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Ants Use Acid to Make "Gardens" in Amazon, Study Says

John Roach
for National Geographic News
Updated March 20, 2006
 
Ants in the Amazon rain forest labor to keep their territory free of all
plants except for one tree species, according to a new study. Scientists
call these cultivated spaces devil's gardens, after the local legends
that hold they're home to evil spirits. Some of the gardens are at least
800 years old.

The ants' gardening tool of choice is a built-in herbicide: formic acid. Formic acid is a simple organic acid, used by humans to preserve foods and by nettles and red ants as a stinging agent.

"To my knowledge, no [other] insects—ants or otherwise—use formic acid as a herbicide," Megan Elizabeth Frederickson, a graduate student in biology at Stanford University in Palo Alto, California, wrote in an e-mail.

Frederickson, who is currently conducting ant research in the Peruvian Amazon, is the lead author of the study, which appeared last September in the journal Nature.

Paul Fine, an evolutionary biologist at the University of Michigan in Ann Arbor, also studies insect and plant interactions in the Amazon. He deemed the finding "very cool."

"I've never heard of that before," he said. Fine, who was not involved with the research, added that future studies should look into whether ants behave similarly in other parts of the world.

Devil's Gardens

According to Frederickson, devil's gardens consist almost exclusively of lemon ant trees. Local legend in the Peruvian Amazon holds that the gardens are cultivated by an evil spirit called the Chuyachaqui.

The Chuyachaqui is a mythical dwarf with one human foot and one hoof. He is able to transform himself to anyone's likeness. He often appears as a friend or family member to lone travelers in the jungle and leads them around in circles until they are lost.

"The devil's gardens are considered his home, and people avoid devil's gardens in fear of him, or at least traverse devil's gardens very cautiously," Frederickson said.

Previous research has suggested that the gardens are created as a result of allelopathy, a process by which one plant prevents another nearby plant from taking root through the secretion of chemicals.

Frederickson, however, noticed that the gardens only exist in the presence of the lemon ant (Myrmelachista schumanni). Lemon ant trees occupied by other ant species, or by no ants, always had other plants growing nearby.

In addition, two previous studies had shown that lemon ants cultivate small stands of other tree species. These observations led Frederickson to test whether the lemon ants are the cultivators of the devil's gardens.

To do so, she set up experimental plots in which saplings of a common Amazon cedar tree were planted inside and outside devil's gardens. Groups of the trees were then either exposed to or protected from the ants.

The ants attacked the exposed cedar trees, but the protected trees went unscathed. The ants attack by injecting formic acid into the tree leaves, which causes them to turn brown and fall off within five days.

Fine, the University of Michigan biologist, said Frederickson and her colleagues successfully show that the lemon ants play a major role in cultivating the devil's gardens. But, he noted, allelopathy should not be ruled out.

Plant chemicals may take weeks or months to kill off a neighboring plant, Fine said, much longer than the current experiment lasted. "It could be that both are happening," he said.

A longer-term experiment is required to completely rule out allelopathy, he added.

Cultivating the Garden

The cultivation of devil's gardens apparently begins when a queen lemon ant colonizes a lemon ant tree. As time passes more trees grow in the areas cleared by the ants, and the ant colony expands to occupy them.

"The largest colony in my study plot, which I estimate to be 807 years old, covers around 1,300 square meters [14,000 square feet]. This colony contains about three million workers and 15,000 queens," Frederickson said.

The ants are believed to prefer the lemon ant trees because they have hollow stems that serve as nest sites. Frederickson thought perhaps the ants decide to attack a plant based on whether it provides nest space.

In a follow-up experiment, however, she found that the ants attacked the Amazon cedar trees regardless of whether they contained nest space.

"I now think it is likely that the ants use chemical cues to distinguish between host and other plant species," she said.

Fine noted that while devil's gardens are mostly limited to the lemon ant tree, other plants are commonly found in the gardens, such as Melastomataceae shrubs and tree ferns. Perhaps, he said, those plants have a defense against formic acid or are in some other way fooling the ants.

Frederickson's research is part of a long-term goal to understand the role of ants in Amazon rain forest ecology.

The combined weight of all the ants in the Amazon is estimated to be four times greater than that of all of the region's mammals, birds, reptiles, and amphibians combined.

"In many ways, ants are the dominant animals in the Amazon," she said.

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