| Press Images
The Nanay River flows through Peru's Allpahuayo-Mishana
Reserve near the site of a study on rainforest diversity.
The research was conducted by biologists from the University
of Utah and the National University of the Peruvian Amazon.
To download
high-resolution click here:
Credit: Jose Alvarez Alonso, Research Institute of the Peruvian
Amazon. |
A new University of Utah study found that diversity of rainforest
trees actually increases because insects, including grasshoppers
like this, eat trees that otherwise would dominate the ecosystem.
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high-resolution click here:
Credit: Paul Fine, University of Utah. |
In a University of Utah experiment, hundreds of Peruvian
rainforest tree seedlings were grown in net enclosures (left)
to keep out insects. Other seedlings were covered only by
a net roof (right) so they got the same amount of sunlight
but were exposed to insects.
To download
high-resolution click here:
Credit: Paul Fine, University of Utah.
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July 29, 2004 -- A University of Utah experiment
conducted in Peru’s Amazon Basin shows insects increase
the diversity of the rainforest when they munch on trees. Such
seemingly destructive behavior keeps dominant tree species under
control but allows other trees to thrive.
“The battle between plants and insects increases the number
of habitats in the rainforest,” thus increasing the diversity
of trees living there, says biology doctoral student Paul Fine,
first author of the study published July 30 in the journal Science.
Study co-author Phyllis Coley, a biology professor at the University
of Utah, says the research sheds light on the amazing diversity
of species in tropical rainforests.
“Understanding diversity is a holy grail in ecology,”
Coley said. “Part of diversity is how many species you can
pack into a given habitat, but part is how many different habitats
exist that can harbor different species” – an issue
clarified by the study.
Fine and Coley conducted the study with Italo Mesones, a student
at the National University of the Peruvian Amazon in Iquitos.
The study was Fine’s Utah doctoral thesis, which he completed
while also working at Chicago’s Field Museum of Natural
History.
Some trees grow on red clay, others on white sand
In the Peruvian rainforest, many trees live on fertile red clay
soil, while others live on nutrient-poor white sand soils. In
the new study, the scientists transplanted red clay tree species
onto white sand soil, and transplanted white sand tree species
onto red clay soil. Net-covered enclosures were built around half
the transplanted trees to protect them from insects. Other transplanted
trees were left unprotected.
Trees that normally grow on red clay soil thrived when transplanted
onto white sand soil – but only if protected from hungry
insects, which included grasshoppers, beetles, caterpillars and
various sap-sucking insects such as cicadas and aphids.
In fact, when protected from insects, the red clay tree species
transplanted onto white sand soil grew roughly twice as tall and
had twice the leaf area as white sand species. That’s because
the white sand species grow slowly because they put more energy
into defenses against insects – either tough leaves or chemical
defenses.
“A plant can’t be extremely well-defended from insects
and grow very fast,” says Fine. “It’s similar
to why you can’t have the heaviest, safest car and the fastest
car.”
Unprotected, the red clay trees transplanted onto white sand soil
died at twice the rate of trees that normally live on white sand
soil.
Meanwhile, white sand trees transplanted onto red clay soil grew
more slowly than the trees that normally live on clay soil. Net
enclosures didn’t increase their survival because they already
had natural defenses against insects.
The findings mean red clay tree species would grow on and dominate
both habitats – red clay soils and white sand soils –
were it not for the presence of insects that normally prevent
the red clay tree species from living on white sand soil.
“My results suggest that if bugs weren’t around, the
faster-growing clay soil plants could live in both soils, and
the white sand tree species would go extinct,” Fine says.
“Therefore, bugs promote diversity in the rainforest by
making it impossible for red clay trees with low defenses [against
insects] to live in poor white sand soils.”
Coley says: “Most people think of herbivores as detrimental
pests – bugs eat plants, which is not good for the plants.
Yet without insects, clay tree species would take over all forest
types. Bugs beget tree diversity.”
Implications for diversity and the origin of species
The study shows different soil types are not adequate by themselves
to define habitats or niches – what trees grow where –
in the Peruvian rainforest. The findings contradicted a hypothesis
that trees normally growing on red clay soil would not survive
on white sand soil due to inadequate nutrients and water.
“People have thought soil alone is enough to explain why
you get two different communities of plants growing on those two
soil types,” says Fine. “But we showed that if it
wasn’t for their insect enemies, it’s possible the
same kind of plants would grow on both soil types. The insects
cause the difference between habitats to become sharper.”
Red clay tree species also are attacked by insects when they grow
on clay soils, but the soil is so rich that the trees grow faster
than they are consumed. And other insects and birds in the lush
red clay forest help control plant-eating insects.
Coley says the study has implications for rainforest conservation:
“White sand soils have unique species, sand patches are
not as extensive as they used to be, and land conversion [development]
therefore could have a big impact on diversity if it hit the relatively
rarer sand forests.”
In fact, during the study Fine and Mesones fended off a land grab
by speculators, even though their study site in the Allpahuayo-Mishana
Reserve supposedly was protected already. Mesones started an environmental
group that helped stop the land grab.
Fine says tropical rainforests are known for biodiversity, with
about 10,000 tree species in the Amazon compared with 500 to 600
in North America’s temperate zone.
Yet the tropics really aren’t different than other regions
in terms of variations in soil, altitude and rainfall. So why
do tropical rainforests have greater species diversity?
“In a world without herbivores, our study predicts there
would be a much lower diversity of trees in the tropics,”
Fine says. “Trees would just follow the physical features
of the environment – whether soil, temperature or rainfall.
Adding pressure from plant-eating insects into the mix increases
the number of different niches or habitats.”
Fine says the study showed that differences between two habitats
– red clay and white sand soil – are magnified by
the effect of insects eating trees. He says plant-eating insects
might have the same impact on accentuating differences between
habitats defined by differences in altitude, rainfall or other
factors.
The study also adds support to a controversial theory of how new
species originate, Fine says. The traditional view has been that
new species can arise when two groups from the same species become
geographically separated, such as when the groups live on separate,
distant islands. But in recent decades, biologists have started
to wonder if a new species can arise even when two groups live
in close contact, such as trees living near each other but on
different soils.
How the study worked
Fine and Mesones conducted the experiment from May 2001 to February
2003.
The study involved 20 species of tree seedlings, matched in 10
pairs. One species in each pair normally lives on red clay soil,
while the other species in the pair was a closely related tree
that normally lives on white sand soil.
The researchers built 22 cage-like enclosures made of nylon netting
to keep insects out. Each cage measured about 10 feet wide, 10
feet deep and 6.5 feet tall. They also built another 22 net roofs
without sides, to make sure trees planted under them were exposed
to insects but got the same amount of light as trees in the enclosures.
One seedling of each of the 20 species was planted in each of
the 22 cages and under each of the 22 net roofs. That meant a
total of 880 seedlings were transplanted: 440 red clay seedlings
transplanted onto white sand soil, and 440 white sand tree seedlings
transplanted into red clay soil. Half of the seedlings were protected
from insects.
The transplanted trees were allowed to grow for 21 months. Every
two months Fine and Mesones determined how fast each tree was
growing by measuring its average leaf area and the height to its
meristem – the part of the tree where new branches and leaves
emerge. They also counted how many seedlings died.
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