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.
The ants have a full banquet and residence on the bull's horn acacia. They nest in the enlarged hollow thorns, entering and exiting from a hole near the tip. They consume energy-rich nectar from specialized extrafloral nectaries at the base of the leaves. The also take the oil and protein packets at the ends of some leaflets back to the nest for their young. The food-producing structures on the plant have no apparent purpose other than to provide for the ants. Did the ants just get lucky, or are they providing the tree with something that it needs in return?
In 1874, in Nicaragua, the naturalist Thomas Belt suggested that the aggressive acacia ants reciprocated by defending the plants against herbivores, using stingers to drive them away. They also defended the plants against plant competitors by chewing away any invading vines or clearing nearby seedlings.
Belt's idea was not tested until Daniel Janzen conducted an experiment in 1966. Janzen selected a population of acacia trees and designated some of them as untreated controls and the others as experimental subjects. To the experimental subjects, he fumigated the trees with insecticide to remove all of the resident ants. He also applied a sticky material to the tree's base to prevent other ants from crawling up and recolonizing the tree.
After 10 months, Janzen determined the percentage of trees still surviving and looked at how much they had grown. He found that the control trees with the ants had considerably higher survival rates. They also had greater growth than did the trees without ant populations. From this experiment, Janzen showed that the relationship between the ant and tree species was indeed mutualistic. The ants benefited, but they did indeed reciprocate, providing substantial survival benefits to the acacia trees.
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.