Trees, ants need threats to their survival to flourish: study

Without a common enemy, organisms let down their guard

Last Updated: Thursday, January 10, 2008 | 5:16 PM ET

CBC News

Plants and animals that depend on each other are more likely to survive when a threat to their collective existence is present, a recent study has concluded.

A report to be released in Friday's edition of the journal Science chronicles the results of an experiment in which acacia trees in Kenya were fenced off from elephants and giraffes for 10 years.

A giraffe is seen near an acacia tree in Kenya. A recent study has indicated that if mammals stop feeding off acacia trees, the trees and the ants that inhabit them suffer.
(Image courtesy of Todd Palmer)

The scientists who conducted the study expected the plant life to flourish since it would not be accessible to wildlife, Todd Palmer, an assistant professor of zoology at the University of Florida, 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 rather than benefit from their newfound distance from the animals that typically feed off them, the plants began to look sickly.

The ants that inhabited the trees, meanwhile, appeared listless and in fewer numbers. Acacia trees produce nectar that the ants feed on. In the wild, animals such as elephants would come to feed on the trees, but the trees in turn would be protected by a swarm of ants that would scare away the predators. No need for protection means less nectar production, the study noted, and as a result, the ants died off.

Some trees also became inhabited by wood-borer beetles, since the ant colonies that normally protect them were diminished and weakened, and a separate ant colony became more dominant.

Palmer, who was assisted by four other scientists, including Jacob Goheen of the University of British Columbia, plans to remove the fencing to determine whether the reintroduction of mammals can reverse the trees' and ants' decline.

"Although this mutualism between ants and plants has likely evolved over very long time-scales, it falls apart very, very rapidly," he said. Performing the experiment in reverse will allow him to "to see how quickly trees will re-induce their investments in symbiotic ants, and in turn, whether such reinvestment will be enough and in time, or too little, too late."