Study
of grasshoppers' diets shows that animals are an important part of organic matter
decomposition
A grasshopper's change in diet to
high-energy carbohydrates while being hunted by spiders may affect the way soil
releases carbon dioxide into the atmosphere, according to research results
published this week in the journal Science.
Grasshoppers like to munch on
nitrogen-rich grass because it stimulates their growth and reproduction.
But when spiders enter the picture,
grasshoppers cope with the stress from fear of predation by shifting to
carbohydrate-rich plants, setting in motion dynamic changes to the ecosystem
they inhabit, scientists have found.
"Under stressful conditions they go
to different parts of the 'grocery store' and choose different foods, changing
the makeup of the plant community," said Oswald Schmitz, a co-author of the
paper and an ecologist at Yale University.
The high-energy, carbohydrate diet also
tilts a grasshopper's body chemistry toward carbon at the expense of nitrogen.
So when a grasshopper dies, its carcass
breaks down more slowly, thus depriving the soil of high-quality fertilizer and
slowing the decomposition of uneaten plants.
"This study casts a new light on
the importance of predation in natural communities," said Saran Twombly,
program director in the National Science Foundation's Division of Environmental
Biology, which funded the research.
"A clever suite of experiments
shows that the dark hand of predation extends all the way from altering what
prey eat to the nutrients their decomposing bodies contribute to soil."
Microbes in the soil require a lot of
nitrogen to function and to produce the enzymes that break down organic matter.
"It only takes a slight change in
the chemical composition of that animal biomass to fundamentally alter how much
carbon dioxide the microbial pool is releasing to the atmosphere while it is
decomposing plant organic matter," said Schmitz.
"This shows that animals could
potentially have huge effects on the global carbon balance because they're
changing the way microbes respire organic matter."
The researchers found that the rate at
which the organic matter of leaves decomposed increased between 60 percent and
200 percent in stress-free conditions relative to stressed conditions, which
they consider "huge."
"Climate and litter quality are considered
the main controls on organic-matter decomposition, but we show that aboveground
predators change how soil microbes break down organic matter," said Mark
Bradford, a co-author of the study and also an ecologist at Yale.
Schmitz added: "What it means is
that we're not paying enough attention to the control that animals have over
what we view as a classically important process in ecosystem functioning."
The researchers took soil from the
field, put it in test tubes and ground up grasshopper carcasses obtained from
environments either with or without grasshopper predators.
They then sprinkled the powder atop the
soil, where the microbes digested it.
When the grasshopper carcasses were
completely decomposed, the researchers added leaf litter and measured the rate
of leaf-litter decomposition.
The experiment was then replicated in
the field at the Yale Myers Forest in northeastern Connecticut.
"It was a two-stage process where
the grasshoppers were used to prime the soil, then we measured the consequences
of that priming," said Schmitz.
The effect of animals on ecosystems is
disproportionately larger than their biomass would suggest.
"Traditionally people thought that
animals had no important role in recycling of organic matter, because their
biomass is relatively small compared to the plant material that's entering
ecosystems," Schmitz said.
"We need to pay more attention to
the role of animals, however. In an era of biodiversity loss we're losing many
top predators and larger herbivores from ecosystems."
Other co-authors are Michael Strickland
of Yale, and Dror Hawlena of the Hebrew University of Jerusalem.
-NSF-
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