Besieged Ants Turn to Topiary

The Hatfields and the McCoys. The Montagues and the Capulets. To this list of arch rivals, biologists can add four species of biting ants in Africa, which all compete for living space in the same kind of tree, a whistling-thorn acacia. Now, they have discovered that entire trees and landscapes can be shaped by the ant wars. One of the ant species has apparently learned how to trim its acacia trees, bonsai-style, into dense bushes that do not touch trees occupied by rivals.

The ant-acacia relationship has long been a textbook example of symbiosis. In both Central America and Africa, acacia trees grow large swollen thorns in addition to their more typical slender thorns. The only apparent function of the swollen thorns is to serve as a home for ants. (When the ants move out, the entryways into the hollow thorns act like the holes on a flute--hence the name "whistling thorn.") Meanwhile, the ants serve the tree by beating back herbivores. A browsing giraffe may avoid the thorns, but it risks an all-out assault on its mouth by thousands of angry, biting ants.

Now scientists are beginning to realize how much the symbiosis is influenced by ant-to-ant combat. Maureen Stanton and Truman Young of the University of California, Davis, wondered why the trees occupied by one of the ant species, Crematogaster nigriceps, appear stunted. When they tied branches of the nigriceps-occupied trees to neighboring branches of a tree hosting another species, they found out. The other species swarmed across the makeshift bridge and usually annihilated the nigriceps colonies. "The ground below the trees was literally covered with dead and dying ants the next day," Stanton recalls. Normally, Stanton and Young report in today's Nature, the nigriceps ants avoid such invasions by pruning their trees, in effect creating a moat of air.

Dan Janzen, a tropical ecologist at the University of Pennsylvania, says that the experiments were "well done" and shed new light on an ecosystem that is literally in the backyard for many Africans. "If you go to Nairobi and get in a taxicab, you're there in two minutes," Janzen says. "You'll see 500 acacias in an acre and nothing else."

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