Which of the following best describes commensalism?

The correct answer is Option (3) → Interaction where one species benefits and the other is neither harmed nor benefited

Two species can affect each other in five ways: mutualism (+/+), competition (−/−), predation and parasitism (+/−), commensalism (+/0) and amensalism (−/0). In commensalism one partner gains and the other is unaffected. An orchid growing on a mango branch and barnacles on a whale’s back are examples.

Option (1) Interaction where both species benefit — Incorrect. When both species benefit, the interaction is mutualism, as in lichens or in fig trees and their wasps.

Option (2) Interaction where one species benefits and the other is harmed — Incorrect. One gains and one is harmed in predation and parasitism. The host or prey is hurt.

Option (3) Interaction where one species benefits and the other is neither harmed nor benefited — Correct. One species benefits while the other is neither harmed nor benefited. This is commensalism, as with the cattle egret feeding beside grazing cattle.

Option (4) Interaction where both species are harmed — Incorrect. When both species lose, the interaction is competition.

NCERT Extract:
Population Interactions – “… Both the species benefit in mutualism and both lose in competition in their interactions with each other. In both parasitism and predation only one species benefits (parasite and predator, respectively) and the interaction is detrimental to the other species (host and prey, respectively). The interaction where one species is benefitted and the other is neither benefitted nor harmed is called commensalism. In amensalism on the other hand one species is harmed whereas the other is unaffected.”