For the first time, scientists have identified an ant species thatproduces its own natural herbicide to poison unwanted plants.
Stanford University biologist Deborah M. Gordon and her co-workersdescribe the findings in the Sept. 22 issue of the journal Nature. Thediscovery was made during a four-year field study led by Stanfordgraduate student Megan E. Frederickson in the Amazon jungle of westernPeru. The research focused on devil's gardens, mysterious tracts ofvegetation that randomly appear in the Amazonian rainforest.
"Devil's gardens are large stands of trees in the Amazonianrainforest that consist almost entirely of a single species, Duroiahirsuta, and, according to local legend, are cultivated by an evilforest spirit," write Frederickson and her colleagues in Nature. "Herewe show that the ant, Myrmelachista schumanni, which nests in D.hirsuta stems, creates devil's gardens by poisoning all plants exceptits hosts with formic acid. By killing other plants, M. schumanniprovides its colonies with abundant nest sites--a long-lasting benefit,as colonies can live for 800 years."
Most tropical rainforests are densely populated with a remarkablediversity of trees, vines, shrubs and wildflowers. But devil's gardensusually consist of a single plant, D. hirsuta, which happens to be thepreferred habitat of the devil's garden ant, M. schumanni.
In addition to the evil-spirit legend, two scientific proposalshave been offered to explain why devil's gardens occur. One hypothesisis that D. hirsuta trees release toxic secretions that kill competingplants--a process botanists call allelopathy. Others argue that devil'sgarden ants are responsible for controlling vegetation, either byextensive pruning or poisoning. "The idea is that by killing otherplants, the insects create a space for young D. hirsuta saplings togrow, thereby allowing the ant colony to expand as it occupies newnesting sites in the saplings," Frederickson explains.
To test this hypothesis, she and her colleagues conducted a series ofexperiments at the Madre Selva Biological Station in the Amazonianrainforest of Loreto, Peru. The research team located 10 devil'sgardens for the study, ranging in size from one to 328 D.hirsutaplants.
Two saplings of a common Amazonian tree called Cedrela odorata, or Spanish cedar, were planted inside each garden near the base of a D. hirsutatree actively patrolled by worker ants. A sticky insect barrier wasapplied to one cedar sapling to exclude ants, while the other saplingwas left untreated. The researchers planted two additionalsaplings--one treated, one untreated--about 150 feet outside of eachgarden but within the primary rainforest.
The results were immediate. Worker ants promptly attacked the untreatedsaplings, injecting a poison called formic acid into the leaves, whichbegan to die within 24 hours. "Most of the leaves on these saplingswere lost within five days, and the proportion lost was significantlyhigher than on ant-excluded saplings," the authors write. On the otherhand, cedars treated with Tanglefoot fared well, whether inside oroutside devil's gardens.
"These results show that devil's gardens are produced by M. schumanniworkers rather than by D. hirsuta allelopathy," the authors conclude.
To find out if worker ants only attack non-hostplants, the scientists decided to mimic D. hirsuta's hollow stems,called domatia, which are the ants' primary nesting sites. Artificialdomatia were constructed out of foil-wrapped test tubes partiallyfilled with cotton. Two cedar saplings, with and without artificialtubes, were planted in devil's gardens near two D. hirsuta saplings,one with and one without domatia. After 24 hours, there was significantleaf death on all of the cedar plants, but none on any D. hirsutasaplings. "We conclude that M. schumanni attacks only non-host plants,such as C. odorata, and that it does not rely on the presence ofdomatia to discriminate between its host and other plant species," theStanford team notes.
Chemical analysis revealed that the only compound produced by the ants'poison glands is formic acid, a toxin that is common in many antspecies and, in fact, got its name from formica, which is Latin forant. "Treatment of leaves with formic acid induced leaf necrosis on allthe plants we tested," the authors write. "To our knowledge this is thefirst record of an ant using formic acid as a herbicide--although it isknown to have bactericidal and fungicidal properties."
The ants employ a very effective system of lethal injection, notesGordon, associate professor of biological sciences at Stanford. "Thesystem harnesses two fundamental tools:formic acid, which many ant species use for other purposes, and thebasic circulatory system of all vascular plants," she says.
A census of the rainforest from 2002 to 2004 revealed thatdevil's gardens grew by 0.7 percent per year. "Using this growth rate,we estimate that the largest devil's garden in our plot, with 351plants, is 807 years old," the authors conclude. They estimate that atypical garden is tended by a single ant colony with as many as 3million workers and 15,000 queens, adding that the presence of multiplequeens "undoubtedly contributes to colony longevity."
"The cultivation of devil's gardens by ants is an excellentexample of niche construction," Frederickson says. "By killing plantsof other species, the ant promotes the growth and establishment of D.hirsuta, thereby gaining more nest sites."
The plants also benefit by increasing their biomass andeliminating the competition, says co-author Michael J. Greene, a formerSanford postdoctoral fellow, now assistant professor of biology at theUniversity of Colorado-Denver. "This work is a truly remarkable exampleof how effectively ants can manipulate their environment in order topromote their own survival," he adds.
A devil's garden begins when a M. schumanni queen colonizes a single D.hirsuta tree, the authors write: "Over time, D. hirsuta saplings growwithin the vegetation-free area created by the ants, and the ant colonyexpands to occupy them. The devilry of M. schumanni today provideshomes for ants in the future."
Frederickson is conducting new field studies to determine whichchemical cues the ants use to discriminate between host plants andother species. She also has begun searching for devil's gardens inother parts of the western Amazon to see how widespread the herbicidephenomenon is. "Megan's work reveals a system that is amazing becausethe ants exert so much control over their environment, creatingsingle-species stands in one of the most diverse places on Earth,"Gordon says.
Funding for the Nature study was provided by the Stanford Center forEvolutionary Studies, the Stanford Center for Latin American Studies,Sigma Xi, the Stanford Graduate Fellowships Program and the NaturalSciences and Engineering Research Council of Canada.
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