AHL: Ecological niches
IB Biology HLΒ· 40 min read
1. Fundamental vs. Realized Nicheβ β ββββ± 10 min
Ecological Niche
The full set of biotic and abiotic interactions that define a species' role in an ecosystem, including resource use, environmental tolerance, and interactions with other organisms.
Example:
A gray wolf's niche includes preying on large herbivores, shaping plant community structure, and occupying large home ranges in temperate and boreal forests.
All species have two measurable niche dimensions: the fundamental niche (the full potential range they can survive in without competition) and the realized niche (the actual range they occupy when competition and other biotic interactions are accounted for). Most species have a smaller realized niche than fundamental niche.
On a rocky intertidal shore, the barnacle Chthamalus can survive at both high and mid tidal levels, but is only found at high tide when the larger barnacle Balanus is present. Identify the fundamental and realized niche of Chthamalus.
- 1
Recall: The fundamental niche is the full range of conditions a species can survive in without competition.
- 2
Chthamalus can survive from high to mid tide, so this full range is its fundamental niche.
- 3
Recall: The realized niche is the actual range a species occupies when competition is present.
- 4
Balanus outcompetes Chthamalus at mid tide, so Chthamalus only occupies high tide, making this its realized niche.
Exam tip:
IB exam questions often ask you to distinguish these two terms. Always mention competition as the key biotic factor that reduces niche size from fundamental to realized.
2. Competitive Exclusion Principleβ β β βββ± 10 min
Competitive Exclusion Principle
Gause's principle stating that two species competing for the same limiting resource cannot coexist indefinitely if they have identical ecological niches. The stronger competitor will eliminate the weaker competitor locally.
This principle was first demonstrated in controlled experiments by Gause (1934) with two species of Paramecium. When grown separately, both species thrived, but when grown together in the same limited resource culture, the faster-growing P. aurelia outcompeted P. caudatum, which went extinct in the culture within 20 days.
Two species of flour beetle that feed on the same stored grain are kept in a sealed container with a fixed amount of grain. After 10 weeks, only one species is alive. Explain this observation using the competitive exclusion principle.
- 1
Both beetle species have identical niches, meaning they rely on the same limiting resource (stored grain) with no niche differentiation.
- 2
The competitive exclusion principle states no two species with identical niches can coexist permanently.
- 3
One species was a stronger competitor, consuming more grain and out-reproducing the weaker species.
- 4
Over time, the weaker competitor was eliminated, leaving only one species in the container.
3. Resource Partitioning and Niche Differentiationβ β β βββ± 12 min
When niches partially overlap, species do not always result in competitive exclusion. Over generations, natural selection favors individuals that compete less, leading to niche differentiation, also called resource partitioning. This allows multiple species with similar niche requirements to coexist.
Resource Partitioning
The evolutionary division of limiting resources between coexisting species, such that each species has a distinct realized niche with reduced overlap, lowering interspecific competition.
Five species of North American warbler all feed on insects in the same coniferous trees. Explain how all five can coexist in the same forest.
- 1
While all warblers feed on insects in the same trees, they partition the habitat resource by occupying different sections of the tree.
- 2
One species feeds only in the top third of the tree, another in the middle third, another on lower branches, another on the trunk, and another on new foliage.
- 3
This partitioning creates distinct realized niches with very little overlap in resource use between species.
- 4
Competition between species is reduced enough to allow all five species to coexist long-term.
Exam tip:
If asked to draw a diagram of resource partitioning, label the niche axis (e.g. resource size, tidal height), draw each species' niche, and shade the small overlapping regions between niches.
4. Niche Overlap and Competition Riskβ β β β ββ± 8 min
Niche overlap is a quantitative measure of how much two species share the same resources or environmental conditions. Higher overlap indicates a higher potential for interspecific competition when resources are limiting.
Niche Overlap Value | Interpretation | Competition Risk |
|---|---|---|
0 | No shared resources | None |
0.1 - 0.4 | Low overlap | Low |
0.4 - 0.7 | Moderate overlap | Moderate |
| High overlap | High |
Test your understanding:
Two native plant species have a measured niche overlap of 0.8 for soil nitrogen. What does this indicate?
No competition between the species is possible
They share most of their niche space and have high competition risk when nitrogen is limited
They have completely separate niches
Competitive exclusion will definitely occur
Reveal answer
They share most of their niche space and have high competition risk when nitrogen is limited βA high overlap value (>0.7) indicates most resources are shared, leading to high competition risk if the resource is limiting. Competitive exclusion is possible but not guaranteed, as resource partitioning may evolve over time.
5. Common Pitfalls
Wrong move:
Confusing habitat and ecological niche
Why:
Many students describe niche as just where a species lives, which is the definition of habitat, not niche.
Correct move:
Remember: Habitat is your address, niche is your job, what you consume, and how you interact with the ecosystem.
Wrong move:
Mixing up fundamental and realized niche definitions
Why:
Students often incorrectly state that realized niche is the full potential range of a species.
Correct move:
Use the mnemonic: F for Fundamental = Full potential range, R for Realized = Range in Reality with competition.
Wrong move:
Claiming any niche overlap leads to competitive exclusion
Why:
Competitive exclusion only occurs when niches are completely identical. Partial overlap leads to resource partitioning instead.
Correct move:
Only 100% identical niches lead to competitive exclusion; most overlapping niches result in resource partitioning and coexistence.
Wrong move:
Stating resource partitioning is a conscious change by individual organisms
Why:
Many students incorrectly describe resource partitioning as a short-term change individual organisms make to avoid competition.
Correct move:
Resource partitioning is an evolutionary change over many generations driven by natural selection, not a conscious change by individuals.
Wrong move:
Assuming high niche overlap always causes competition
Why:
Competition only occurs when resources are limiting. If resources are abundant, high overlap does not lead to competition.
Correct move:
Connect overlap to resource availability: high overlap + limited resources = high competition risk, but high overlap + abundant resources = little competition.
6. Quick Reference Cheatsheet
Term | Key Definition | Exam Tip |
|---|---|---|
Ecological Niche | Role + resource use + interactions | Not the same as habitat (location only) |
Fundamental Niche | Full potential range, no competition | F = Full potential |
Realized Niche | Actual range, with competition | R = Reality, usually smaller than fundamental |
Competitive Exclusion | No two species with identical niches can coexist | Use Gause's Paramecium as example |
Resource Partitioning | Division of resources to reduce competition | Enables coexistence of overlapping niches |
7. Frequently Asked
What is the key difference between habitat and niche?
A habitat is only the physical location where an organism lives, while an ecological niche describes the entire role of the organism, including what it eats, what resources it uses, how it interacts with other species, and what environmental conditions it tolerates.
When this came up on past exams
AI-estimated based on syllabus patterns β cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2022 Β· Paper 1
Fundamental vs realized niche
- 2023 Β· Paper 2
Explain resource partitioning
- 2021 Β· Paper 1
Competitive exclusion principle
Going deeper
What's Next
Understanding ecological niches is foundational for all further study of community ecology, conservation biology, and evolutionary biology. Niche theory explains global patterns of biodiversity, predicts the impact of invasive species on native communities, and helps us understand how species respond to climate change as they shift their realized niches to track suitable habitat. It also underpins our understanding of adaptive radiation, where multiple new species evolve from a common ancestor by partitioning resources into distinct niches. Mastery of this topic prepares you to analyze human impacts on ecosystems and species conservation strategies.