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.
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This, in turn, stunts the trees' growth and causes them to die twice as often than when they are being regularly eaten by giraffes, elephants and other large African herbivores. "The trees are actually making a shortsighted decision by defaulting on their end of the mutualism bargain," Pringle says. "If they sustained production of ant rewards in the absence of large mammals, they would reduce their probability of being taken over by this somewhat nasty antagonistic ant."
This counterintuitive result may apply only to the whistling thorn acacia, one of the only species of that genus in Africa that relies on ants as bodyguards rather than thorns and / or chemical defenses. After all, in the wake of disappearing large mammals across Africa, these other types of acacia have proliferated, says ecologist Jacob Goheen of the University of British Columbia.
But it does provide an example of how the disappearance or extinction of elephants, giraffes, zebras and other large herbivores in a region can have unexpected and unintended consequences—much like the boom in leaf-eating beetles and the lizards that prey on them shown in earlier work—whereas the decline of such mammals continues nearly continent-wide through the loss of habitat and overhunting.
"Large herbivores are tremendously important players in these systems," Pringle says. "Not just because of the direct effects they have upon plants, but also because of the myriad effects they exert on smaller, less conspicuous components of biodiversity." For want of an elephant, a protective ant species diminished and left the whistling thorn acacia in dire straits.
