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Ants Protect African Trees From Elephants: Study

September 3, 2010

A species of acacia tree in Eastern Africa seems to be protected from elephants by ants, according to new research from scientists at the Universities of Wyoming and Florida.

The researchers conducted a series of studies in Kenya, and found that the acacia trees in areas heavily trafficked by elephants simply get pummeled without protection from their tiny ant bodyguards.

“It really is a David-and-Goliath type of story, where these little ants are up against these huge herbivores, protecting trees and having a major impact on the properties of the ecosystems in which they live,” said Todd Palmer of the Mpala Research Centre in Kenya and the University of Florida.

“In the words of the renowned biologist E.O. Wilson, ‘It’s yet another example of how the little things run the world.’”

The ants seem to be major ecosystem players in the African savanna, said Palmer and his colleague Jacob Goheen at the University of Wyoming.

The researchers stumbled onto the finding after observing that one species of acacia tree, Acacia drepanolobium, which houses ant guardians, did not appear to be bothered by elephants.

“We thought that was really interesting, because we often see elephants feeding on other species of trees that do not harbor ants,” Palmer said.

“In fact, the number of elephants in the central highlands of Kenya has become high enough in recent years that we see severely elephant-damaged trees all over the place these days.”

The researchers wanted to further investigate their observation.  They began by testing whether the elephants would eat those acacia trees if they were stripped of their ant defenders.

The answer, they found, was yes.  In fact, in the absence of ants, the elephants enjoyed eating the ant-plants just as much as they did their favorite tree food.

“When either tree species had ants on them, the elephants avoided those trees like a kid avoids broccoli,” Palmer said.

When ants were removed from the acacia trees in their natural environment on the African savanna, the elephants did much more damage than they otherwise would have over the course of a year.

“We had solved that part of the mystery””swarming groups of ants that weigh about five milligrams each can and do protect trees from animals that are about a billion times more massive,” said Palmer.

It appears that an elephant’s long trunk, which has a highly sensitive interior loaded with nerve endings, is its Achilles’ heel.

“It seems that elephants simply do not like ants swarming up the insides of their trunks, and I can’t say I blame them,” Palmer said.

By comparison, the ants don’t provide much protection against giraffes, which merely swipe the ants away with their long, rough tongues.

The study’s results have larger implications, the researchers say, because elephants at high enough densities can “literally convert woody areas into areas of open grassland.”

It might be that ants could prevent that type of long-term change in savannas. 

Indeed, the researchers showed that ant-acacia numbers don’t decline when elephants move in.

“Because tree cover strongly regulates a host of ecosystem processes, including carbon storage, fire-return intervals, food web dynamics, nutrient cycling, and soil-water relations in our [study] system and others, these tiny bodyguards likely exert powerful indirect effects at very large spatial and temporal scales,” the researchers wrote in a report about their study.

“As elephants and other large mammals in Africa exhibit chronic declines in some habitats and overabundance in others, identifying the ecological consequences of such landscape change remains an important challenge for wildlife managers in the future.”

The study was published online on September 2 in Current Biology, a Cell Press publication. 

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