BiologyCBSE Class 12 · Organisms and Populations

Competition and Competitive Exclusion

Q: What is Gause's competitive exclusion principle?

Two closely related species competing for the same limiting resource cannot coexist indefinitely; the competitively inferior species is eventually eliminated.

Q: What is resource partitioning?

A way competing species coexist by using a resource differently, such as feeding at different times or in different ways; e.g. MacArthur's five warbler species on one tree.

Q: What is competitive release?

The expansion of a species' range when a superior competitor that restricted it is removed, as with Chthamalus after Balanus is removed.

Q: Can unrelated species compete?

Yes. Flamingoes and resident fishes in some South American lakes compete for zooplankton.

Q: Is competition always for limited resources?

No. In interference competition, one species reduces another's feeding efficiency even when food and space are abundant.

Updated 15 October 2026 · Fact-checked

Competition is an interaction in which the fitness (intrinsic rate of increase, r) of one species is significantly lower in the presence of another. It can occur between unrelated species and even when resources are not limiting (interference). Gause's principle says two closely related species competing for the same limiting resource cannot coexist indefinitely; resource partitioning lets species coexist.

Understand Competition and Competitive Exclusion

Competition (−/−). Darwin was convinced that interspecific competition is a potent force in evolution. It is commonly thought to occur only when closely related species compete for the same limited resources, but this is not entirely true:
1. Totally unrelated species can compete: in some shallow South American lakes, visiting flamingoes and resident fishes compete for zooplankton.
2. Resources need not be limiting: in interference competition, the feeding efficiency of one species is reduced by the interfering and inhibitory presence of another, even when food and space are abundant.
So the best definition is fitness-based: competition is a process in which one species' fitness, measured as its 'r' (intrinsic rate of increase), drops significantly when another species is present.

Evidence for competition.

Laboratory: Gause and other ecologists showed that when resources are limited, the competitively superior species eventually eliminates the other. In nature, evidence for such exclusion is not always conclusive, but there is strong circumstantial evidence.
Abingdon tortoise of the Galapagos Islands became extinct within a decade after goats were introduced, apparently because the goats browsed more efficiently.
Competitive release: a species restricted to a small area by a superior competitor expands its range dramatically when the competitor is experimentally removed.
Connell's field experiments on the rocky sea coasts of Scotland: the larger, competitively superior barnacle *Balanus* dominates the intertidal area and excludes the smaller barnacle *Chthamalus* from that zone.
In general, herbivores and plants seem more adversely affected by competition than carnivores.

Gause's competitive exclusion principle. Two closely related species competing for the same resources cannot coexist indefinitely, and the competitively inferior one will eventually be eliminated. This may hold when resources are limiting, but not otherwise.

Coexistence: resource partitioning. Recent studies do not support sweeping generalisations. Species facing competition may evolve mechanisms that promote coexistence rather than exclusion. Resource partitioning: competitors avoid competition by choosing, for example, different times for feeding or different foraging patterns. MacArthur found that five closely related warbler species sharing one tree coexist without competing because they forage differently (behavioural differences, such as feeding in different parts of the tree).

Competition vs other interactions. In competition both species lose (−/−); in amensalism one is harmed and the other unaffected (−/0). Predation and parasitism are +/−.

Flow chart: two species use the same resource → resource limiting? → yes: superior competitor's r stays high, inferior's r falls → inferior excluded (Gause) unless they partition the resource (time, place, food type) → coexistence. Resource abundant but one species interferes → interference competition → r of the other still falls.

Draw and label: Connell's barnacle zones. Checklist: a vertical rocky shore with three bands: upper zone (exposed longer), middle zone, lower zone (submerged longer); small barnacles drawn in the upper zone, large barnacles crowding the middle and lower zones; an arrow showing Chthamalus spreading downwards when Balanus is removed. Labels: Chthamalus (smaller), Balanus (larger, superior), intertidal zone, high tide, low tide, competitive release.

Competition and Competitive Exclusion: key facts

Definition by fitness: $r_{with competitor} < r_{alone} (significantly)$; r = intrinsic rate of increase.
Sign pair: $competition = - / -; amensalism = - /0$; Both species are harmed in competition.
Gause's principle: $same limiting resource + closely related \Rightarrow no indefinite coexistence$; Inferior competitor is eliminated (when resources are limiting).
Coexistence: $resource partitioning: different time, place or way of feeding$; MacArthur's five warbler species on one tree.
Percentage reduction in r: $% reduction = \frac{r _{alone} - r _{with}}{r _{alone}} \times 100$; A way to quantify competition in data questions.

How to answer Competition and Competitive Exclusion questions

Competition questions test the definition, Gause's principle, and examples. Use the NCERT evidence in the right place.

1Define competition through fitness (r), not just 'fighting for food'.
2Correct the two misconceptions with examples: unrelated species (flamingoes and fishes) and non-limiting resources (interference competition).
3State Gause's principle with its condition (closely related, same limiting resource) and outcome (inferior eliminated).
4Give evidence: laboratory experiments, Abingdon tortoise and goats, competitive release, Connell's barnacles.
5For coexistence, explain resource partitioning with MacArthur's warblers.
6In data questions, compare r (or numbers) of each species alone and together; the one whose r falls more is the inferior competitor.

Competition and Competitive Exclusion in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Measuring competition through r

Suppose an ecology club grows two species of duckweed in tanks. Grown alone, species P has r = 0.30 per day and species Q has r = 0.25 per day. Grown together in a tank with limited nutrients, P has r = 0.27 and Q has r = 0.05.

Calculate the percentage reduction in r for each species, identify the superior competitor, and predict the long-term outcome. What change might allow coexistence?

Try it first, then show the solutionSolution

P: (0.30 − 0.27)/0.30 × 100 = 10% reduction.
Q: (0.25 − 0.05)/0.25 × 100 = 80% reduction.
Both are harmed (−/−), but Q far more; P is the superior competitor.
With limiting nutrients, Gause's principle predicts Q will eventually be eliminated. If the two used different nutrients, depths or growth times (resource partitioning), or if nutrients were not limiting, they might coexist.

Answer: P loses 10%, Q loses 80% of its r; P is superior; Q is likely excluded unless resources are partitioned or not limiting.

Memory hook: Whoever's r falls most loses the race.

Tricks to remember Competition and Competitive Exclusion

Short memory aids. Say each one aloud once, then use it on a question the same day.

Complete competitors cannot coexist
Gause: same need + same place + scarce supply = one must go.
Use it when: Stating the principle.
Big Balanus bullies
Bigger Balanus pushes small Chthamalus out.
Use it when: Connell's experiment.
Warblers share the tree
Five warblers, one tree, different branches: partitioning.
Use it when: Resource partitioning example.

Competition and Competitive Exclusion: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Define competition · 3 marks · Short answerBoard level

How is competition best defined? Why is the older idea 'competition occurs only between closely related species for limited resources' not entirely true?

Try it first, then show the solutionSolution

Competition is a process in which the fitness of one species (its intrinsic rate of increase, r) is significantly lower in the presence of another species (1 mark).
Unrelated species can compete: flamingoes and resident fishes in South American lakes compete for zooplankton (1 mark).
Resources need not be limiting: in interference competition one species reduces the feeding efficiency of another even when food and space are abundant (1 mark).

Answer: Lower r in presence of another species; unrelated species and non-limiting resources can still involve competition.

Example 2: Gause's principle · 2 marks · Very short answerBoard level

State Gause's competitive exclusion principle. Under what condition does it hold?

Try it first, then show the solutionSolution

Two closely related species competing for the same resources cannot coexist indefinitely; the competitively inferior one will eventually be eliminated (1 mark).
It holds when the resources are limiting, but not otherwise (1 mark).

Answer: Closely related species on the same resource cannot coexist indefinitely; true when resources are limiting.

Example 3: Evidence from nature · 2 marks · Very short answerHarder board level

Give two pieces of evidence for competition in nature.

Try it first, then show the solutionSolution

The Abingdon tortoise of the Galapagos became extinct within a decade of goats being introduced, apparently because goats browsed more efficiently (1 mark).
Connell's barnacles: on Scottish rocky coasts the larger Balanus excludes the smaller Chthamalus from the intertidal zone; competitive release occurs when the superior competitor is removed (1 mark).

Answer: Abingdon tortoise and goats; Connell's Balanus and Chthamalus (competitive release).

Example 4: Resource partitioning · 3 marks · Short answerHarder board level

Explain resource partitioning with the example studied by MacArthur. Why does it not contradict Gause's principle?

Try it first, then show the solutionSolution

Resource partitioning: competing species avoid competition by using the resource differently, e.g. different feeding times or foraging patterns (1 mark).
MacArthur found five closely related warbler species living on the same tree coexisting because of behavioural differences in their foraging activities (1 mark).
Gause's principle applies to species competing for exactly the same limiting resource; by partitioning, the warblers no longer use exactly the same resource, so they can coexist (1 mark).

Answer: Different foraging behaviour lets five warblers share one tree; they no longer compete for identical resources.

Example 5: Competitive release · 3 marks · Short answerHarder board level

What is competitive release? Explain how Connell's experiment demonstrates it.

Try it first, then show the solutionSolution

Competitive release: a species restricted to a small area by a superior competitor expands its range dramatically when the competitor is removed (1 mark).
On Scottish rocky coasts, the larger barnacle Balanus dominates the intertidal zone and excludes the smaller Chthamalus (1 mark).
When Balanus is removed experimentally, Chthamalus spreads into the zone it was excluded from, showing that competition limited its distribution (1 mark).

Answer: Range expansion after removing a superior competitor; Chthamalus spreads when Balanus is removed.

Example 6: Two flasks · 3 marks · Short answerHOTS / competency

Suppose two closely related protozoan species are grown separately and together in flasks with a fixed daily food supply. Alone, both reach a stable population. Together, species X stabilises while species Y declines to zero by day 16. (a) What principle does this illustrate? (b) What would you change in the setup to test whether the outcome depends on limited food? (c) Predict the result if the two had used different food particles.

Try it first, then show the solutionSolution

(a) Gause's competitive exclusion principle: X is the superior competitor and Y is eliminated (1 mark).
(b) Supply food in excess (non-limiting) and repeat; if both persist, exclusion depended on limited food (1 mark).
(c) With different food particles they would partition the resource and could coexist, as both would reach stable populations (1 mark).

Answer: Competitive exclusion; test with abundant food; different foods → coexistence.

Where students lose marks in Competition and Competitive Exclusion

The slips examiners see most often, and what to write instead.

Mistake: Defining competition only as a fight for limited food
Older textbook idea.
Wrong working
Competition is when related species fight for limited food.
Right working
Fitness (r) of one species is significantly lower in the presence of another; resources need not be limiting and species need not be related.
Do this: Use the r-based definition.
Usually costs: 1 mark.
Mistake: Stating Gause's principle without its condition
Students omit 'limiting resources'.
Wrong working
Two species can never share a resource.
Right working
Closely related species competing for the same limiting resource cannot coexist indefinitely.
Do this: Include 'closely related', 'same', 'limiting'.
Usually costs: 1 mark.
Mistake: Swapping Balanus and Chthamalus
Names are unfamiliar.
Wrong working
Chthamalus excludes Balanus.
Right working
Larger Balanus excludes smaller Chthamalus.
Do this: Big Balanus bullies.
Usually costs: 1 mark.
Mistake: Calling competition +/−
Students think the winner benefits.
Wrong working
Competition: +/−.
Right working
Competition: −/− (both lose fitness; one may lose much more).
Do this: Learn the sign table.
Usually costs: 1 mark.

Exam tips

Start any competition answer with the r-based definition.
Quote the NCERT evidence by name: flamingoes and fishes, Abingdon tortoise, Connell's barnacles, MacArthur's warblers.
Write 'competitive release' and 'resource partitioning' as technical terms.
For Gause's principle, add the qualifier that it holds when resources are limiting.
Draw the barnacle zonation sketch for 3-mark questions on Connell.

Practice questions on Competition and Competitive Exclusion

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
Which pair shows that unrelated species can compete for the same resource?
(a) Balanus and Chthamalus (b) flamingoes and fishes in South American lakes (c) five warbler species (d) cuckoo and crow
Hint
Zooplankton.
Answer
(b) flamingoes and fishes in South American lakes.
Q2 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Competition can occur even when food and space are abundant.
Reason (R): In interference competition, one species reduces the feeding efficiency of another by its inhibitory presence.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Interference.
Answer
(a).
Q3 · 1 mark · Very short answerBoard level
Which group appears to be more adversely affected by competition: herbivores and plants, or carnivores?
Hint
NCERT generalisation.
Answer
Herbivores and plants.
Q4 · 3 marks · Short answerHOTS / competency
Suppose goats are introduced to an island where a native tortoise browses on shrubs. Using the Galapagos example, predict what may happen, explain the type of interaction, and suggest a conservation measure.
Hint
Abingdon tortoise.
Answer
Goats browse more efficiently, reducing food for the tortoise; the tortoise's fitness (r) falls and it may go extinct, as the Abingdon tortoise did within a decade of goats arriving in the Galapagos. This is interspecific competition (−/−, though the tortoise suffers far more). Conservation: remove or control the introduced goats and protect the tortoise's habitat.
Q5 · 4 marks · Case-basedHarder board level
Read and answer.
Suppose researchers count barnacles on a Scottish rocky shore. In plots with both species, Chthamalus occurs only in the upper zone. In plots where Balanus is removed, Chthamalus is found in the upper and middle zones within two years.
(a) Name the phenomenon seen after removal. (1)
(b) Which species is the superior competitor and why? (1)
(c) What does this experiment show about the actual vs potential range of Chthamalus, and which scientist did similar work? (2)
Hint
Competitive release; Connell.
Answer
(a) Competitive release. (b) Balanus: it is larger and dominates the intertidal zone, excluding Chthamalus. (c) Chthamalus can live in the middle zone (potential range) but is confined to the upper zone by competition (actual range); Connell carried out such field experiments.
Q6 · 2 marks · Very short answerHarder board level
Why is evidence for competitive exclusion easier to obtain in the laboratory than in nature?
Hint
Controlled limited resources.
Answer
In the lab, resources can be kept limited and other factors controlled, so the superior competitor clearly eliminates the other (as Gause showed). In nature, resources vary, species may partition resources or move, and other interactions (predation, disturbance) act, so evidence is often only circumstantial.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.How is competition best defined?Show answer
As a process in which the fitness (r) of one species is significantly lower in the presence of another.
2.What happened to the Abingdon tortoise?Show answer
It became extinct within a decade after goats were introduced to its Galapagos island.
3.Which barnacle is the superior competitor in Connell's study?Show answer
Balanus.
4.Who showed resource partitioning in warblers?Show answer
MacArthur.
5.What is interference competition?Show answer
Reduction in feeding efficiency of one species by the inhibitory presence of another, even when resources are abundant.
6.Sign pair for competition?Show answer
−/−.

Competition and Competitive Exclusion: frequently asked questions

What is Gause's competitive exclusion principle?

Two closely related species competing for the same limiting resource cannot coexist indefinitely; the competitively inferior species is eventually eliminated.

What is resource partitioning?

A way competing species coexist by using a resource differently, such as feeding at different times or in different ways; e.g. MacArthur's five warbler species on one tree.

What is competitive release?

The expansion of a species' range when a superior competitor that restricted it is removed, as with Chthamalus after Balanus is removed.

Can unrelated species compete?

Yes. Flamingoes and resident fishes in some South American lakes compete for zooplankton.

Is competition always for limited resources?

No. In interference competition, one species reduces another's feeding efficiency even when food and space are abundant.