Stanford University biologist Deborah M. Gordon and her
co-workers describe the findings in the Sept. 22 issue of the
journal Nature. The discovery was made during a four-year field
study led by Stanford graduate student Megan E. Frederickson in the
Amazon jungle of western Peru. The research focused on devil's
gardens, mysterious tracts of vegetation that randomly appear in the
Amazonian rainforest.
"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," write Frederickson and her colleagues in Nature.
"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--a
long-lasting benefit, as colonies can live for 800 years."
Devilish ants
Most tropical rainforests are densely populated with a remarkable
diversity of trees, vines, shrubs and wildflowers. But devil's
gardens usually consist of a single plant, D. hirsuta, which happens
to be the preferred habitat of the devil's garden ant, M. schumanni.
In addition to the evil-spirit legend, two scientific proposals
have been offered to explain why devil's gardens occur. One
hypothesis is that D. hirsuta trees release toxic secretions that
kill competing plants--a process botanists call allelopathy. Others
argue that devil's garden ants are responsible for controlling
vegetation, either by extensive pruning or poisoning. "The idea is
that by killing other plants, the insects create a space for young
D. hirsuta saplings to grow, thereby allowing the ant colony to
expand as it occupies new nesting sites in the saplings,"
Frederickson explains.
To test this hypothesis, she and her colleagues conducted a
series of experiments at the Madre Selva Biological Station in the
Amazonian rainforest of Loreto, Peru. The research team located 10
devil's gardens for the study, ranging in size from one to 328
D.hirsuta plants.
Two saplings of a common Amazonian tree called Cedrela
odorata, or Spanish cedar, were planted inside each garden near
the base of a D. hirsuta tree actively patrolled by worker
ants. A sticky insect barrier was applied to one cedar sapling to
exclude ants, while the other sapling was left untreated. The
researchers planted two additional saplings--one treated, one
untreated--about 150 feet outside of each garden but within the
primary rainforest.
The results were immediate. Worker ants promptly attacked the
untreated saplings, injecting a poison called formic acid into the
leaves, which began to die within 24 hours. "Most of the leaves on
these saplings were lost within five days, and the proportion lost
was significantly higher than on ant-excluded saplings," the authors
write. On the other hand, cedars treated with Tanglefoot fared well,
whether inside or outside devil's gardens.
"These results show that devil's gardens are produced by M.
schumanni workers rather than by D. hirsuta allelopathy," the
authors conclude.
Domatia
To find out if worker ants only attack non-host plants, the
scientists decided to mimic D. hirsuta's hollow stems, called
domatia, which are the ants' primary nesting sites. Artificial
domatia were constructed out of foil-wrapped test tubes partially
filled with cotton. Two cedar saplings, with and without artificial
tubes, were planted in devil's gardens near two D. hirsuta saplings,
one with and one without domatia. After 24 hours, there was
significant leaf death on all of the cedar plants, but none on any
D. hirsuta saplings. "We conclude that M. schumanni attacks only
non-host plants, such as C. odorata, and that it does not rely on
the presence of domatia to discriminate between its host and other
plant species," the Stanford 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
ant species and, in fact, got its name from formica, which is Latin
for ant. "Treatment of leaves with formic acid induced leaf necrosis
on all the plants we tested," the authors write. "To our knowledge
this is the first record of an ant using formic acid as a
herbicide--although it is known to have bactericidal and fungicidal
properties."
The ants employ a very effective system of lethal injection,
notes Gordon, associate professor of biological sciences at
Stanford. "The system harnesses two fundamental tools: formic acid,
which many ant species use for other purposes, and the basic
circulatory system of all vascular plants," she says.
A census of the rainforest from 2002 to 2004 revealed that
devil'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
351 plants, is 807 years old," the authors conclude. They estimate
that a typical garden is tended by a single ant colony with as many
as 3 million workers and 15,000 queens, adding that the presence of
multiple queens "undoubtedly contributes to colony longevity."
Niche construction
"The cultivation of devil's gardens by ants is an excellent
example of niche construction," Frederickson says. "By killing
plants of 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 and
eliminating the competition, says co-author Michael J. Greene, a
former Sanford postdoctoral fellow, now assistant professor of
biology at the University of Colorado-Denver. "This work is a truly
remarkable example of how effectively ants can manipulate their
environment in order to promote 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 grow within the vegetation-free area created by the ants,
and the ant colony expands to occupy them. The devilry of M.
schumanni today provides homes for ants in the future."
Frederickson is conducting new field studies to determine which
chemical cues the ants use to discriminate between host plants and
other species. She also has begun searching for devil's gardens in
other parts of the western Amazon to see how widespread the
herbicide phenomenon is. "Megan's work reveals a system that is
amazing because the ants exert so much control over their
environment, creating single-species stands in one of the most
diverse places on Earth," Gordon says.
###
Funding for the Nature study was provided by the Stanford Center
for Evolutionary Studies, the Stanford Center for Latin American
Studies, Sigma Xi, the Stanford Graduate Fellowships Program and the
Natural Sciences and Engineering Research Council of Canada.
Editor's Note: The original news release can be found here.
.gif)
.gif)