Ants and Trees Rub Each Others' Backs

Jan. 10, 2008 -- Call it the rule of unintended consequences -- drop your guard because one threat goes away and an unexpected menace jumps up and smacks you. And new research shows it even applies to African acacia trees.

For thousands of years these thorny shrubs have provided food and shelter to aggressive biting ants, which protect the trees by attacking animals that try and eat the acacia leaves.

Called mutualism, it's a good deal for both the trees and the ants.

Scientists studying the decline in large animals in Africa wondered what would happen if they no longer were eating the leaves. So they fenced off some of the acacias, so elephants, giraffes and other animals couldn't get to them.

Surprisingly, 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 no longer bothered to take care of their ants -- they reduced nectar production and made fewer swollen thorns that the ants could live in.

The result: The protective ants either began damaging the plant or were replaced by other insects that ate holes in the bark.

"Although this mutualism between ants and plants has likely evolved over very long time-scales, it falls apart very, very rapidly," said Todd Palmer, an assistant professor of zoology at the University of Florida.

"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, who is currently in Kenya, explained in an interview via e-mail. Palmer's findings are reported in Friday's edition of the journal Science.

"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 instead, 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 its colonies become smaller.