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Ecologyecological relationship

Facilitation

A nurse plant shelters a seedling until the seedling is big enough to kill it.

One organism making conditions better for another, without needing anything back. A shrub shading a seedling is not helping it — it is casting a shadow, and the seedling is using it.

For most of the twentieth century, ecologists asked of two neighbouring plants which was suppressing the other. Competition was the assumed interaction, and where a plant did well beside another the explanation was that it was competing successfully. The alternative — that the neighbour was improving conditions — was under-studied for long enough that its rehabilitation counts as one of the field’s useful corrections. The prediction that made it testable is that facilitation should matter more as conditions get harsher, because there is more to ameliorate: shade in heat, shelter in wind, moisture in drought, salt uptake in a salt marsh. And it can fail. So somebody removed plants’ neighbours at high and low elevation on mountains across several continents, using an identical protocol. High up, removal made target plants do worse. Lower down, on the same mountains, removal made them do better. The sign of the interaction reversed with stress, exactly as predicted. Nurse plants are the case people picture: a seedling establishing in the shade of an established shrub in a desert or on a scree, protected from heat, from desiccation and often from grazing. What makes them a good illustration is the ending. The nurse’s benefit is greatest when the seedling is small and vulnerable, and as the plant grows the relationship inverts — the same neighbour that made establishment possible becomes a competitor for water and light, and in some systems the nurse is eventually killed by the plant it sheltered. Facilitation is not a nicer kind of relationship. It is a net effect that happens to be positive, for now, under these conditions.

Early coverage · 36% complete · reviewed 2026-09-04

What this page covers

Best documented in plants, particularly in harsh environments — alpine, arid and salt-marsh systems — and in animals that create structure others shelter in.

Often confused with: Mutualism, which requires both parties to gain; facilitation does not; Cooperation, which implies something the facilitating organism is not doing

Quick facts

No reciprocity required
One gains; the other need not, and usually is not doing anything for it
The sign flips
Removing neighbours hurt plants high on a mountain and helped them lower down
Harsher means more of it
A stated prediction, tested by identical removals on several continents
And it reverses with age
The nurse that enabled establishment becomes the competitor

The interaction ecology forgot to look for

Because it had already decided the answer was competition.

Remove a plant’s neighbours high on a mountain and it does worse. Do the same lower down and it does better. The sign of the interaction reverses with how hard the conditions are.

Established

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

Paired neighbour-removal experiments at high and low elevation across mountain sites on several continents found reduced target survival and growth following removal at high elevation and increased survival and growth at low elevation, consistent with a shift from net facilitation to net competition as physical stress decreases.

Who this applies to
Alpine plant communities across multiple mountain ranges and continents.
Studied in
Plantae
Why we rate it this way, and what the caveats are
EstablishedHigh confidence

A stated prediction, tested by identical manipulation at many independent sites, with the sign of the effect reversing as predicted.

How far it can be extended

The same protocol was applied at many sites on several continents, with consistent direction of effect.

Caveats

  • Facilitation and competition operate simultaneously; what reverses is the net effect, not the presence of either.
  • Elevation bundles cold, wind and exposure together, and the design does not separate which stress matters.

Still unanswered

  • Whether the reversal holds over decades, since the experiments ran for one or two growing seasons.

Last reviewed 2026-09-04

The evidence (2 studies)

How we know

Pulling out the neighbours, high and low

Are neighbouring plants competitors or shelter? And does the answer change with how harsh the conditions are?

At mountain sites on several continents, target plants had their neighbours removed while paired targets nearby kept theirs. The same paired design was run at high elevation and at lower elevation on the same mountains, using an identical protocol everywhere, and target survival and growth were recorded.

What happened

At high elevation, removing neighbours reduced survival and growth. At lower elevation on the same mountains, removing neighbours improved them. The sign of the effect reversed.

What it shows

That whether a neighbour helps or harms is not a property of the two species but of the conditions they are in. Where it is cold and exposed, a neighbour shelters more than it competes; where conditions ease, the same neighbour is a competitor. It also demonstrates a stated prediction being tested and confirmed, rather than a pattern being explained after the fact.

What it does not show

The experiments ran over one or two growing seasons, and long-term outcomes may differ. Elevation also bundles cold, wind and exposure together, so the design shows that stress matters without identifying which stress. And facilitation and competition operate at once — what reverses is the net effect, not the presence of either.

The controls — what makes this evidence rather than a story
  • Paired targets with neighbours intact at every site, so each removal has its own comparison.
  • The same protocol at high and low elevation on the same mountain, which is what isolates stress from everything else about a site.
  • Many independent sites on several continents, so a result cannot be a property of one mountain.

From Positive interactions among alpine plants increase with stress

Diagram

Remove the neighbours, and see what happens

The same manipulation, at two elevations on one mountain.

Remove the neighbours, and see what happensLow elevation — milderHigh — harsherTarget does betterTarget does worseEffect ofremovalThe neighbour was competing below, and sheltering above.Same species, same manipulation — the sign of the interaction reverses with stress.
The same explanation in words

A horizontal axis runs from low elevation and milder conditions on the left to high elevation and harsher conditions on the right. At the low-elevation end, removing a plant’s neighbours makes the target do better. At the high-elevation end, marked out as the contrasting result, removing them makes the target do worse. A note records that the neighbour was competing below and sheltering above, and that with the same species and the same manipulation the sign of the interaction reverses with stress.

What makes this a good piece of science rather than a good idea is the structure of the test. The prediction — that facilitation should increase with stress — was stated in advance and could have failed. The same manipulation was run at many independent sites. And the result was not a difference in magnitude but a reversal in sign, which is much harder to produce by accident.

Nurse plants, and how the favour ends

The shelter that becomes the competition.

What a nurse plant provides, and when it stops mattering
What the seedling getsWhy it mattersWhen it turns
ShadeLower temperature and water lossOnce the plant needs full light
MoistureSoil under a canopy dries more slowlyOnce roots compete for the same water
Shelter from grazingA spiny nurse deters browsersOnce the plant outgrows the canopy
Improved soilLitter and shade build organic matterRarely — this one usually persists

The third column is what makes nurse plants worth a section rather than a sentence. This is an interaction whose sign changes over the life of one participant: facilitation while the seedling is small, competition once it is not, and in arid systems the sheltered plant sometimes outlives and outcompetes the shrub that made its establishment possible. Nothing about either species changed. The balance did.

Related

Why this changes how a community is read

Two plants growing together are not necessarily fighting.

If facilitation is common in stressful environments, then the structure of those communities cannot be inferred from competition alone — and the plants growing together in a desert or on a mountainside may be there because of each other rather than despite each other. That has practical consequences for restoration: planting into the shelter of existing vegetation can work far better than clearing ground first, which is the intuitive approach and the wrong one where conditions are harsh.

The research behind this page

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

2008Science

Breakdown of an ant-plant mutualism follows the loss of large herbivores

With large herbivores excluded, trees reduced their production of swollen thorns and nectar.

2002Nature

Positive interactions among alpine plants increase with stress

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

2001Ecology Letters

The exploitation of mutualisms

Exploitation is present in most well-studied mutualisms, by partners and by outsiders, and mutualisms nevertheless persist.

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.

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 36% completeness against what we would call a finished subject, and was last reviewed on 2026-09-04. It carries 2 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
  • Salt-marsh and arid-system facilitation, both major literatures, are mentioned rather than covered.
  • Facilitation between animals, and animals facilitating plants, are under-covered.
  • How facilitation and competition combine into a net effect is described qualitatively only.