A mutualism is a type of interaction between species that benefits both. There are few, if any, taxonomic limits on the formation of mutualisms; animals, for example, can form mutualistic associations with other animals, with plants, and with a wide range of microorganisms. Mutualistic interactions often arise in environments where resources are in short supply. Consequently, many mutualisms involve an exchange of food for housing or defense. In another type of mutualism, many sessile organisms, particularly flowering plants, rely on more mobile species for mating or dispersal. In the animation, mutualistic partners exchange food and housing for defense.
Bull's horn acacia trees grow numerous structures that provide food and shelter for acacia ants. The tree and the ant have evolved a mutualistic interdependency. The ants provide the tree, in return, with defense against herbivores and neighboring plants. This mutualism was experimentally demonstrated by Daniel Janzen, who showed that the trees benefit greatly from their association with these ants, and that the energy expended in growing ant-attractive structures is repaid with increased growth and survival.
Textbook Reference: Concept 43.4 Interactions within and among Species Can Result in Evolution