Tree-ant bond alive thanks to mammals

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.

It turns out that without animals eating their leaves, the trees reduced nectar production and made fewer swollen thorns that the ants could live in. The result: The protective ants began damaging the trees or were replaced by other insects that ate holes in the bark.

“Although this mutualism between ants and plants has likely evolved over a very long time scales, it falls apart very, very rapidly,” said Todd M. Palmer, a University of Florida zoologist who reported the findings Friday in the journal Science.

“Over the course of only 10 years, we found that when mammals could not eat plants, the plants began to have less use for the ants, and therefore began to reduce their ‘payments’ to the ants, in the form of nectar,” Palmer said.

“If you had asked me 10 years ago ‘What would happen if you took large mammals out of the system?’ I would have answered: ‘I’ll bet the trees would be really happy!’ ” he said.

But, because the browsing animals are the driving force behind the tree paying out benefits to the ants, when the payments diminish, the ants that protect the tree begin to starve, and their colonies become smaller.