There is the niche a species could occupy, and the niche it actually gets. The gap between the two is filled entirely by other living things — and that gap is what this page is about.
📚 What you need to know
The fundamental niche is the full range of conditions and resources in which a species could survive and reproduce, based on its adaptations and tolerance limits.
The realised niche is the conditions and resources in which a species actually exists, because of biotic interactions.
The realised niche is always smaller than the fundamental niche.
A niche can only be occupied by one species, so every species has its own unique niche.
If two species try to occupy the same niche, one out-competes the other, which is then forced into a different niche or made locally extinct.
Eliminating a competing species from its niche is competitive exclusion.
The niche, one more time
A niche is the role of a species within its habitat. That role takes two kinds of interaction into account:
Biotic interactions — the organisms it feeds on, and the organisms that feed on it.
Abiotic interactions — for example, how much oxygen and carbon dioxide it exchanges with the atmosphere.
Fundamental and realised niche
Fundamental niche
The full range of conditions and resources in which a species could survive and reproduce, based on its adaptations and tolerance limits.
Realised niche
The actual conditions and resources in which a species exists, due to biotic interactions.
The difference is caused by other organisms. On its own, a species can live anywhere its physiology allows. Add competitors and predators, and it gets squeezed into whichever part of that range it can hold on to.
The barnacle example
The barnacle Chthamalus dalli has a wide fundamental niche. It can attach to a variety of substrates across a wide range of rocky intertidal areas in the Pacific Northwest, and it tolerates a broad range of temperature and salinity.
In reality its realised niche is much smaller, because it competes with another barnacle, Balanus glandula, for space and resources. Its actual range is restricted to areas where Balanus is absent or scarce — higher up the shore, where it is exposed to air for longer periods and can avoid the competition.
The upper shore is harsher, not better. Chthamalus is not choosing it — it is the only stretch where Balanus cannot follow.
Fundamental niche
Realised niche
The niche a species would occupy if there were no limiting factors and it could use every resource available
The niche a species actually occupies in the presence of competitor species
The potential distribution of a species
The actual distribution of a species
No competition for resources, and no predation
Competition for resources and predation both occur
Large
Smaller
A quick way to keep them straight: fundamental is what the body can do; realised is what the neighbours allow. The species has not changed at all between the two panels — only its company has.
Competitive exclusion
A niche can only be occupied by one species, which means every species has its own unique niche. If two species try to occupy the same one, they compete for exactly the same resources, and something has to give.
🧩 The three possible endings
One species out-competes the other, and the loser is forced into a new, slightly different niche.
One species out-competes the other, and the loser is made locally extinct.
Both species are pushed into a smaller part of their fundamental niches, and both persist in reduced form.
The elimination of a competing species from its niche — ending 2 — is called competitive exclusion.
The Paramecium experiment
The classic demonstration uses two species of single-celled, free-living protozoans: Paramecium aurelia and Paramecium caudatum.
Grown apart, both species thrive. Put them in one flask and P. aurelia out-competes P. caudatum for the same resources, until P. caudatum is eliminated.
An important caveat. This experiment was done under laboratory conditions, where both species were deliberately given identical resources. In nature it is rare for two species to occupy exactly the same niche — which is precisely why complete exclusion is rarely seen in the wild.
🤔 So why does the world not end up with one species per habitat?
Because outcome 1 and outcome 3 are far more common than outcome 2. Competition usually pushes species apart rather than wiping one out: each ends up specialising on the resources it happens to be better at.
Go back to the warblers in the spruce tree. The most likely reason each species feeds in its own band is exactly this — competition pushed them into slightly different niches, and now they coexist. What looks like peaceful sharing is the fossilised result of an old fight.
Worked examples
WORKED EXAMPLE
Distinguish between the fundamental niche and the realised niche of a species. [3 marks]
Step 1: the fundamental niche
The full range of conditions and resources in which the species could survive and reproduce, based on its adaptations and tolerance limits.
Step 2: the realised niche
The conditions and resources in which it actually exists, given the biotic interactions it faces.
Step 3: the distinction
The realised niche is always smaller than the fundamental niche, because competition and predation exclude the species from part of the range it could otherwise occupy.
Potential vs actual, with the reason for the differenceThe word “biotic” is doing real work here. The difference is caused by other organisms, not by conditions.
WORKED EXAMPLE
Using the graphs above, describe and explain what happens to P. caudatum when it is grown with P. aurelia. [4 marks]
Step 1: describe, using the data
Grown separately, P. caudatum rises and levels off at a density of about 78. Grown together it rises to only about 32 by day 4, then falls steadily to almost zero by day 16.
Step 2: explain the fall
The two species occupy the same niche in the culture, so they are in direct competition for the same resources.
Step 3: name what is happeningP. aurelia obtains resources more successfully → fewer are left for P. caudatum → its death rate exceeds its birth rate → the population fallsCompetitive exclusion: P. caudatum is eliminated“Describe and explain” means data first, biology second. Quote figures off the graph in the describe part.
WORKED EXAMPLE
Chthamalus can survive across the whole shore in the laboratory, but on a real shore it is found only high up. Explain this. [3 marks]
Step 1: identify which niche is which
The whole shore is its fundamental niche, since its tolerance limits allow it to survive there. The upper shore is its realised niche.
Step 2: name the biotic factor
On a real shore it competes with Balanus for space and resources, and Balanus out-competes it on the lower shore.
Step 3: explain why the upper shore is different
Higher up, the rock is exposed to air for longer. Chthamalus tolerates that better, so Balanus is absent or scarce there and cannot exclude it.
Out-competed low down; survives high up because its competitor cannot
💡 Exam tip
Learn both niche definitions word for word. They are short and they come up often.
State clearly that the realised niche is smaller, and say why — biotic interactions.
Use the term competitive exclusion by name whenever a species is eliminated.
In graph questions, describe with numbers first, then explain the biology. The marks are usually split that way.
Remember there are three possible outcomes of competition, not just extinction.
If you can add the laboratory caveat about the Paramecium experiment, do — it shows real understanding.
⚠ Common mix-up
Realised niche described as larger. It can never be bigger than the fundamental niche.
Blaming abiotic factors for the difference. The gap between the two niches is caused by biotic interactions.
Assuming competition always ends in extinction. Most often both species are simply restricted.
Saying the losing species “moves away”. It is excluded from that niche, which is not the same as migrating.
Treating the Paramecium result as typical of nature. It was a laboratory set-up designed to force identical niches.
Using habitat and niche interchangeably in a competition answer. Two species can share a habitat happily.
That completes Ecological Niches & Nutrition. The through-line: a niche is a role, the role is mostly about food, and where two roles overlap too far, one species gives way.
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