4.2.13—Competitive exclusion
November 2016Paper3 ["SL"] · TZ011(c)[ 3 ]B4.2.13—Competitive exclusion
Practice this objective
Coverage 2012–2025 · Updated 15 Jul 2026
Competitive Exclusion Limits Complete Niche Overlap
Competitive exclusion occurs when species with very similar niches compete for the same limiting resource strongly enough that stable complete overlap cannot persist.
One possible outcome is elimination of the weaker competitor from the habitat. Another is restriction of both species to different parts of their fundamental niches through resource, spatial or temporal partitioning.
Gause's mixed cultures illustrate the principle: Paramecium aurelia outcompeted P. caudatum under shared conditions, whereas reduced niche overlap can allow coexistence.
If two bird species use the same limiting prey in the same place and time, one may decline; feeding at different heights can reduce overlap and restrict each realized niche rather than eliminate either.
Coexistence does not prove absence of competition, and competitive exclusion requires a limiting shared resource—not merely two species living in the same habitat.
Competitive exclusion
Assessment in practice
2–3 marksHow it is assessed
This objective is assessed through multiple choice, structured response, commonly using Explain / Outline / Describe.
Command terms
Explain / Outline / Describe
What earns marks
Build the answer around this relationship: Two species cannot occupy identical niches indefinitely when resources are limiting.
Watch for
Saying competitive exclusion always means worldwide extinction rather than local exclusion from a niche.
Representative question
Question 1
Paramecium aurelia and Paramecium caudatum are two species of paramecium that grow well individually. Scientists grew these two species of paramecium together, and the result is shown in the graph.
What could be deduced from this data?
A
Predation results in P. caudatum consuming P. aurelia.
B
Allelopathy results in P. caudatum outcompeting P. aurelia.
C
Interspecific competition results in P. aurelia outcompeting P. caudatum.
D
Intraspecific competition results in P. aurelia eliminating P. caudatum.
Niches, Nutrition, And Competition
A niche is the role of a species in a community, including habitat, abiotic tolerances, nutrition, activity, reproduction, and interactions. The evidence should match the question: nutrition mode, oxygen tolerance, feeding adaptation, predator-prey interaction, plant light strategy, fundamental versus realized niche, or competitive exclusion.
- Definition answers should include role plus habitat/resources/interactions, not only place.
- Nutrition modes explain how organisms obtain carbon, energy, or organic matter.
- Adaptation examples should be linked to feeding, defence, light harvesting, or competition.
- Fundamental versus realized niche and competitive exclusion are tested together because interactions narrow where species actually live.
Concept essentials
- Two species cannot occupy identical niches indefinitely when resources are limiting.
- Greater niche overlap increases interspecific competition.
- The better competitor may exclude the other species locally.
- Paramecium mixed-culture data can demonstrate competitive exclusion.