Attack of the tree-killing Peruvian ants

Nature

October 14, 2005|By DENNIS O'BRIEN | DENNIS O'BRIEN,SUN STAFF

A Peruvian ant has developed an unusual survival mechanism: It kills any tree that it can't use for a home.

The ant's habit has created eerie landscapes in patches of the western Amazon rain forest that locals refer to as "devil's gardens." Legends attribute the phenomenon to an evil spirit.

"You know it the minute you see one of these areas. They look extremely different from everything around it," said Megan Frederickson, a Stanford University biologist.

Most rain forests have a canopy comprising a variety of tall trees, with an understory of smaller trees, she said. But in a devil's garden, the taller trees are absent and the landscape is dominated by the same of type of shorter tree.

Native to the western Amazon, the tree is a broad-leafed evergreen that grows about 12 feet high with branches that serve as nesting sites for the ant, Myrmelachista schumanni.

Researchers suspected for years that the ants had been attacking competing saplings. But exactly how they killed the competitors remained a mystery until Frederickson planted saplings of a common tree inside and outside one of the gardens and watched for an attack.

The ants went to work within a day, biting small holes in the leaves of the unwanted tree and releasing formic acid into the leaves. The acid turns the leaves brown within a few hours and kills the tree in a few weeks, she said.

Her results were published in the journal Nature last month.

There are 15,000 to 20,000 species of ants. But M. schumanni is the only ant known to use the acid as an herbicide, Frederickson said.

The ants nest in the hollows of D. hirsuta's branches. By attacking other types of trees, they increase their nesting sites by reducing the tree's competition for soil nutrients, light and space in the rain forest, Frederickson said.

The strategy apparently works: Frederickson analyzed 26 gardens and found one with 350 trees, tended by a colony of 3 million ants. Based on the age of the garden's trees, the ants have been tending the site for 800 years, she said.

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