IB Biology HLPopulations & CommunitiesPaper 1 & 2~13 min read
Interspecific Competition
When two species want the same things, one of them usually does it slightly better. The loser does not simply die out — it gets pushed into a smaller corner of the habitat. That squeeze has a name, a testable prediction, and a very visible example running through British woodland right now.
📘 What you need to know
Two species occupying very similar niches will compete for resources: interspecific competition.
One species may be better adapted, so the other is outcompeted.
The outcompeted species may alter its distribution so it no longer directly competes. If that is not possible, it may become locally extinct.
Being forced out of one niche into another is competitive exclusion.
The ideal niche is the fundamental niche; the one it is squeezed into is the realised niche.
Removing a competitor and watching for a change in distribution tests for competition — but it does not prove it.
An invasive species is a non–native species that causes harm to the environment it has been introduced to.
Endemic species are found in a particular place and nowhere else, which makes them especially vulnerable.
Fundamental and realised niches
If a species is outcompeted, it is forced out of its niche into an alternative niche. That is competitive exclusion, and it produces two niches worth naming.
The realised niche is always the same size or smaller than the fundamental niche. Remove the competitor and the species should expand back — which is exactly what the test below relies on.
Testing for interspecific competition
The competitive exclusion effect gives you a way to test whether competition is happening. Remove one species and see whether the other’s distribution changes.
🧩 The two ways to run the test
In the lab: culture two bacterial species on their own and then together, measuring how this affects population size or colony distribution.
In the field: first carry out random quadrat samples, recording the presence or absence of one or both species at different locations around the habitat.
Then remove one species from a small area (this is the manipulation).
Measure the effect this has on the distribution of the second species.
If the distribution changes, competition was likely taking place. If removal has no effect, distribution is probably down to another factor.
Read this next sentence twice, because it is the sort of precision that separates a 6 from a 7. This effect does not prove the presence of interspecific competition — it only indicates that it could be occurring. Something else may have changed at the same time. Science indicates; it rarely proves.
Nature of science: experiments and observations
A hypothesis is a proposed explanation for an observation that can be tested by scientific investigation. As the test above shows, hypotheses can be tested in a laboratory or in the field, and the two have opposite strengths.
Laboratory
Field
Conditions
Controlled, on a small scale
A real–life setting, on a large scale
Example
Growing bacteria in a lab, or plants in a greenhouse
Growth of plants in a forest, or species distribution on a rocky shore
Control of variables
High — only the independent variable is changed
Not possible to control factors beyond the independent variable
Weakness
Results can never be directly applied to a real–life situation; organisms may not behave as they do naturally
May not give a perfectly valid set of results, and is very hard to replicate exactly
Strength
Confident that the independent variable caused the effect
A more realistic representation of the real world
Invasive species
An invasive species is a non–native species that causes harm to the environment into which it has been introduced. They are sometimes called alien species.
Invasion can happen naturally when a species migrates or expands its habitat, but most recorded incidents are caused by humans:
Knowingly collecting and trading species between countries — attractive plants brought into gardens, or animals kept in zoos or as pets that then escape into the wild.
Unknowingly providing transport to a new ecosystem, e.g. rats on board ships.
Deliberately introducing alien species as biological control for pests.
Why they spread so fast. In a new ecosystem an invasive species has little or none of the natural population controls that existed in its previous one — no natural predators or competitors. As a result it can increase in number at a rapid rate.
Effect on the ecosystem
What happens
Competition
Occurs between the invasive species and native species occupying a similar niche, with the native species displaced or pushed to extinction
Predation
Many invasive species are successful predators, causing a massive decline in their prey species
Disease
Invasive species can introduce new diseases to which native species have no natural immunity
Grey squirrels were introduced to the UK from North America in the 1800s. They have been highly successful and have largely outcompeted the native red squirrel.
Endemic species
Endemic species are found in a particular place and in no other location in the world. The Scottish crossbill is found in the conifer forests of Scotland and nowhere else, so it is endemic to Scotland.
Endemic species are especially vulnerable to invasive species, for a stark reason: local extinction means the species has gone entirely extinct. There is no population elsewhere to recolonise from.
Australia: the case study
Detail
Why it has so many endemics
Its species evolved over many years of isolation from other continents
The invasive predators
The European red fox, introduced in the 1800s by settlers for fox hunting, and the domestic cat, likely travelling on ships and introduced by accident
The scale of the damage
Evidence suggests more than 10 % of Australia’s endemic mammal species have already gone extinct since European settlers arrived
Who is worst affected
Small mammals, being the best food source for the invasive predators
What is left
Some species now survive only on islands around mainland Australia where foxes and cats have not yet arrived
Worked examples
WE 1
Fundamental and realised niches
Distinguish between the fundamental niche and the realised niche of a species. (2 marks)
Point 1: fundamental
The ideal niche a species could occupy in the absence of competition.
Point 2: realised
The smaller niche it actually occupies once a better–adapted competitor has forced it out of part of its range, through competitive exclusion.
What it could occupy, versus what it is left withthe realised niche is never larger than the fundamental one
WE 2
Interpreting a removal experiment
A researcher removes barnacle species A from part of a rocky shore. Species B then spreads into that area. Evaluate what this shows. (3 marks)
Point 1: what it indicates
Species B expanded into space it previously did not occupy, which indicates that interspecific competition with species A may have been restricting its distribution.
Point 2: the niches
Species B was previously confined to its realised niche and has now expanded towards its fundamental niche.
Point 3: the limitation
This does not prove competition was occurring. Other factors may have changed at the same time, so the result only indicates competition could be taking place.
Strong evidence, not proof“evaluate” is asking for the limitation — do not stop at the conclusion
WE 3
Why invasive species spread
Explain why an introduced species is often able to increase in number far more rapidly in its new habitat than in its native one. (3 marks)
Point 1: controls are missing
In the new ecosystem it has little or none of the natural population controls that existed in its previous ecosystem.
Point 2: name them
In particular it has no natural predators and no established competitors adapted to compete with it.
Point 3: the consequence
With few density–dependent factors limiting it, survival and reproduction are high, so numbers increase at a rapid rate and native species occupying a similar niche may be displaced.
No predators, no competitors, no brakestying this back to density–dependent factors shows the examiner you have linked the unit together
💡 Exam tips
Fundamental = ideal niche; realised = what is left after competition.
Name competitive exclusion when describing a species being pushed out.
Removal experiments indicate competition; they do not prove it.
For lab versus field, pair control against realism.
Define invasive as non–native and causing harm — both halves matter.
Endemic species matter because local extinction is total extinction.
⚠ Common mistakes
Saying any non–native species is invasive. It must also cause harm.
Saying the realised niche is bigger than the fundamental niche. It never is.
Claiming a removal experiment proves competition. It indicates it.
Saying the outcompeted species always dies out. It usually alters its distribution first.
Saying lab experiments are simply better. They trade realism for control.
Confusing endemic with endangered. Endemic means found nowhere else.
Up next: Chi–Squared Test (Skills). You have just seen that two species can be found in different parts of a habitat. The final page is the statistics that tell you whether that pattern is real or just chance.
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