When two species want the same thing, one usually does it slightly better. Over time that small edge pushes the other species out of part of its range — or out of the habitat altogether. This is also the topic where the IB asks you to think like a scientist about what an experiment can and cannot prove.
📘 What you need to know
Two species with similar niches compete for the same resources.
The fundamental niche is the full range a species could occupy; the realised niche is the smaller range it actually occupies once competitors are present.
Competitive exclusion is one species being pushed out of part of its niche — or out of the habitat entirely, causing local extinction.
You can test for competition by removing one species and seeing whether the other spreads.
A change after removal indicates competition but does not prove it.
Invasive species often outcompete natives; endemic species are especially at risk.
Niches: fundamental and realised
A niche is the set of conditions and resources a species is adapted to — where it lives, what it eats, when it is active, what it needs to survive. Two species with almost identical niches are, by definition, after the same things.
Grow a species on its own and it spreads across everywhere it can physically cope with. That full range is its fundamental niche. Add a competitor that is slightly better in some conditions and the species gets squeezed into what is left. That smaller, real-world range is its realised niche.
The realised niche is always the same size or smaller than the fundamental niche — never bigger.
Competitive exclusion
Two species cannot occupy exactly the same niche indefinitely. One is pushed into a smaller niche, or out altogether.
So what happens to the losing species? Two possible endings:
It shifts. It changes where or how it lives in the habitat so it no longer competes directly. Its realised niche shrinks, but it survives.
It disappears locally. If there is nowhere else for it to go, the population dies out in that habitat — local extinction.
The word “exclusion” makes students picture fighting. There is usually none. The stronger competitor simply grows faster, breeds sooner or feeds more efficiently, and the other species slowly fades from that part of the habitat.
Testing for competition
Suppose two species are found in different parts of a shore. Is that competition, or just different preferences? You cannot tell by looking. You have to change something and watch.
🧩 A removal experiment in the field
Sample first. Use randomly placed quadrats to record the presence or absence of both species across the habitat.
Remove one species from a small area, leaving the rest of the habitat untouched as a control.
Wait, then re-sample the same way, using the same quadrat size and the same number of samples.
Compare the distributions. If the remaining species spreads into the cleared area, competition was probably limiting it.
If nothing changes, something else — soil, moisture, light — was controlling the distribution instead.
The honest conclusion. A change after removal shows competition could have been happening. It does not prove it. Removing a species disturbs the habitat in other ways too, and something else you did not measure may have changed at the same time.
Lab or field? (NOS)
Hypotheses can be tested by controlled experiments and by careful observation. Both are proper science, and each has a cost.
Laboratory investigation
Field investigation
Small scale and tightly controlled, e.g. growing two bacterial species alone and together
Real-life scale, e.g. recording species along a rocky shore
Only the independent variable changes, so any difference is easier to attribute
Most variables cannot be controlled, so results are harder to attribute
Easy to repeat exactly, which makes results reliable
Very hard to repeat exactly, since conditions never recur
Organisms may not behave as they would in the wild, so results may not transfer
Gives a more realistic picture of what actually happens in nature
Invasive species: competition at its most one-sided
An invasive species is a non-native species that causes harm in the place it has been introduced. Most arrivals are down to humans: species traded deliberately for gardens, zoos or pest control and then escaping, or carried accidentally, like rats on ships.
In their new home, invasive species often have no natural predators, parasites or competitors — the population controls that limited them back home are simply absent. So their numbers can climb very fast, and three things follow:
Competition with native species occupying a similar niche, which may be displaced or driven to extinction.
Predation, if the invader is an effective hunter, causing sharp declines in its prey.
New diseases, to which native species have no immunity.
The crossover point matters in data questions. Read off the year where the two lines meet and describe what happens either side of it.
Endemic species are the most vulnerable
An endemic species is found in one place and nowhere else on Earth — the Scottish crossbill in Scotland’s conifer forests, for example. That makes local extinction catastrophic: if it goes there, it is gone completely.
Australia shows the pattern painfully clearly. Its native species evolved in isolation for millions of years. Introduced predators — the European red fox, brought over in the 1800s for hunting, and domestic cats — have caused huge declines. Evidence suggests more than 10 % of Australia’s endemic mammal species have already gone extinct since European settlement, with small mammals worst hit. Some now survive only on offshore islands the foxes and cats have not reached.
WORKED EXAMPLE
Describing and explaining the squirrel graph
Using the graph above, describe what happens to the two squirrel populations over the twenty years, and explain the pattern. [4]
Describe: use figures from the graphgrey squirrels rise from near zero to about 190; red squirrels fall from about 160 to about 10Describe: name the crossoverthe two lines cross at around year 8 to 9, after which greys are more numerousExplain: the mechanismthe two species occupy a similar niche, so they compete for the same food and territoryExplain: the outcomegreys are the better competitors, so reds get less food, survive and breed less, and are displaceddescribe with numbers, explain with biology — two different jobs in one question
WORKED EXAMPLE
Evaluating a removal experiment
A student removes barnacle species A from an area of shore. Six weeks later, barnacle species B has spread across the cleared area. The student concludes that species A was outcompeting species B. Evaluate this conclusion. [3]
What the result does supportspecies B spread once A was gone, which is consistent with A limiting its distributionWhy it falls short of proofremoval changes other things too, so another factor may have caused the spreadHow to strengthen ituse a control area where nothing was removed, and repeat at several sitesthe safe wording is “indicates that competition could be occurring”, not “proves”
💡 Exam tip
Learn the pair together: fundamental = could occupy, realised = does occupy.
Never write that an experiment proves competition. Use indicates or suggests.
In removal experiments, always mention a control area where nothing is removed.
For invasive species, the key phrase is no natural predators or competitors in the new ecosystem.
Define endemic properly: found in one place and nowhere else in the world.
Describe questions want numbers off the graph; explain questions want biology. Do both when asked for both.
⚠ Common mix-up
Competitive exclusion is not predation. Grey squirrels do not eat red squirrels.
The realised niche cannot be larger than the fundamental niche. If your answer implies that, the labels are swapped.
Endemic does not mean endangered. It means restricted to one location — which is what makes it vulnerable.
Invasive is not just “non-native”. The species must also cause harm.
Local extinction is not global extinction, unless the species is endemic to that one place.
A niche is not a habitat. Habitat is the place; niche is the place plus the role and the conditions needed.
Up next: Chi-Squared Test — the statistics that tell you whether an apparent association between two species is real or just chance.
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