Fenced-off trees drop their friends

Science News, Jan 26, 2008 by Susan Milius

On Kenya's savanna, good fences make bad neighbors. Protecting acacia trees there from giraffes and other browsers sets off a chain reaction that ruins the partnership between trees and their bodyguard ants, says Todd Palmer of the University of Florida in Gainesville.

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He first noticed something odd about the protected trees when walking among whistling-thorn acacias cordoned off since 1995. With hole-riddled thorns, the wind whines through these acacias, which are "spindly, knobby trees" at best, Palmer says. Those behind fences looked even worse.

The acacias offer ant amenities: modified thorns for shelter and specialized nectaries for food. "Ants duke it out for host trees," Palmer says. A colony that wins a tree may grow 100,000 strong. When an intruder jostles a branch, the colony boils out to bite and sting unwelcome guests.

Fences undercut that pact, Palmer and his colleagues report in the Jan. 11 Science. Trees fenced for a decade skimped on ant-coddling features. Even trees with the best bodyguards, Crematogaster mimosae, provided a third fewer nectaries than usual.

In turn, the ants in those miserly trees were sluggish to attack and more likely to cultivate miniherds of scale insects. The scales ooze edible waste products for ants but suck sap from trees. And the prime bodyguard species was more likely to lose out to a nonprotective ant species. Trees that housed uncooperative ants grew more slowly and had greater mortality than trees with better tenants, the researchers conclude.