Facultative mutualism refers to a biological relationship in which both organisms benefit from the association, but the relationship is not essential. If the organisms must live mutually, then instead of being facultative mutualism, it is called obligate mutualism.
An example of organisms that experience facultative mutualism would be aphids and ants. Both aphids and ants are capable of living completely separately. However, when the two insects live in the same area, the ants protect the aphids from predation, and the aphids provide a sugary fluid that the ants can live off of.
Another common example of facultative mutualism is the relationship between small cleaner fish and large fish. Neither of the species need to live together, but the smaller fish can feed off of the parasites on the larger fish. In this case, the small fish are gaining nutrition, and the large fish are receiving protection from parasites.
According to the biology department of McDaniel College, mutualism is thought to have evolved as a response to parasitic interactions. When an organism was unable to get away from a parasite, it may have evolved to gain in the relationship.
There are different types of mutual interactions, which can all be either facultative or obligate. The organisms may gain shelter, nutrition, defense, transportation or pollination. However, it is not necessary that the organisms gain the same benefit in the mutual relationship. In the example of the ants and aphids, the aphids are gaining defense from the ants and the ants are gaining nutrition.