The Niche a Species Could Fill, and the One It Does
A niche is not a place. It is a species’ whole way of making a living: what it eats, what conditions it can stand, when it is active and what it needs to breed. The HL twist is that the niche a species could occupy is nearly always bigger than the one it actually gets, because competitors are in the way.
📚 What you need to know
A niche is the full set of biotic and abiotic conditions and resources a species responds to and depends on.
The fundamental niche is the full range a species could occupy with no competitors or other limiting factors.
The realised niche is the narrower range it actually occupies because of biotic interactions.
The realised niche results from both adaptations and competition.
Competitive exclusion: two species cannot occupy the same niche indefinitely — one is pushed out or into a different niche.
Species sharing a habitat avoid this by partitioning resources, for example feeding at different heights.
Habitat is the address, niche is the job
This one sentence solves most niche questions. A badger’s habitat is the woodland. Its niche is being a nocturnal omnivore that digs setts in well-drained soil, eats earthworms and fruit, and breeds in late winter. Two species can share a habitat quite happily. They cannot share a niche.
The two definitions
fundamental niche = what a species could use realised niche = what it actually uses
Fundamental and realised niches on a shore
The classic study is on barnacles. Some species can survive across almost the whole rocky shore, from the high-tide mark down to the low-tide mark, because they tolerate a wide range of temperature and salinity and will settle on many surfaces. That whole range is their fundamental niche.
In reality they are only found on the upper part of the shore. Lower down, a larger barnacle species grows faster and takes the space. The smaller species is squeezed into the upper zone, where it has to put up with longer periods out of the water — but where the competitor cannot survive. That squeezed band is its realised niche.
The upper shore is the harsher place to live. Species A survives there not because it prefers it, but because that is where the competitor cannot follow.
A neat way to remember it: the fundamental niche is what your CV says you could do; the realised niche is the job you actually got once everyone else had applied too.
Competitive exclusion
If two species need exactly the same resources in exactly the same way, one of them will always be slightly better at getting them. That species gets more food, breeds more successfully, and its numbers rise while the other’s fall. There are only three possible endings:
The weaker competitor is driven out of the area.
The weaker competitor shifts into a slightly different niche and survives there.
The weaker competitor goes extinct locally.
This is why the statement “no two species occupy the same niche” is not just an observation. It is the outcome of competition doing its work.
Sharing a habitat by splitting the resources
Species that look like they should be in direct competition usually turn out to be doing subtly different things. Several warbler species live in the same conifer trees and eat the same kinds of insect, yet they coexist because each feeds in a different part of the tree.
Looking at the birds alone, you would say they share a niche. Watching where they feed shows three separate niches inside one habitat.
Worked examples
WORKED EXAMPLE
Applying the idea to a new species
In laboratory tanks, a freshwater shrimp survives at temperatures from 4 °C to 26 °C. In the river where it lives it is only found in stretches above 18 °C, where a second shrimp species is absent. Identify the fundamental and realised niches and explain the difference. [4]
Step 1: the fundamental nichethe full range it could tolerate with nothing in the wayfundamental niche: 4 °C to 26 °CStep 2: the realised nichethe narrower range where it is actually found in the riverrealised niche: above 18 °CStep 3: the reasonthe second species outcompetes it in the cooler stretchesthe lab result gives the fundamental niche because competitors are removed
WORKED EXAMPLE
Predicting the effect of removing a competitor
A disease wipes out species B from the lower shore. Predict what happens to species A and justify your answer. [3]
Step 1: what was limiting species Acompetition for space with species B, not the physical conditionsStep 2: the predictionspecies A spreads down into the lower shoreStep 3: the justificationthe lower shore was already inside its fundamental niche, so it can survive therethe realised niche expands towards the fundamental nichenever say the fundamental niche gets bigger — only the realised one changes
💡 Exam tip
Start any niche answer with “role”, not “place”. That single word choice signals you know the difference.
Fundamental is bigger, realised is smaller. If your answer says the opposite, check it.
Lab or greenhouse data usually gives the fundamental niche; field data gives the realised niche.
Name the limiting factor when explaining a realised niche — usually competition, sometimes predation.
Practise applying the idea to unfamiliar species. Exams rarely use the example from the textbook.
Mention adaptations as well as competition. The realised niche comes from both.
⚠ Common mix-up
Defining a niche as a habitat. The habitat is where; the niche is how it lives.
Saying the realised niche can be larger than the fundamental one. It never is.
Thinking a species chooses its realised niche. It is forced into it by competition.
Assuming the realised niche is the best conditions available. It is often the harsher end of the range.
Claiming two species never share resources. They can overlap — they just cannot overlap completely for ever.
Forgetting abiotic tolerance. A niche includes temperature, pH and salinity ranges, not just food.
Up next: Life Cycles and Survivorship Curves — the last HL page, on the two very different strategies species use to stay in the game.
Want this explained one-to-one?
Book a free session with an experienced IB ESS tutor and get your trickiest topics made simple.
