MUTUALISM

Examples of mutualism are of fundamental importance in determining the structure of biological communities. In the tropics, for example, leafcutter ants are often so abundant that they can remove a quarter or more of the total leaf surface of the plants in a given area. They do not eat these leaves directly; rather, they take them to their underground nests, where they chew them up and inoculate them with the spores of particular fungi. These fungi are cultivated by the ants and brought from one specially prepared bed to another, where they grow and reproduce. In turn, the fungi constitute the primary food of the ants and their larvae. The relationship between the leafcutter ants and these fungi, fueled by material cut from the leaves of plants, is an excellent example of mutualism.
Another relationship of this kind involves ants and aphids. Aphids, also called greenflies, are small insects that suck fluids from the phloem of living plants with their piercing mouthparts. They extract a certain amount of the sucrose and other nutrients from this fluid, but much runs out in an altered form through their anus. Cer-tain ants have taken advantage of this habit-in effect domesticating the aphids by carrying the aphids to new plants where they come into contact with new sources dl food and using the "honeydew" that the aphids excrete as food.
A particularly striking example of mutualism involving ants concerns certain Latin American species of the plant genus Acacia. In these species, the leaf pan', called stipules are modified as paired, hollow thorns; consequently, these particular species are called "bull's horn acacias." The thorns are inhabited by stinging ants oi the genus Pseudomyrmex, which do not nest anywhere else. Like all thorns that occur on plants, they serve to deter herbivores.
At the tip of the leaflets of these acacias are unique, protein-rich bodies called Beltian bodies-after Thomas Belt, a nineteenth-century British naturalist whofirsi wrote about them based on his experiences in Nicaragua. Beltian bodies do not occur in species oi Acacia that are not inhabited by ants and their role is clear: they serve as a primary food for the ants. In addition, the plants secrete nectar from glands near the bases of their leaves. The ants consume this nectar also, and feed it and the Beltian bodies to their larvae as well.
Apparently this association is beneficial to the ants, and one can readily see why they inhabit acacias of this group. The ants and their larvae are protected within the swollen thorns and the trees provide a ready source of a balanced diet, including [he sugar-rich nectar and the protein-rich Beltian bodies. What, if anything, do the ants do for the plants? This had been the question that had fascinated observers for nearly a century until it was answered by Daniel Janzen, then a graduate student at the University of California, Berkeley, in a beautifully conceived and executed series of field experiments.
Whenever any herbivore lands on the branches or leaves of an acacia that is inhabited by such ants, the ants immediately attack and devour it. Thus the ams protect the acacias from being eaten, and the herbivore also provides additional food for the ants, which continually patrol the branches. Related species of acacias that do noi have the special features of the bull's horn acacias and are not protected by ants have
bitter-tasting substances in their leaves that the bull's horn acacias lack. Evidently, ihese bitter-tasting substances protect the acacias in which they occur as do the ants that inhabit other species of the same genus of plants.
The ants that live in the bull's horn acacias also help their hosts to compete with other plants. The ants cut away any branches of other plants that touch the bull's hom acacia in which they are living-creating, in effect, a runnel of light through which the acacia can grow, even in the lush deciduous forests of lowland Central America, Without the ants, as Janzen showed experimentally by poisoning (he ant colonies that inhabited individual plants, the acacia is unable to compete successfully in this habitat. Finally, the ants bring organic material into their nests, and the part that they do not consume, together with their excretions, provides the acacias with an abundant source of nitrogen, which is an essential nutrient.