The big idea: A species' niche is its full role — the conditions it can tolerate, the resources it uses and how it interacts with other species.
When two species need the same limited resource, they compete. The competitive exclusion principle says two species cannot occupy the exact same niche indefinitely — one is driven out, or they divide the resource between them.
Because of this, the niche a species could occupy (its fundamental niche) is often bigger than the niche it actually occupies once competitors are present (its realized niche).
Species A alone occupies its full fundamental niche; add competitor B and each species is restricted to a smaller realized niche along the resource gradient — the competitive exclusion principle.
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- Ecological niche
- An organism's role in its ecosystem: the abiotic conditions it tolerates, the resources it uses, and its interactions with other species.
- Competition (interspecific)
- An interaction in which two different species both need the same limited resource, so each reduces the amount available to the other.
- Competitive exclusion principle
- Two species cannot occupy exactly the same niche in the same place indefinitely; the stronger competitor excludes the weaker, unless they divide the resource.
- Fundamental niche
- The full range of conditions and resources a species COULD use if there were no competitors present.
- Realized niche
- The smaller part of the fundamental niche a species ACTUALLY occupies once competition from other species restricts it.
- Resource partitioning
- When competing species divide a shared resource (by space, time or type) so each uses a different part and they can coexist.
Fundamental = potential, realized = actual: Think of it as could versus does.
The fundamental niche is what a species could occupy with no rivals.
The realized niche is what it actually occupies once a competitor squeezes it into a smaller space. The realized niche is therefore never larger than the fundamental one.
When a species lives alone, it spreads across its whole fundamental niche — every part of the gradient (light, depth, temperature, food size) that it can tolerate.
Add a competitor that needs the same resource, and the overlap becomes a battleground. Whichever species is the better competitor in a given part of the gradient wins that part; the other is pushed out of it.
Two ways competition can end: There are two possible outcomes when species compete for the same niche:
1. Competitive exclusion — one species is the stronger competitor everywhere, so the weaker one cannot persist and disappears from that habitat. Only one species remains.
2. Resource partitioning — each species is the better competitor in a different part of the gradient, so they split the resource and coexist in separate zones. Each ends up in a realized niche smaller than its fundamental one.
| Outcome of competition | What happens | Result you see in the field |
|---|---|---|
| Competitive exclusion | The stronger competitor takes the shared resource; the weaker one cannot persist there | Only one species is found in that habitat / zone |
| Resource partitioning | Each species shifts to the part of the resource it uses best | The two species coexist but in SEPARATE, non-overlapping zones or habitats |
Why species end up in separate zones: This is the key cause–effect chain examiners want:
two species need the same resource → they compete where their tolerances overlap → each is the better competitor in one part of the gradient → each is excluded from the part it loses → the result is separate, non-overlapping zones or habitats, with each species in its realized (not fundamental) niche.
Fundamental niche (alone)
- The full range the species could use
- Measured with no competitors present
- Larger — the species' whole potential
- Set only by the species' own abiotic tolerances
Realized niche (with competitor)
- The part the species actually uses
- Measured with a competitor present
- Smaller — squeezed by competition
- Set by tolerances plus competition from other species
Reading the data: On a graph or transect, the separate, non-overlapping distribution of two species is the fingerprint of competition.
Where one species grows alone you are seeing its fundamental niche; where the two grow together and each is squeezed into a narrower band, you are seeing their realized niches.
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How this is tested: This micro is a data-question favourite. On Paper 3 you may be given graphs of two species grown separately and together and asked to identify which niche type each graph shows — alone is the fundamental niche, together is the narrower realized niche.
A 2-mark Outline can ask for the factors that lead two related species to occupy separate habitats — competition for the same resource, with each excluded from the part it loses (resource partitioning).
On Paper 1B a 3-mark Discuss question gives a transect and asks why two species occupy different, non-overlapping zones — competitive exclusion and zonation.
IB-style question — identify the niche from the data
Two barnacle species were studied on a rocky shore. When grown on its own, species P spread across the whole height of the shore. When species Q was also present, species P was found only on the upper shore. Identify the niche type shown by species P in each case, and explain your answer. [3]
How to score all three marks
- Name the niche when alone. When species P grows alone it covers the whole shore — this is its fundamental niche (the full range it can tolerate with no competitor).
- Name the niche when the competitor is present. With species Q present, P is restricted to the upper shore — this smaller range is P's realized niche.
- Explain the cause. Species Q is the better competitor on the lower shore, so it excludes P from there; P only persists where it competes successfully. (Mark 1: fundamental = whole shore. Mark 2: realized = upper shore only. Mark 3: Q out-competes / excludes P from the lower shore.)
Final answer
Alone, P shows its fundamental niche (the whole shore); with Q present, P shows a smaller realized niche (upper shore only) because Q out-competes and excludes P from the lower shore.
✓ Why this scores full marks: It pairs the data pattern with the correct term: whole range alone = fundamental, restricted range with a competitor = realized.
The third mark comes from the cause — the competitor excludes the species from the part it loses. Naming the niches without explaining the exclusion would drop a mark.
| Feature | Fundamental niche | Realized niche |
|---|---|---|
| What it is | The full range of conditions and resources a species COULD use | The smaller part a species ACTUALLY uses |
| Competitors present? | Imagined with NO competitors | With competitors present |
| Size | Larger — the species' full potential | Smaller — restricted by competition |
| Seen when | A species grows ALONE | A species grows ALONGSIDE a competitor |
| Set by | Abiotic tolerances of the species itself | Tolerances PLUS competition from other species |