In parts of Africa, colonies of ants live on certain species of acacia trees. The ants feed on leaf structures rich in lipids, while the tree is protected from predation since the ants attack and scare off any predators. The ant and acacia tree relationship is best described by the word or phrase:

coevolution

Explanation:

The answer is "coevolution".

When two or more organisms or species affects their evolution collectively as a result of interaction between them. The term "coevolution" is first given by Peter H. Raven and Paul R. Ehrlich.

Charles Darwin stated the interconnection between the insects and the flowering plant as a similar scenario of coevolution.

Thus the ants who lives on the acacia trees and feeds on its leaf structures rich in lipids and protects the trees from other predators. They shows coevolution relationship.

Thus the answer is "coevolution".