"Ants can have dramatic and long-lasting impacts on their environment. The largest devil's garden that I studied measures 1,300 square meters in size and is an estimated 800 years old. Yet the garden is created by tiny ants, measuring only a few millimeters long," Frederickson told Animal Planet News.
The researchers located 10 devil's gardens, ranging in size from one to 328
D. hirsuta plants, at the Madre Selva Biological Station in the Amazonian rainforest of Loreto, Peru.
Then they planted two saplings of a common Amazonian cedar tree (
Cedrela odorata) inside each garden near the base of an ant-infested
D. hirsuta tree.
A sticky insect barrier was applied to one cedar sapling to prevent ants reaching it, while the other sapling was left untreated.
Worker ants immediately attacked the untreated samplings by injecting formic acid from their poison glands into the leaves, which began to to die within 24 hours.
"Most of the leaves on these saplings were lost within five days," wrote the researchers.
But cedars treated with the insect barrier thrived.
The results rule out one of the main theories of devil's gardens formation, known as allelopathy. Under this process, trees release toxic secretions that kill competing plants.
Named for formica, the Latin word for ant, formic acid is a toxin used by many ant species to defend themselves against insect attack. But this is the first time that ants have been found to use it as an herbicide, said the researchers.
They estimated that a typical garden is tended by a single ant colony, comprising as many as three million workers and 15,000 queens. The presence of multiple queens contributes to colony longevity.
"By killing plants of other species, the ant promotes the growth and establishment of
D. hirsuta, thereby gaining more nest sites," Frederickson and colleagues wrote.
It is unclear how an ant can tell whether a plant is the same species as its host or a different one. Her current working hypothesis is that ants recognize their
D. hirsuta home trees through species-specific chemical cues.
"This is an excellent study. It is a fantastic example of how complex biotic interactions can play the primary role in these small ant-created, species-poor communities within the hyper-diverse Amazonian rainforest," Paul Fine, an evolutionary biologist at the University of Michigan in Ann Arbor, told Animal Planet News.
