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.
Competitive exclusion: 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.
Fundamental niche
- The niche a species could occupy
- Often bigger than the realized niche
Realized niche
- The niche it actually occupies once competitors are present
- Smaller, because of competition
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.
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The big idea: The fundamental niche is set by a species' adaptations and tolerance limits. The realized niche is the part it actually occupies when competitors are present.
The points to remember
- Fundamental niche: the potential range, with no competitors.
- Realized niche: the actual range, with competitors present.
- The realized niche is never larger than the fundamental niche.
- In data: the species alone shows its fundamental niche; with a competitor, its realized niche.
Remember it as: Fundamental = could; realized = does.
Real example: Connell's barnacles. On the Isle of Cumbrae in Scotland in the 1950s, Joseph Connell removed Balanus barnacles from patches of rock: Chthamalus barnacles then settled and survived lower down the shore than before.
Trap: Do not call the fundamental niche 'perfect' or 'ideal': say it is the potential niche, limited only by adaptations and tolerance.
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How it is asked: Expect graphs or descriptions of a species alone and with a competitor, or a request to distinguish the two niches.
A grass grows from 0 to 60 cm above a stream when alone, but only from 0 to 20 cm when a taller grass grows beside it. Identify the type of niche shown in each case.
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