There is the niche a species could occupy, and the niche it actually gets. The gap between those two is filled by other organisms. Understanding that gap explains why a barnacle that can survive across a whole shore ends up squeezed into a strip at the top of it.
📘 What you need to know
A niche is the role of a species within its habitat, covering both biotic and abiotic interactions.
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 actual conditions and resources in which it exists, due to biotic interactions.
The realised niche is smaller than the fundamental niche, and competition is why.
A niche can only be occupied by one species. Two species trying to share one will compete.
Competitive exclusion is the elimination of a competing species from its niche.
What a niche takes into account
The definition is the same one from the start of this topic, but it is worth restating with the two kinds of interaction spelled out. A species’ role takes 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.
Keep that split in mind, because it is exactly what separates the two kinds of niche below. The fundamental niche is set by abiotic tolerance. The realised niche is what is left after the biotic interactions have had their say.
Fundamental and realised niches
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
Fundamental niche
Realised niche
The niche a species would occupy if there were no limiting factors in the environment or on the resources it could use
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
The barnacle example
The standard case is the barnacle Chthamalus dalli on rocky shores in the Pacific Northwest.
Its fundamental niche covers a wide range of rocky intertidal areas. It can attach to a variety of substrates and tolerate a wide range of temperature and salinity.
In reality its realised niche is much smaller, because of competition with another barnacle, Balanus glandula, for space and resources.
The actual range of Chthamalus 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 competition.
So the realised niche of Chthamalus is smaller than its fundamental niche because of biotic interactions with another species. Nothing about the physical conditions lower down stops it living there.
This is the point of the whole diagram: the lower shore is perfectly liveable for Chthamalus. It is kept out by a competitor, not by the conditions.
If a question ever asks why a species is missing from somewhere that looks suitable, “competition with another species has restricted its realised niche” is a strong answer. Suitable conditions are not enough on their own.
Competitive exclusion
Now the rule that ties it together:
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, they compete for the same resources.
Eventually one species out-competes the other, until the loser is either forced into a new, slightly different niche, or made locally extinct.
Alternatively, both species end up forced into a smaller part of their fundamental niches.
DefinitionCompetitive exclusion is the elimination of a competing species from its niche.
The Paramecium experiment
The classic demonstration uses two species of single-celled, free-living protozoans: Paramecium aurelia and Paramecium caudatum.
Grown separately, both species thrive.
Grown in the same habitat, they compete for resources, and eventually P. aurelia outcompetes P. caudatum, leading to the elimination of P. caudatum.
Read the left graph first. It rules out the obvious alternative explanation — that the conditions simply did not suit P. caudatum.
An honest caveat, and it earns marks. This experiment grows both species under laboratory conditions. In nature it is rare for two species to occupy exactly the same niche, because natural selection has already pushed them apart.
What exclusion looks like in the wild
Total elimination is the extreme outcome. More often, the competing species end up pushed into slightly different niches. The three warblers from the start of this topic are the visible result: the most successful competitor takes the best feeding position, and the others shift to feeding elsewhere in the tree.
That is why species that look like they share a niche never quite do. Competition has already done the sorting.
Worked examples
WORKED EXAMPLE
Distinguish between the fundamental niche and the realised niche of a species. [3]
Define the firstFundamental: the full range of conditions and resources in which the species could survive and reproduce, based on its adaptations and tolerance limits.Define the secondRealised: the actual conditions and resources in which it exists, given the biotic interactions it faces.State the relationshipThe realised niche is smaller, because competition and predation restrict itthe words “could” and “actual” are doing the work. Use them.
WORKED EXAMPLE
Using the Paramecium graphs, explain what they show about competition. [4]
Step 1: describe the separate culturesGrown separately, both species increase and level off, so both can survive in these conditions.Step 2: describe the mixed cultureGrown together, P. aurelia still reaches a high population, while P. caudatum rises briefly then falls to zero.Step 3: give the reasonThe two occupy the same niche, so they compete for the same resources and P. aurelia is the better competitor.This is competitive exclusion — P. caudatum is eliminated from the nicheuse the left graph as your control. It proves the conditions were not the problem.
WORKED EXAMPLE
Chthamalus can tolerate the conditions on the lower shore, but is only found on the upper shore. Explain. [3]
Point out what the tolerance tells youIts fundamental niche includes the lower shore, since the abiotic conditions there are within its tolerance.Now the biotic factorBalanus outcompetes it for space and resources lower down.State the outcome in the right languageIts realised niche is restricted to the upper shore, where Balanus is absent or scarcethe question hands you the abiotic half. The marks are in the biotic half.
💡 Exam tip
Learn both niche definitions word for word. They are short and they are marked strictly.
Fundamental = could; realised = does. That one-word contrast will keep you straight.
Say what causes the difference: biotic interactions, mostly competition and predation.
For graph questions, always describe the data first, then explain it. Quote values off the axes.
Mention the laboratory conditions caveat for Paramecium — it shows evaluative thinking.
Remember the two possible outcomes of competition: the loser moves to a different niche, or it is made locally extinct.
⚠ Common mix-up
Saying the realised niche is bigger. It is always the smaller of the two.
Thinking abiotic factors set the realised niche. They set the fundamental one; biotic interactions shrink it.
Saying competitive exclusion means global extinction. It is usually local — elimination from that niche in that habitat.
Assuming two species can share a niche if resources are plentiful. One niche, one species.
Reading the Paramecium result as “the conditions killed it”. The separate culture rules that out.
Forgetting that both species can lose. Sometimes both end up in a smaller part of their fundamental niche.
Treating a lab result as what always happens in nature. Exact niche overlap is rare in the wild.
That is the end of the Ecological Niches chain. Up next: Ecological Niches — start again from the top and see how much more of the competition argument makes sense now.
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