THE mystery of the Amazon rainforest’s “Devil’s gardens”, in which only a single species of tree survives for hundreds of yards around, has been unravelled by scientists.

The enigmatic stands of Duroia hirsuta trees are not tended by evil forest spirits, as local legend would have it, but are the work of an industrious variety of ant.

Research has shown that ants of the species Myrmelachista schumanni, which live in the hollow stems of D. hirsuta trees, produce Devil’s gardens by poisoning all the other plants that spring up around their woody homes. By destroying the competition to the trees they rely on for food and shelter, they allow D. hirsuta saplings to thrive and provide them with fresh nests into which ant colonies can expand.

Their arboriculture is so successful that the longest-lived Devil’s gardens are thought to be more than 800 years old, and to house as many as three million workers and 15,000 queens.

Tropical rainforests are generally populated with an abundant variety of trees, vines, shrubs and wildflowers, but Devil’s gardens, which appear apparently at random in the Amazon, consist purely of the D. hirsuta tree.

“Devil’s gardens are large stands of trees in the Amazonian rainforest that consist almost entirely of a single species, Duroia hirsuta, and, according to local legend, are cultivated by an evil forest spirit,” said Megan Frederickson. of Stanford University in California, who led the study.

“Here we show that the ant, Myrmelachista schumanni, which nests in D. hirsuta stems, creates Devil’s gardens by poisoning all plants except its hosts with formic acid. By killing other plants, M. schumanni provides its colonies with abundant nest sites.”

Aside from the evil spirit theory, two serious scientific explanations have been advanced for the phenomenon. One hypothesis is that the D. hirsuta trees themselves kill competing plants, a process known as allelopathy, while the other suggests it is their symbiotic ants that are responsible.

Dr Fredrickson’s work, which is published today in the journal Nature, has demonstrated clearly that it is the M. schumanni ants — also known as Devil’s garden ants — that deserve the credit.

To test the ant hypothesis, the researchers conducted a series of experiments at the Madre Selva Biological Station in the Amazonian rainforest of Loreto, Peru. They used ten Devil’s gardens, the largest of which contained 328 D. hirsuta trees or saplings.

The team planted two saplings of a common Amazonian tree called Cedrela odorata, or Spanish cedar, inside each Devil’s garden near the base of a D.hirsuta tree actively patrolled by worker ants. One of the cedars was protected from the ants by a sticky substance that prevented them reaching it, while the other was left untreated.

The scientists found that worker ants immediately attacked the untreated saplings, injecting formic acid into the leaves to kill them. The plants began to die within 24 hours. The treated saplings, however, survived — indicating that it was the ants, not the D. hirsuta trees, that caused the damage.

Chemical analysis revealed that the only compound produced by the ants’ poison glands is formic acid, common in many ant species.

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