The big idea: Every species has an ecological niche — its full role in a community: the conditions it can tolerate, what it eats, and how it interacts with other species.
When two species need the same limited resource, they compete. The competitive exclusion principle says two species cannot share the same niche indefinitely — the better competitor wins, and the other is excluded.
Some organisms compete by releasing chemicals that harm their rivals: a plant doing this is showing allelopathy; a microbe doing it is showing antibiosis.
- Ecological niche
- The full role of a species in its community — the range of abiotic conditions it tolerates, the resources it uses, and its interactions with other species.
- Competitive exclusion principle
- Two species that require the same limited resource cannot coexist indefinitely; the more successful competitor excludes the other from that niche.
- Fundamental niche
- The entire niche a species could occupy if no competitors were present.
- Realized niche
- The smaller part of the niche a species actually occupies once competitors restrict it.
- Allelopathy
- When a plant releases a chemical that inhibits the growth or germination of other plants nearby, reducing competition.
- Antibiosis
- When a microorganism releases a chemical that inhibits the growth of other microorganisms, reducing competition.
Why niche overlap matters: If two species overlap only partly, they can coexist by using slightly different resources.
But if their niches overlap completely for a limited resource, one must lose — that is competitive exclusion.
On its own, a species can spread across its whole fundamental niche — every set of conditions and resources it is able to use.
Introduce a competitor that needs the same resource, and the picture changes: the weaker competitor is pushed out of the contested area, so each species ends up occupying only its smaller realized niche.
Alone, species A fills its full FUNDAMENTAL niche. Add competitor B and A is squeezed into a smaller REALIZED niche — two species cannot share one niche indefinitely.
Interactive diagram
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Competitive exclusion — cause and effect: Cause: two species need the same limited resource (the same food, the same space, the same light).
Effect: the better competitor obtains more of the resource, grows faster and reproduces more. The poorer competitor gets less, so its population declines.
Outcome: the poorer competitor is excluded — it dies out locally, or it survives only by shifting to a different niche (using a different resource). The two species cannot share one niche indefinitely.
| Term | What it means | Key idea to remember |
|---|---|---|
| Ecological niche | The full role of a species — its abiotic tolerances, what it eats and how it interacts with others | Two species with the SAME niche cannot coexist for long |
| Fundamental niche | The whole niche a species COULD occupy with no competitors present | The 'best case', before competition |
| Realized niche | The smaller part of the niche a species ACTUALLY occupies once competitors are present | Competition shrinks the fundamental niche to the realized niche |
| Competitive exclusion | Two species needing the same limited resource cannot coexist indefinitely; the better competitor excludes the other | One species wins; the loser dies out locally or shifts to a different niche |
Chemical competition — allelopathy & antibiosis: Competition is not always a passive race for resources. Some organisms release chemicals that directly harm their rivals.
A plant that releases a growth-inhibiting chemical into the soil is showing allelopathy — for example, the black walnut tree releases a chemical that stops many other plants growing beneath it.
A microorganism that releases a chemical to inhibit other microbes is showing antibiosis — for example, a mould producing an antibiotic that kills nearby bacteria.
| Feature | Allelopathy | Antibiosis |
|---|---|---|
| Who releases the chemical | A plant (e.g. black walnut, certain grasses) | A microorganism (e.g. a fungus or bacterium) |
| What the chemical does | Inhibits the growth/germination of nearby plants | Inhibits the growth of nearby microorganisms |
| Why it benefits the releaser | Reduces competition for water, light and minerals | Reduces competition for nutrients and space |
| Everyday example | Black walnut releasing juglone, which harms plants growing under it | A mould producing penicillin, which kills competing bacteria |
| Shared idea | Chemical competition — a chemical weapon, not direct contact | Chemical competition — a chemical weapon, not direct contact |
Competition for a shared resource
- Both species need the same limited resource
- The better competitor obtains more of it
- The poorer competitor's population declines
- Ends in competitive exclusion (one is excluded)
Chemical competition
- An organism releases a chemical to harm rivals
- Plant → plant inhibition is allelopathy
- Microbe → microbe inhibition is antibiosis
- Reduces competition without direct contact
A memory hook: Allelopathy = plants at war chemically (think allelopathy, a for the soil chemical the walnut adds).
Antibiosis = anti-microbe chemicals (think antibiotics from moulds).
And: Fundamental = the full niche; Realized = what is really left after competition.
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How this is tested: On Paper 1A a multiple-choice question often asks you to identify which statements correctly describe competitive exclusion between two species — the key marking points are same niche / same resource and one species is excluded (cannot coexist indefinitely).
Another favourite Paper 1A item gives a short scenario — a plant releasing a chemical that inhibits nearby plants, or a microbe inhibiting other microbes — and asks you to name the process (allelopathy or antibiosis).
A classic data scenario describes one Paramecium species declining when grown together with another, and asks you to deduce the cause — interspecific competition leading to competitive exclusion.
IB-style question — deduce the cause of a population decline
Two species of the ciliate Aquanella are grown in flasks containing the same bacterial food. Grown separately, both populations thrive. Grown together in one flask, species 1 increases while species 2 falls to zero. Deduce the cause of the decline of species 2. [3]
How to score all three marks
- Identify the relationship. Both species feed on the same food (the same bacteria), so they occupy the same niche and compete for that limited resource.
- Explain the cause and effect. Species 1 is the better competitor, so it obtains more of the food, grows faster and reproduces more; species 2 obtains too little food, so its population declines.
- Name the principle. Two species cannot share the same niche indefinitely — this is competitive exclusion, so species 2 is excluded and falls to zero. (Mark 1: same food / same niche → competition. Mark 2: species 1 is the better competitor / gets more resource. Mark 3: competitive exclusion of species 2.)
Final answer
Both species compete for the same limited food (the same niche); species 1 is the better competitor and obtains more of it, so species 2 is starved out — competitive exclusion drives species 2 to zero.
✓ Why this scores full marks: It links the cause (same niche / same food → competition) to the effect (one species is the better competitor) and to the principle (competitive exclusion).
A common slip is to say 'one ate the other' — this is competition for shared food, not predation.
Exam Tips:
- When a population falls to zero only WHEN GROWN TOGETHER, the cause is interspecific competition / competitive exclusion — not predation or parasitism.
- Always name the principle: 'competitive exclusion — two species cannot share one niche indefinitely'.
- Spell out which species is the better competitor and why it gets more of the resource.