Ants known to defend certain species of Acacia trees from elephant predation deter the massive herbivores so effectively that they are impacting entire savanna ecosystems, according to a study published online today (2nd September) in Current Biology.

Ants on a whistling-thorn tree Image: Todd Palmer

"I don't think any one had suspected how strong an effect the ants [had] in terms of driving elephants to avoid the Acacia," said ecologist David Augustine of the US Department of Agriculture, who was not involved in the study. "It's a very nice demonstration of [how] this small-scale mechanism can explain a large-scale and pretty important pattern in the savannas in this region."

Savannas are made up of a mixture of trees and grass, and the amount of tree cover in a given area is largely variable, depending on such factors as rain, fire and herbivory. Elephants can kill large expanses of trees as they tear them apart and uproot them during feeding. But satellite images of the savannas in Kenya showed that parts of the tree coverage between 2003 and 2008 was surprising stable, despite a dramatic increase in elephant populations in those areas.

The swaths of savanna that showed this stability all turned out to be areas where ants defend particular species of trees, such as the whistling-thorn tree, Acacia drepanolobium, growing in a soil type composed primarily of clay, said community ecologist Jacob Goheen of the University of Wyoming. "Could it actually be," he wondered, "that these ants are doing a good job protecting the trees from elephants?"

Image: Jacob Goheen

Goheen and Todd Palmer of the University of Florida performed two simple experiments to determine the answer to this question. First, they removed ants from Acacia trees to see if elephants would eat the ant-free trees and ensure that there wasn't anything inherent to the tree itself that was acting as a deterrent. And second, they added ants to other species of trees that elephants frequently eat to see if the insects could work as an effective defense for these other species.

"And sure enough," Goheen said, "the elephants were choosing to eat more or less entirely based on whether or not these trees were defended by ants." That is, elephants steered clear of the ant-laden trees, but they had no problem chowing down on the Acacia trees once they were cleaned of ants.

"I think [it's] interesting just how good the ants are at deterring herbivory by megafuana, especially the largest megafauna herbivore on the continent of Africa," said savanna ecologist T. Michael Anderson of Wake Forest University, who was not involved in the research. "This promotes stability in what is otherwise a biome that is highly dynamic under typical circumstances."

Image: Rob Pringle

As for how the ants deter elephant predation, Goheen suspects that the ants crawl into the elephants' nostrils and bite them, and the longer an elephant stands there and eats, the more ants swarm. But "that's actually a bit mysterious to us," he said.

The implications of these findings are significant, as "savannas make up a huge percentage of the global landmass -- up to 20 percent," added Anderson, who wrote an accompanying commentary in Current Biology. "They're important because there's a huge number of people that directly get their livelihood from savannas," including those who raise livestock. Furthermore, "[savannas] are able to store and release a tremendous amount of carbon, and [thus] can be reservoirs for quite a bit of the CO₂ that's in the atmosphere," he said "If we were to lose them, the ability to store carbon in this landmass goes away."

Another lingering question is why those tree species that harbor ants that defend them against herbivory seem restricted to the clay soils. "Why don't they extend to other types of soils and other portions of the landscape and that is still a big unknown," Augustine said. "[It's] an interesting question" that will take more research to sort out.

J.R. Goheen and T.M. Palmer, "Defensive plant-ants stabilize megaherbivore-driven landscape change in an African savanna," Current Biology, 20, 1-5, 2010.


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