Mutualism is an interaction in which both organisms benefit from the relationship, a positive interaction form of symbiosis. There are two common types of mutualism: obligate mutualists and facultative mutualist. An obligate mutualist is an organism that must have its partner in order to survive. A facultative mutualist is an organism that can survive on its own but for which there is an added benefit if it remains with another organism.
An example of complex obligate mutualism is leaf cutter ants, fungus, and bacteria. The ants cut and harvest leaves from different species of plants. These leaves are carried back to their colony, where the ants grind up the plant matter. The ants inoculate the leaves with a fungus that produces fruiting bodies, which are used as a food source for the ant colony. The ants use bacteria that lives on their body to protect this fungus garden against pathogens that would destroy their fungal partner. The relationship between the ant, fungus, and bacteria is one of mutualism.
An example of facultative mutualism is a plant that produces a fruit that is eaten by a bird. The fruit helps the bird survive, and the bird helps with dispersing seeds when they move through its digestive system and are deposited in locations that the plant could not reach on its own. Both organisms could survive on their own, but there is a benefit to each if they remain in the other's environment.