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Competitive exclusion

The principle is not wrong. It is conditional — and a lake in summer meets almost none of its conditions.

Two species limited by exactly the same thing, in an unchanging environment, cannot coexist for ever. Almost nothing about a real environment is unchanging — which is why the world is full of similar species living together.

The principle is usually stated as a fact about nature and is better understood as a piece of arithmetic. If two populations are limited by one identical resource, and nothing else varies, then whichever converts that resource into offspring slightly more efficiently will end up with all of it. The other does not need to be beaten in any dramatic sense; it simply grows a fraction more slowly, indefinitely, and indefinitely is a long time. Gause demonstrated it in culture tubes, and the result is real. What is usually left out is that the same programme produced the opposite outcome. Two paramecia that used the same tube differently — one feeding in suspension, the other on the floor of the vessel — persisted together indefinitely at lower densities. The exclusion and the coexistence came from the same experiments, and together they say something more useful than either alone: what matters is not whether two species share a resource but whether anything at all differs in how they are limited. Then there is the case that broke the tidy version. Open water is about as unstructured a habitat as exists, with a handful of limiting nutrients — and it supports enormous numbers of coexisting phytoplankton species, far more than the principle permits. The paper that pointed this out was written by the ecologist who had done most to formalise the niche in the first place, which is a piece of scientific conduct worth noticing. His suggestion was that the environment never sits still long enough for exclusion to finish: conditions shift, the advantage moves, and the outcome is repeatedly reset. Modern coexistence theory adds the mechanisms — fluctuation, spatial structure, self-limitation, predation, disturbance — and turns "why do so many species coexist" from a paradox into a question with testable answers.

Early coverage · 45% complete · reviewed 2026-09-04

What this page covers

A theoretical result, demonstrated in laboratory culture with protists and applied across taxa. Its interest lies in how often its assumptions fail.

Often confused with: A law about nature, when it is a consequence of specific assumptions; "Two species cannot use the same resource", which any pond in summer contradicts; A prediction that similar species will be found apart, which is not reliably what happens

Quick facts

The conditions
One identical limiting resource, unchanging environment, equilibrium
Gause got both results
One species excluded; two coexisted when they used the tube differently
The plankton problem
Far more species coexist in open water than the principle allows
What coexistence needs
Each species held back more by its own kind than by the other

What the principle actually claims

With the conditions attached, which is where it usually goes wrong.

Two species limited by exactly the same thing, in an unchanging environment, cannot coexist indefinitely. Nature rarely offers an unchanging environment — which is why so many similar species do coexist.

Established

Specialists would state this without hedging. Multiple independent lines of evidence agree.

Competitive exclusion follows where two species are limited by an identical single resource in a spatially and temporally homogeneous environment at equilibrium. Relaxing any of those assumptions — fluctuation, spatial structure, additional limiting factors, predation, disturbance — permits coexistence.

Who this applies to
A theoretical result with specific assumptions, demonstrated in laboratory culture.Do not extend this beyond the taxa listed — the popular version over-reaches.
Studied in
Animalia, Plantae, Protista
Why we rate it this way, and what the caveats are
EstablishedHigh confidence

Both halves are well established: the exclusion result in controlled culture, and the abundant coexistence that the assumptions fail to cover.

How far it can be extended

The principle is about what follows from its assumptions. Applying it to a field system requires showing the assumptions hold there, which is usually not done.

Caveats

  • The principle is not wrong; it is conditional, and the conditions are strong.
  • Demonstrating that two field species are limited by the same single resource is much harder than assuming it.

Where researchers disagree

  • Open water supports far more phytoplankton species than its few limiting resources should allow — the paradox of the plankton, published by the same author who formalised the niche.

Still unanswered

  • Which coexistence mechanisms dominate in which systems, which is largely unresolved for most communities.

Last reviewed 2026-09-04

The evidence (3 studies)

  • Supports · primary

    The Struggle for Existence

    Gause, 1934 · Williams & Wilkins

    The culture experiments, including the case where two species coexisted.

  • Challenges · primary

    The paradox of the plankton

    Hutchinson, 1961 · The American Naturalist

    The observation that plankton diversity far exceeds what the principle permits, which locates the failure in the assumptions.

  • Supports · primary

    Mechanisms of maintenance of species diversity

    Chesson, 2000 · Annual Review of Ecology and Systematics

    Sets out which mechanisms permit coexistence and why.

How we know

Two species, one tube of food

If two species make their living the same way, can they share a resource — or does one of them go?

Paramecium species were grown on a bacterial food supply in controlled culture. Each was first grown alone, establishing the density it reaches by itself. Then pairs were grown together in the same tube on the same supply, with the medium renewed on a fixed schedule, and population densities counted over time.

What happened

Where both species fed the same way, one declined to extinction and the other persisted. Where one fed in suspension and the other on the vessel floor, both persisted together at reduced densities.

What it shows

That the outcome depends on how the resource is used, not on which species is stronger in general. Both results come from the same programme, and the second is the one usually left out: two species in one tube can coexist if they use it differently.

What it does not show

It does not show that two species can never share a resource in nature. A culture tube is constant, undisturbed, single-resourced and offers no refuge — which is precisely the condition under which exclusion is most likely, and precisely what a pond or a shore is not.

The controls — what makes this evidence rather than a story
  • Each species grown alone in the same conditions, so the mixed result is compared against what each does by itself.
  • A constant renewal regime, removing food supply variation as an explanation for any decline.
  • Replicated cultures, so a single crash cannot carry the conclusion.

From The Struggle for Existence

Diagram

The same experiment produced both results

Exclusion and coexistence, from one programme.

The same experiment produced both resultsSame way of feedingBoth suspension feeders, onerenewed food supply→ one species goes extinctThis is the famous resultDifferent way of feedingOne in suspension, one on thefloor of the same vessel→ both persist, at lower densityThis one is usually left outSo the principle is about how a resource is used, not whether it is shared.A culture tube is constant, undisturbed and offers no refuge. A pond is none of those.
The same explanation in words

Two panels. On the left, marked as the failing case for coexistence: two species feeding the same way on one renewed food supply, where one goes extinct — the famous result. On the right: two species feeding differently in the same vessel, one in suspension and one on the floor, where both persist at lower density — the result usually left out. Beneath, a summary states that the principle is about how a resource is used rather than whether it is shared, and that a culture tube is constant, undisturbed and offers no refuge, while a pond is none of those.

Note what the exclusion required: a constant environment, one renewed resource, no refuge, no predator, no disturbance, and enough time. A culture tube supplies all of that by design. It is a good experiment precisely because it is artificial — it isolates the mechanism — and it is a poor model of a shore, a pond or a forest for the same reason.

The case that did not fit

Published by the person with most to lose from it.

Open water has little structure and few limiting nutrients, and it holds dozens of coexisting phytoplankton species. On the tidy reading of the principle that should be impossible. The paper making the point came from the ecologist who had formalised the niche a few years earlier, and it did not attempt to explain the difficulty away: it named it, and proposed that the environment simply never stays still long enough for exclusion to complete.

Species coexist stably when each is held back more by its own kind than by the other. Being different is not enough on its own — what matters is whether a species recovers when it becomes rare.

Well supported

Good evidence backs this, though some details remain open.

Stable coexistence requires intraspecific competition to exceed interspecific competition, producing a positive growth rate for each species when rare. Mechanisms achieving this are termed stabilising; those merely reducing fitness differences are equalising and do not by themselves produce stable coexistence.

Who this applies to
Coexistence theory as applied across communities.
Studied in
Animalia, Plantae
Why we rate it this way, and what the caveats are
Well supportedModerate confidence

The theory is settled and internally rigorous. Measuring the required quantities in field systems is difficult, so empirical coverage is thinner than the framework’s prominence suggests.

How far it can be extended

A general theoretical result, with empirical support in a growing but still limited set of systems.

Caveats

  • A framework for asking the question rather than an answer to it; which mechanism operates must be established system by system.
  • The measurements it requires are demanding, and many field applications approximate them.

Still unanswered

  • How much of observed coexistence is stabilised at all, as against species declining slowly enough that exclusion has not yet finished.

Last reviewed 2026-09-04

The evidence (1 study)

The modern answer is a list of mechanisms rather than a single one. Environments fluctuate and species respond differently, so the advantage moves. Space is patchy and dispersal limited, so a poorer competitor persists where the better one has not arrived. Predators and disturbance knock back whichever species is winning. Each of these can be measured, and the useful question stopped being "are these species too similar to coexist" and became "does each recover when it becomes rare, and why".

Related

How to use the principle without misusing it

It predicts what follows from conditions, not what a field looks like.

The principle is a tool for reasoning, and the reasoning runs one way. If you can establish that two species are limited by the same single thing in a constant setting, you can predict that one will go. You cannot run it backwards: finding two similar species living together does not show that they must differ in some undiscovered way, though it is frequently used that way. It may mean the environment fluctuates, or that a predator is holding both down, or that exclusion is underway and has not finished.

That last possibility is the one most often forgotten. Coexistence observed over a few years does not establish that it is stable. Some of what looks like species sharing a resource may be a slow exclusion that nobody has watched for long enough.

The research behind this page

6 studies, newest first. Each one has a page explaining what it found and what it could not show.

2006Science

Evolution of character displacement in Darwin’s finches

After the larger species established and drought reduced the large seeds both depended on, beak size in the resident species shifted downward within a generation, with survival favouring smaller-beaked birds.

2000Annual Review of Ecology and Systematics

Mechanisms of maintenance of species diversity

Stable coexistence requires that intraspecific competition exceed interspecific competition.

1994Science

Experimental evidence that competition promotes divergence in adaptive radiation

With the competitor present, generalists resembling it grew more slowly, while those differing from it grew better.

1961The American Naturalist

The paradox of the plankton

Observed plankton diversity substantially exceeds the number of limiting resources, so the conditions under which competitive exclusion operates are evidently not met in open water.

1958Ecology

Population ecology of some warblers of northeastern coniferous forests

The species used the trees differently.

1934Williams & Wilkins

The Struggle for Existence

Grown alone, each species reached a stable density.

This page is a stop on a longer route

A guided journey reads several subjects in a deliberate order, with an argument for why one follows another. You can join in the middle.

Where to go from here

Each of these follows from something on this page — a relationship in the evidence, a claim people ask about, or the next mechanism along.

How complete this page is, and what it is still missing

NatureHQ publishes its own gaps. This page is at 45% completeness against what we would call a finished subject, and was last reviewed on 2026-09-04. It carries 4 claims and answers 0 mapped search questions.

  • no research from the last few years is attached — check for newer work
  • no popular claim about this subject has been checked yet
  • The Lotka–Volterra formalism behind the principle is described in words only.
  • Empirical tests of modern coexistence theory in field systems are mentioned but not surveyed.
  • Priority effects, where arrival order decides the outcome, are noted only in passing.