It's a David versus Goliath kind of story, with an ecological twist: In African savannas (regions with both trees and grass), acacia-dwelling ants can repel voracious, tree-eating elephants, according to new research by published online September 2 in Current Biology.

This ant-driven tree protection has large-scale implications for savanna landscapes, report zoologists Jacob Green and Todd Palmer, of the University of Wyoming and University of Florida, respectively. That's because elephants are one of the primary shapers of tree cover in sub-Saharan Africa—too few tree-grazing elephants, and savannas may transform into woodlands; too many, and open grasslands may soon result. It looks like ants may help to strike a balance between these two extremes, helping to preserve one of Africa's largest yet endangered ecosystems.

The scientists made their discovery after noticing that elephants avoid eating one species of acacia tree, Acacia drepanolobium, but happily munch on other acacias in the same region. So what's different about these tree types? One disparity the team noted is that A. drepanolobium provides food and housing for symbiotic ants, which in turn defend their hosts against tree-damaging insect intruders.

To determine whether the ants were influencing elephants' dining preferences, the researchers used smoke to remove all or just a portion of ants from A. drepanolobium trees and applied a sticky barrier at the base of each plant (to ensure their desired levels of ant colonization were sustained). They monitored how frequently elephants ate from the trees during a 12-month period and found that the pachyderms would actually eat A. drepanolobium when fewer ants were around.

In fact, given their choice of food, the team found that elephants ate ant-free A. drepanolobium just as often as their more typical chow, A. mellifera (another type of acacia).

The researchers suspect that the animals' trunk, with its thick outer skin belying a more sensitive interior lining, makes elephants vulnerable to ants, which swarm aggressively in response to disturbances and attack by biting thin skin and mucous membranes. In the words of the researchers, "attack by scores of biting ants probably serves as a strong deterrent."

Because tree cover influences many key ecosystem processes in savannas and other habitats, such as carbon storage, food webs and wildfires, the authors conclude that these tiny ant bodyguards "likely exert powerful indirect effects" through their impacts on elephants. Sometimes, it really is the little things that count.

Photo: Courtesy of iStockphoto