Acacia in the Rainforest & Ants

The Rainforest Environment

• The acacia trees and ants live in a very competitive environment. Plants grow quickly in the heat and humidity of the forest, so trees are always in competition with other plant life. The trees also are under threat from insects and animals, which eat foliage, bark and sap, or may damage the tree to make nests. The ants need safe and secure nesting sites to raise their offspring, and they need a reliable supply of nutrients that will meet their dietary needs.

Benefits to Trees

• Acacia trees inhabited by the acacia ant generally are avoided by herbivores, or plant-eating animals. This is because the animals can smell the ant's pheromone, or scent. Animals know that if they go near an acacia tree, the ants will respond by rushing out and stinging the animal. Acacia ants are very aggressive and give a painful sting. They also will attack insects that seek to colonize a tree. Another benefit that the acacia ants give is clearing seedlings from around the base of the tree so that the tree will not have to compete for water, soil nutrients or light with other plants.

Benefits to Ants

• Ants need protein, sugar and water in their diets. The bull's horn acacia grows protein-rich balls called Beltian bodies on the tips of its leaves, which the ants eat. There is no other known function for these structures. The trees also excrete nectar, which is a sugar-rich liquid. It is formed in depressions in the leaf stalks, and the ants drink it. The ants live in the thorns of the tree, which are hollow. The ants chew a hole at the tip of the thorn quite easily, then are protected from the rain and sun.

Adaptations

• Some studies in Africa have shown that when the acacia trees no longer need the ants' protection, they reduce their production of sugar-rich nectar. Researchers studied trees that had been fenced off to protect them from herbivores. They found that the ants responded by allowing sap-sucking insects such as aphids and scale insects to colonize the tree. The ants then ate the sugar-rich liquid that these insects exude, derived from the sap of the tree.