Sometimes size really doesn't matter. A new report published online on September 2 in Current Biology, a Cell Press publication, shows that puny ants can be the best defense against hulking elephants. Without their insect bodyguards, acacia trees in areas that are heavily trafficked by elephants simply get pummeled.
"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 findings reveal tiny ants to be major ecosystem players in the African savanna, according to Palmer and his colleague Jacob Goheen, now at the University of Wyoming.
Goheen and Palmer stumbled onto the finding after noticing that one species of acacia tree (Acacia drepanolobium), which houses ant guardians, didn't seem 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."
To investigate further, the researchers first tested whether the elephants would eat those acacia trees if they were stripped of their ant defenders. And it turned out the answer was yes. In fact, in the absence of ants, the elephants like to eat the ant-plants just as much as they like to eat 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 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." Palmer said.
It appears that an elephant's long trunk is its Achilles' heel. The outside of the trunk may be tough, but the inside is extremely sensitive and loaded with nerve endings. "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 added.
In contrast, the ants don't offer much protection against giraffes, which simply swipe swarming ants away with their long, tough tongues.
The findings matter in the bigger picture, 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 sort of widespread, long-term change in savannas. In fact, 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 write. "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 researchers include Jacob R. Goheen, University of Wyoming, Laramie, WY; University of British Columbia, Vancouver, BC, Canada; Mpala Research Centre, Nanyuki, Kenya; and Todd M. Palmer, Mpala Research Centre, Nanyuki, Kenya, University of Florida, Gainesville, FL.