Species compete when their ecological niches overlap and they require the same resource at the same place and time, provided that the resource is limited. Each species reduces the amount available to the other, potentially lowering survival, growth, or reproductive success. This relationship is called interspecific competition.
If two species use all limiting resources in essentially the same way under stable conditions, the competitive exclusion principle predicts that they cannot coexist indefinitely. One species may exclude the other locally, or competition may restrict one or both species to narrower realized niches. This article explains why species compete for a niche, how competition changes ecological communities, and how to express the principle accurately in an IB Biology exam.
What does it mean to occupy the same niche?
An ecological niche describes how a species exists within an ecosystem. It includes the resources the species uses, the abiotic conditions it tolerates, its location and activity patterns, and its interactions with other organisms. A niche is therefore much more than the physical place where an organism lives.
A habitat answers the question, “Where does the species live?” A niche also addresses how it survives and reproduces there. Several species can share a forest habitat while occupying different niches because they eat different foods, forage at different heights, or are active at different times.
Important niche dimensions include:
- food type and feeding method
- access to water, mineral ions, light, oxygen, or nesting space
- temperature, salinity, moisture, and pH tolerance
- the area or vertical layer of a habitat used
- seasonal or daily activity patterns
- predators, prey, parasites, mutualists, and competitors
- conditions required for mating and reproduction
Two species do not need identical niches to compete. Competition can occur wherever their niches partly overlap, but it is generally stronger when the overlap involves an important limiting resource.
For wider syllabus context, use RevisionDojo’s IB Biology Ecology Explained resources and the focused B4.2 Ecological Niches topic page. This article concentrates specifically on niche overlap and competitive exclusion rather than duplicating the full ecology topic.
Why species compete for the same niche
Species do not deliberately choose to compete. Niche overlap develops because organisms living in the same environment may have similar adaptations and biological requirements. Two seed-eating birds, for example, may both depend on seeds of the same size because their beaks and digestive systems are suited to that food.
Competition occurs when three conditions are present:
- The species use the same resource. Examples include food, water, light, territory, nesting sites, or mineral nutrients.
- The resource is limited. If enough is available for every individual, sharing it may have little measurable effect.
- Use by one species reduces access or success for the other. The effect may be caused by resource depletion or direct prevention of access.
This final point is important. Mere overlap in distribution or diet does not automatically prove competition. If food is temporarily abundant, two species may consume the same food without either experiencing a meaningful reduction in fitness.
Competition is commonly described as a negative-negative interaction, written as −/−. Both species would, in principle, grow, survive, or reproduce more successfully if the competitor were absent. The effects do not need to be equal, however, and one species may be affected much more severely than the other.
Why limitation changes the interaction
Suppose two herbivore species feed on the same plant. If plant biomass is renewed faster than the herbivores consume it, food may not limit either population. If drought reduces plant growth, the same dietary overlap can produce intense competition because every unit consumed by one species leaves less for the other.
Limiting factors can therefore change across seasons and locations. Competition may be strong in a dry season but weak in a wet season, or intense in one part of a habitat but negligible elsewhere. This helps explain why natural competition is more dynamic than a simple statement that two species “eat the same thing.”
Niche overlap and the two forms of competition
Niche overlap is the extent to which different species use the same resources or environmental conditions. Greater overlap creates a higher potential for competition, especially if the shared resource controls population growth.
Ecologists distinguish two main mechanisms:
| Type of competition | Mechanism | Example |
|---|---|---|
| Exploitative competition | Species indirectly affect each other by consuming or depleting a shared resource | Two plant species absorb nitrate ions from the same soil |
| Interference competition | One species directly blocks, attacks, displaces, or otherwise prevents another from accessing a resource | Territorial birds drive another species away from feeding sites |
In exploitative competition, the organisms may never meet. A nocturnal animal can reduce food available to a diurnal species by consuming the resource earlier. In interference competition, direct behavior or physical contact affects access.
The distinction may appear in data-based questions. A graph showing declining food followed by reduced population growth suggests exploitation, while observations of aggression or physical displacement suggest interference. Both mechanisms can operate simultaneously.
The competitive exclusion principle
The competitive exclusion principle, also called Gause’s principle, states that two species occupying the same niche and competing for the same limiting resources cannot coexist indefinitely under stable conditions. If one species has a consistent competitive advantage, its population will increase relative to the other until the weaker competitor is excluded from that niche.
“Excluded” should be interpreted carefully. It usually means local elimination or displacement, not the global extinction of the entire species. The excluded population might survive elsewhere, migrate, or use a different part of its fundamental niche.
The principle is most accurate when its conditions are included:
- the species have very high or complete niche overlap
- they depend on the same limiting resource
- environmental conditions remain sufficiently stable
- one species has a consistent competitive advantage
- no process produces enough niche differentiation for stable coexistence
This is why the statement “two species can never share a niche” is too absolute. Natural environments fluctuate, niches are multidimensional, and species commonly differ in at least one aspect of resource use. The principle predicts the fate of complete competitors, not every pair of species that shares some food or space.
OpenStax’s community ecology explanation summarizes the principle and shows how resource partitioning can prevent exclusion. More advanced ecological treatments similarly emphasize that competition is strongest in overlapping parts of species’ niches, as explained in Georgia Tech’s resources and competition overview.
Fundamental and realized niches
The current IB Biology guide distinguishes between the fundamental niche and the realized niche. This distinction allows students to explain how competition changes the actual distribution of a species.
| Concept | Meaning | Main determining factors |
|---|---|---|
| Fundamental niche | The potential range of resources and conditions a species could use based on its adaptations and tolerance limits | Physiology, behavior, adaptations, and abiotic tolerance |
| Realized niche | The actual extent of the niche occupied when the species interacts and competes with other species | Competition and other biotic interactions acting alongside abiotic limits |
Competition can prevent a species from occupying all of its fundamental niche. Its realized niche is then narrower because competitors restrict its access to particular resources or parts of the habitat.
For example, a plant might tolerate both sunny and partly shaded soil. Its fundamental niche includes both conditions, but a faster-growing competitor may dominate sunny patches. The first plant consequently persists mainly in shade, making the shaded region part of its realized niche.
RevisionDojo’s fundamental versus realized niche explainer and B4.2.12 study notes provide additional exam-focused comparisons.
Possible outcomes of competition
The IB Biology guide for first assessment in 2025 specifically expects students to appreciate two possible outcomes of competition between species: elimination of one competitor or restriction of both species to part of their fundamental niches. The second outcome permits coexistence because the realized niches are no longer identical.
Competitive exclusion
One species may use a limiting resource more efficiently, reproduce faster under the prevailing conditions, tolerate lower resource concentrations, or interfere more effectively. Its advantage reduces the other species’ population until that competitor disappears locally.
Exclusion does not imply that the winning species is universally “better adapted.” Competitive ability depends on the environment. A species that wins at one temperature or resource level may lose when conditions change.
Resource partitioning and coexistence
Species can coexist if they use resources differently. Resource partitioning, also called niche partitioning, reduces interspecific competition by separating resource use along one or more dimensions.
Partitioning may occur through:
- spatial separation, such as feeding at different heights in a tree
- temporal separation, such as feeding during the day versus at night
- dietary separation, such as consuming different seed sizes
- microhabitat separation, such as occupying different soil depths
- behavioral separation, such as using different hunting methods
When each species uses a different portion of the available resources, individuals may compete more strongly with members of their own species than with the other species. This difference can stabilize coexistence. The process and its ecological importance are explored further in Nature Education’s resource partitioning overview and RevisionDojo’s article on how niche differentiation reduces competition.
Character displacement
Over generations, competition can create natural selection favoring heritable traits that reduce overlap. If two bird species compete for seeds, individuals specialized for different seed sizes may obtain more food and leave more offspring. The populations may eventually differ more strongly in beak size where they coexist than where each occurs alone.
This evolutionary divergence is called character displacement. Resource partitioning describes a pattern of different resource use, whereas character displacement specifically involves evolutionary change in traits associated with that separation. Students should not use the terms as exact synonyms.
Evidence supporting competitive exclusion
Gause’s Paramecium experiments
Georgy Gause cultured species of Paramecium separately and together under controlled laboratory conditions. Paramecium aurelia and Paramecium caudatum could each grow when cultured alone. When they were placed together and depended on the same food supply, P. aurelia increased while P. caudatum eventually disappeared from the mixed culture.
The result supports the competitive exclusion principle because the two species competed strongly for a shared limiting resource in a stable, simplified environment. It does not show that P. caudatum must disappear in every natural habitat. Environmental variation and niche differentiation can alter the result.
Connell’s barnacle study
Joseph Connell investigated barnacles on the Scottish coast. Chthamalus barnacles normally occupied a higher intertidal zone, while Semibalanus, historically called Balanus, occupied a lower zone. When Connell removed Semibalanus, Chthamalus expanded downward, showing that it could physiologically survive there but was normally restricted by competition.
The experiment demonstrated competitive release. Removing a competitor allowed Chthamalus to occupy more of its fundamental niche, providing evidence that its usual realized niche was narrowed by interspecific competition. Connell’s original barnacle field study is particularly valuable because it used experimental manipulation in a natural community rather than relying only on correlation.
What IB Biology students need to know
In the current course, ecological niches appear under B4.2 Ecological niches. The official IB Biology course page identifies the current course as first taught in 2023, with first assessment in 2025. The Biology guide specifies B4.2.12 Fundamental and realized niches and B4.2.13 Competitive exclusion and the uniqueness of ecological niches.
A strong exam explanation should form a causal chain:
- State that the species have overlapping niches.
- Identify the shared limiting resource.
- Explain that use by one species reduces availability to the other.
- State that this lowers growth, survival, or reproductive success.
- Predict exclusion or restriction to different parts of the fundamental niches.
For example:
The two bird species have overlapping niches because both consume the same limited seeds in the same habitat. Consumption by either species reduces the seed supply available to the other, producing interspecific competition. If one species uses the seeds more efficiently, it may reduce the population of the other until that species is locally excluded. Alternatively, the species may coexist if they use different seed sizes or forage in different locations, producing narrower and distinct realized niches.
Use explain questions to connect mechanisms and outcomes rather than listing vocabulary. After reviewing the concept, practise with the B4.2 Ecological Niches Questionbank and use the B4.2 flashcards to recall definitions precisely. Jojo AI can help identify missing causal links in a written response.
Common mistakes to avoid
- Confusing a niche with a habitat: A habitat is where a species lives; a niche includes resource use, conditions, and ecological interactions.
- Assuming any shared resource proves competition: The resource must be limiting, and the presence of one species must negatively affect the other.
- Saying one species is always superior: Competitive advantages depend on environmental conditions.
- Treating exclusion as global extinction: Competitive exclusion usually describes local disappearance from a niche or community.
- Claiming partial overlap always causes exclusion: Partial overlap can lead to competition without eliminating either species.
- Ignoring coexistence: The IB guide includes restriction of both species to parts of their fundamental niches as an important possible outcome.
- Confusing individual change with evolution: An individual changing where it feeds is behavioral adjustment; character displacement requires heritable change across generations.
Conclusion
Species compete for the same niche because their requirements overlap and a shared resource is limited. Competition can reduce the fitness of both species, but unequal competitive ability may lead to local exclusion. Alternatively, resource partitioning can restrict both species to different realized niches and allow coexistence.
For IB Biology, the essential link is niche overlap → competition for a limiting resource → reduced success → exclusion or niche restriction. RevisionDojo’s ecology notes can consolidate the wider topic, while its Questionbank, Flashcards, and Jojo AI are useful for practising precise, evidence-based explanations.
Sources and referenced URLs
External sources
- International Baccalaureate: Biology in the Diploma Programme
- OpenStax Biology 2e: Community Ecology
- Georgia Tech Biological Principles: Resources and Competition
- Nature Education: Resource Partitioning and Why It Matters
- Connell’s 1961 barnacle competition study