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Commensalism vs. Amensalism: What's the Difference?

Edited by Aimie Carlson || By Janet White || Published on January 1, 2024
Commensalism is a relationship where one organism benefits and the other is unaffected, while amensalism is where one is harmed and the other unaffected.

Key Differences

Commensalism is an ecological interaction where one species gains a benefit, such as food or shelter, while the other species is not significantly affected, neither harmed nor helped. In contrast, amensalism involves an interaction where one species is inhibited or harmed by the presence of another, which remains unaffected by the interaction.
An example of commensalism is a small fish swimming close to a larger fish, gaining protection from predators without affecting the larger fish. Amensalism, on the other hand, is exemplified by a tree shading out smaller plants beneath it; the tree is unaffected, while the plants suffer from lack of sunlight.
Commensal relationships are often seen as a form of coexistence where one species capitalizes on another's lifestyle without causing detriment. In amensal relationships, one species inadvertently or passively suppresses the growth or well-being of another without deriving any benefit.
The impact of commensalism on ecosystems is generally neutral for one species and beneficial for the other. However, in amensalism, the impact is detrimental to one species without any corresponding gain or loss to the other.
Commensalism often evolves in ecosystems where species are in close proximity, leading to opportunistic benefits. Amensalism, by comparison, often arises from competitive exclusion where the presence of a dominant species inadvertently suppresses the development of others.

Comparison Chart

Impact on Species

One benefits, the other unaffected
One harmed, the other unaffected


A bird nesting in a tree; the tree is unaffected
A large plant shading out a smaller plant

Ecological Role

Opportunistic coexistence
Competitive exclusion or suppression

Typical Interaction

Beneficial for one without affecting the other
Detrimental to one without affecting the other

Evolutionary Outcome

Often leads to symbiotic relationships
Can lead to ecological displacement or suppression

Commensalism and Amensalism Definitions


A non-harmful coexistence where one species takes advantage of another's existence.
Remoras riding on sharks exhibit commensalism, benefiting from leftovers.


A relationship where the presence of one organism impedes another without benefit.
Large animals trampling smaller plants without noticing is an example of amensalism.


An ecological interaction benefiting one species without harming the other.
Birds nesting in trees is a form of commensalism where trees remain unaffected.


An interaction in nature where one organism inhibits another without gaining.
In amensalism, a fungus can overshadow a plant, harming it but not benefiting the fungus.


A relationship where one organism benefits and the other is neutral.
In commensalism, barnacles attach to whales, gaining transport without affecting the whales.


An ecological relationship where one organism is harmed, and the other is unaffected.
In amensalism, a tree's shade can inhibit the growth of grass below.


A symbiotic relationship where one party gains and the other is neither helped nor harmed.
The presence of certain skin bacteria living on humans is an example of commensalism.


A negative interaction for one species with no impact on the other.
The secretion of antibiotics by some fungi is amensalism, harming bacteria.


An interaction in nature where one organism benefits from another's presence.
Commensalism is seen in epiphytes growing on trees without harming them.


A one-sided negative ecological impact where one species suffers, and the other is neutral.
Amensalism occurs when walnut trees release chemicals that inhibit other plant growth.


A symbiotic relationship between two organisms of different species in which one derives some benefit while the other is unaffected.


An association between two organisms of different species in which one is harmed or inhibited while the other is unaffected.


(ecology) A sharing of the same environment by two organisms where one species benefits and the other is unaffected. An example is barnacles on whales.


(ecology) A form of symbiosis in which one species is harmed or impeded and the other is unaffected.


The act of eating together; table fellowship.


The act of eating together; table fellowship.


The relation between two different kinds of organisms when one receives benefits from the other without damaging it


What is a simple definition of commensalism?

A relationship where one organism benefits, and the other is unaffected.

Is commensalism common in marine environments?

Yes, it's quite common, like with remoras and sharks.

Are human activities an example of commensalism?

Sometimes, like when urban animals thrive without impacting humans.

What is a simple definition of amensalism?

A relationship where one organism is harmed, and the other is unaffected.

Is amensalism intentional?

Usually, it's unintentional and a byproduct of another organism's existence.

How does amensalism differ from competition?

In competition, both species are affected, but in amensalism, only one is harmed.

Can amensalism be reversed or stopped?

Sometimes, if the suppressing factor is removed or reduced.

Can amensalism lead to the extinction of a species?

Potentially, especially if the harmed species can't adapt or move.

Can amensalism be beneficial for biodiversity?

Not usually, as it often suppresses certain species.

Can commensalism evolve into mutualism?

Yes, if both organisms start benefiting, it can evolve into mutualism.

Does commensalism affect the population dynamics of an ecosystem?

It can, especially if it significantly benefits one species.

Can amensalism occur in agricultural settings?

Yes, like when large crops overshadow smaller plants.

Are there human-induced examples of amensalism?

Yes, like pollution affecting wildlife while humans remain largely unaffected.

How does commensalism affect the host organism?

In commensalism, the host is not affected positively or negatively.

Is it easy to identify commensal relationships in nature?

It can be challenging, as the impact on the host may not be immediately apparent.

Are there any risks in a commensal relationship?

Generally, there are minimal risks to the unaffected organism.

How does amensalism differ from predation?

In predation, one organism feeds on another, while in amensalism, there's no consumption.

Can amensalism be a form of chemical warfare in nature?

Yes, when one species releases toxins that harm another.

Are there examples of amensalism in urban environments?

Yes, like building construction disrupting local wildlife habitats.

Does commensalism always involve physical contact?

Not necessarily, it can also occur through indirect benefits.
About Author
Written by
Janet White
Janet White has been an esteemed writer and blogger for Difference Wiki. Holding a Master's degree in Science and Medical Journalism from the prestigious Boston University, she has consistently demonstrated her expertise and passion for her field. When she's not immersed in her work, Janet relishes her time exercising, delving into a good book, and cherishing moments with friends and family.
Edited by
Aimie Carlson
Aimie Carlson, holding a master's degree in English literature, is a fervent English language enthusiast. She lends her writing talents to Difference Wiki, a prominent website that specializes in comparisons, offering readers insightful analyses that both captivate and inform.

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