Skip to content
NatureHQ

Ecologyecological relationship

Competition

Most competition involves no contact at all. Something was there, and then it was not.

Two organisms needing the same limited thing. Most of it involves no contact at all — one simply gets there first. And the strongest competition a individual faces is usually from its own species, not another one.

Competition splits into two kinds that look nothing alike. In exploitation competition nobody meets: one organism consumes a resource and it is no longer there for the next, and the entire interaction is mediated by the thing that has gone. A plant taking up soil water competes with its neighbours without any of them ever touching. Interference competition is the version people picture — one organism directly preventing another from getting at something, by fighting, by chemical suppression, or in the barnacle case by physically growing over it and levering it off the rock. The distinction matters because they leave different evidence: interference can be watched, exploitation has to be inferred from what is missing. The more useful correction is about who competes with whom. Competition within a species is usually the fiercest, because two individuals of one kind need the same things in almost exactly the same amounts and at the same time. Two species, however similar, differ somewhere. This is not a footnote — it is the condition that makes coexistence possible at all, and modern coexistence theory is built on precisely that comparison: a species can persist alongside another if it is held back more by its own kind than by the other. The final thing worth holding onto is evidential. Competition is the most over-applied explanation in ecology, because it explains so much so easily. Two similar species with adjoining ranges, or using a habitat differently, are exactly what competition would produce — and also exactly what you would see if the two places simply differ, or if the species have always been unalike. The pattern does not discriminate. The studies that carry weight here are the ones where somebody removed a competitor, or added one, and watched.

Early coverage · 56% complete · reviewed 2026-09-04

What this page covers

Competition occurs wherever organisms need something in limited supply, which is everywhere. The best evidence comes from systems where a competitor can be removed and the result watched.

Often confused with: Fighting, which is one form of it and not the common one; The default explanation for any two species with adjoining ranges; Something that happens only between species, when competition within a species is usually stronger

Quick facts

Two kinds
Exploitation — take it first; interference — stop the other reaching it
Your own kind is worse
Intraspecific competition is usually the stronger of the two
And that is why coexistence works
Species persist when each limits itself more than it limits the other
The evidence problem
A distribution pattern fits competition — and fits other things equally well

Taking it first, or keeping you off it

Two mechanisms that leave very different evidence.

The forms of competition, and how each shows itself
FormWhat happensWhat you can observe
ExploitationOne organism consumes the resource; the other finds less of itNothing directly — only the absence, and slower growth
InterferenceOne organism physically or chemically prevents accessThe act itself: fighting, overgrowth, chemical suppression
Within a speciesIndividuals need identical things at identical timesDensity-dependent growth, survival and territory size
Between speciesOverlapping but never identical requirementsRange edges, and changes when a competitor is removed

Exploitation is the common case and the harder one to study, because there is nothing to watch. The barnacles are unusual in being an interference system where the mechanism is visible: one species grows underneath the other and levers it off the rock. Most competition is a resource that simply is not there any more, and establishing that it was taken by a particular competitor requires removing that competitor and seeing whether the resource comes back.

Where a species lives has boundaries of two kinds: what it can physically stand, and what another organism will let it have. You cannot tell them apart by looking — you have to remove the neighbour.

Established

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

Removal experiments on rocky shores show a species’ lower distributional limit set by interspecific competition and its upper limit by physical tolerance. The distribution occupied in the presence of competitors (the realised niche) is a subset of that occupiable in their absence (the fundamental niche).

Who this applies to
Demonstrated experimentally in intertidal barnacles; the distinction applies generally, the particular boundaries do not.
Studied in
Chthamalus stellatus, Semibalanus balanoides, Animalia
Why we rate it this way, and what the caveats are
EstablishedHigh confidence

A direct field manipulation with matched controls, in a system where the mechanism — crushing, undercutting, smothering — was observed rather than inferred.

How far it can be extended

Removal experiments in many systems show distributions expanding when competitors are excluded.

Caveats

  • Removing a competitor changes more than competition; here the crushing and smothering were directly observed, which not every system allows.
  • Many range edges are set by neither of these — dispersal limitation and predation also draw boundaries.

Still unanswered

  • How often range edges attributed to climate are in fact set by an unexamined competitor.

Last reviewed 2026-09-04

The evidence (2 studies)

Why competition is the most over-applied explanation in ecology

Because it fits everything, including things it did not cause.

Suppose two similar species occupy adjoining areas, or feed in different parts of the same tree. Competition would produce that. So would the two areas differing in some way nobody measured, and so would the two species having been unalike long before they met. The observation does not distinguish between them, and a great deal of mid-century ecology treated it as though it did.

How we know

Measuring what it costs to resemble your competitor

Similar species often differ more where they overlap. Does competition actually push them apart, or does that pattern have another explanation?

Generalist sticklebacks were introduced to experimental ponds. Half the ponds also received a specialist competitor feeding on open-water plankton; half did not. Growth of individual generalists was then measured against their body shape, so the relationship between resembling the competitor and doing well could be estimated directly.

What happened

With the competitor present, generalists most resembling it grew worst and those differing from it grew best. Without the competitor, the relationship between shape and growth disappeared.

What it shows

That competition generates selection against similarity — the mechanism character displacement needs, measured rather than assumed. The usual evidence for displacement is that species differ more where they co-occur, which is consistent with competition and equally consistent with the two places simply being different. This design removes that ambiguity.

What it does not show

Growth over one season is a component of fitness, not the whole of it, and reproduction was not measured. Nor does one experimental system show that displacement is a common outcome in nature — only that the selection it requires is real.

The controls — what makes this evidence rather than a story
  • Competitor present or absent by design rather than by geography, which is what removes the alternative explanations.
  • Growth measured on individuals against their own morphology, so selection is estimated rather than inferred from group means.
  • Ponds otherwise matched, so the difference between treatments is the competitor.

From Experimental evidence that competition promotes divergence in adaptive radiation

Diagram

The shape of the strongest evidence in ecology

Before, manipulation, comparison, and the limitation that always attaches.

The shape of the strongest evidence in ecologyBeforeRecord what is thereRemoveTake one species out, keepCompareAgainst a matched untoucheInterpretWhat changed, and what elsThe control plot is the whole argument. Without it, the change is a coincidence.And removal always changes more than the one thing removed — which is why thelast step is a limitation and not a conclusion.
The same explanation in words

Four steps in sequence. Before: record what is there. Remove: take one species out and keep it out — the step marked out as the manipulation. Compare: against a matched untouched plot. Interpret: what changed, and what else could have. Two notes follow. The control plot is the whole argument, because without it the change is a coincidence. And removal always changes more than the one thing removed, which is why the final step is a limitation rather than a conclusion.

The remedy is not scepticism about competition, which is real and pervasive. It is insisting on the manipulation. Where a competitor has been removed, added, or excluded by a cage, competition stops being a story that fits and becomes a measured effect — and sometimes the effect is absent, which is equally worth knowing.

Related

Your own species is the real competition

And that is what makes sharing possible.

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)

Two individuals of one species need the same foods, at the same sizes, in the same places, at the same time of year. Two species overlap and differ. That asymmetry is the foundation of coexistence: if being common hurts you more than it hurts your competitor, then whenever you become rare you recover, and whenever the other becomes rare it recovers too. Neither can drive the other out. Being different is not what does the work — being self-limiting is.

The research behind this page

10 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.

2002Nature

Positive interactions among alpine plants increase with stress

At high elevation, removing neighbours reduced target plant survival and growth.

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.

1994Trends in Ecology & Evolution

Positive interactions in communities

Facilitation is widespread, particularly in physically stressful environments, and communities in such settings may be structured as much by amelioration as by competition.

1961Ecology

The influence of interspecific competition and other factors on the distribution of the barnacle Chthamalus stellatus

Where the competitor was removed, the high-shore species survived and grew well below its usual limit.

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.

1957Cold Spring Harbor Symposia on Quantitative Biology

Concluding remarks

The niche can be treated as a multidimensional volume rather than a place, which makes it possible to ask precise questions about overlap, breadth and limiting similarity.

1934Williams & Wilkins

The Struggle for Existence

Grown alone, each species reached a stable density.

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 56% completeness against what we would call a finished subject, and was last reviewed on 2026-09-04. It carries 6 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
  • Plant competition, including below-ground competition for water and nutrients, is under-covered relative to its importance.
  • Allelopathy — chemical interference between plants — is mentioned only in passing.
  • Competition between microbes, where much of the recent experimental work is, is not treated.