From Scientific American:
Acacia trees are the iconic shrub of the East African savanna. Their thorny thickets house a host of creatures and provide sustenance to the local charismatic megafauna, from elephants to zebras. In light of this continual foraging, the plants have struck a mutually beneficial bargain with several species of ants. The insect armies swarm intrusive browsers in exchange for housing and food. But according to new research in Science, it appears that without such browsing—a state of affairs the acacia might be thought to long for—the trees suffer.
Zoologist Todd Palmer and his colleagues examined the interdependence of one such acacia species—the whistling thorn tree, Acacia drepanolobium—the ants it hosts and the herbivores that eat it. He compared six such trees in Kenya that have been surrounded by an electrified fence since 1995 (by entomologist Truman Young of the University of California, Davis) with six trees open to local giraffes, elephants and other acacia-eaters. In the absence of herbivores, the whistling acacia stopped producing little ant houses in hollow thorns—known as domatia—and excreting the sweet nectar that its bodyguard ants eat. But instead of spurring more growth, the acacias found themselves more than twice as likely to be providing a home to another type of ant—Crematogaster sjostedti—which do not defend the trees and rely on invasions of the bark-boring cerambycid beetle larvae to build the holes in which they dwell. “The cavity-nesting antagonistic ants actually promote the activities of the stem-boring beetle,” says biologist Robert Pringle of Stanford University.