In Battle of Elephants and Ants, Trees Win Big

Nathan Gregory

Elephants gather around the ant-plant Acacia drepanolobium.

Todd Palmer

Ants help prune the whistling thorn tree, which is a favorite of African elephants.

In the savannahs of Africa, a long-running conflict pits elephants against enormous swarms of angry ants that live in trees the elephants would like to eat.

According to a new study, a peace accord between the warring species would be one of the worst ways this war could end.

Ecologist Todd Palmer of the University of Florida says this war is being fought over a small African tree that has learned how to fend off giant, hungry elephants.

"Elephants can eat entire trees in much the same way we eat cupcakes," Palmer says.

But in Kenya, there's one kind of tree that hires ants to help drive the elephants away. Palmer says this tree secretes a tasty sap that draws as many as 100,000 ants into a single tree.

When a pachyderm attempts to chow down on the branches, the ants get mad in hurry, Palmer says.

"They come swarming out and they release this horrible [smell] that tells the tree that it is under attack," he says. "Then the entire tree mounts this huge defense and eventually enough is enough."

Palmer has seen angry ants charge straight into the famously sensitive trunks of elephants. He says many pachyderms run away with tens of thousands of ants swarming all over their faces.

Ecologists refer to these kinds of alliances as "mutualisms." But Palmer says they're not as peaceful as they sound.

"It's a co-evolutionary war," he says. "People used to think of mutualisms as friendly situations but it's really more of a battlefield."

This particular battlefield is notable, in part, because it's where a tiny insect has been trumping the biggest animal on earth. It's also the site of an experiment set up to show what would happen to the ant-tree alliance if the common enemy — elephants — disappeared.

In a paper published in the journal Science, Palmer says he found the answer by enclosing some of his trees in elephant-proof fences and measuring the changes that took place over the next decade.

Palmer thought the trees inside the fences would flourish but that's not what happened. After 10 years, many of the trees were dead and others had begun to grow more slowly.

Palmer says all of the trees had stopped producing tasty sap, which drove off the ants that had once protected the trees from hungry elephants.

"It was as if the trees said, 'Look, the elephants are gone, there is no need for us to pay our bodyguards,'" he says.

But in the end, those sap reductions backfired, Palmer says. That's because the guard ants were replaced by other insects that attacked the trees and filled them full of ugly holes and tunnels.

This is an experiment that shows just how powerful and complicated natural alliances can be, according to biologist Mark Bertness of Brown University in Rhode Island. More importantly, he says, it's a reminder that you never know what you are going to get when you mess with Mother Nature.

"This is a great example of how, without these mutualisms, a community collapses," he said.

Bertness says it's possible that similarly unexpected ecosystem collapses could take place soon in parts of Africa where elephants are in trouble.