The acacias provide food and home to the insects which protect the leaves. But if there is no threat, the deal is off and both suffer.
For thousands of years, thorny African acacia trees have provided food and shelter to aggressive biting ants, which protected the trees by attacking animals that try to eat the acacia leaves.
Called mutualism, it's a good deal for the trees and the ants.
Scientists studying the decline in large animals in Africa wondered what would happen if the animals no longer were eating the leaves. So they fenced off some of the acacias from elephants, giraffes and other animals.
After a few years, the fenced-in trees began looking sickly and grew slower than their unfenced relatives.
Edited by Administrator for consistency.