Jun 18, 2012
Scared grasshoppers make plants rot more slowly
Despite their position at the top of the food web, predators in many ecosystems are being lost faster than other species. Now, it seems, they can affect even the rate of plant decomposition in soil litter, not through their consumption of other species but by altering the body chemistry of their herbivore prey. As the bodies of those prey that escape being eaten decompose, the balance of nutrients reaching the soil microbial community changes.
"We explore a whole new pathway that shows how herbivores and their predators may regulate ecosystem functioning via control over the function of the microbial community," Dror Hawlena of Yale University, US, and the Hebrew University of Jerusalem, Israel, told environmentalresearchweb. "This new mechanism contrasts classic thinking in ecosystem ecology and biogeochemistry that animals can alter nutrient cycling mainly via regulation of plant communities."
Hawlena and colleagues from Yale and Virginia Polytechnic Institute and State University found that Melanoplus femurrubrum grasshoppers scared by the presence of predator spiders contained a slightly higher carbon:nitrogen ratio than grasshoppers reared in the absence of Pisuarina mira. When the previously frightened grasshoppers were left to decompose and grass litter was added to the soil, the grass rotted significantly more slowly than in a similar experiment using grasshoppers that were unbothered by spiders.
"In the last couple of years, I have developed and empirically tested a new theoretical framework that uses prey stress responses to predation risk to link food–web interactions to biogeochemical processes at the ecosystem level," said Hawlena. "This approach is needed because we currently lack predictive theory that links organismal ecology that focuses on species identity and functional traits with the more holistic ecosystem ecology that focuses on energy and material fluxes."
During this research, Hawlena found that stressed prey crave more sugar to meet their high-energy requirements, and consume fewer proteins. This reduces the amount of nitrogen in their waste products and bodies.
"We predicted that such changes in material and energy are going to affect ecosystem processes by altering the function of the soil microbial community when the prey body and waste material decomposed," he said. "The rationale behind this hypothesis is that microbes that get nitrogen start to produce exoenzymes that work outside the cells. Those enzymes break down complex bio-molecules to simpler compounds, generating a positive feedback."
Hawlena believes that in this way a small addition of nitrogen to nitrogen-limited soils can generate a much larger effect on the decomposition of low-quality detritus such as plant litter.
"In the same way that different intensities of fire determine the way different parts of a homogenous field will recover, leading to different plant communities, one would expect to have similar responses when exposing microbe communities to detritus of different nutritional quality – stressed versus non-stressed prey," he said.
Hawlena believes that the research could help predict how human-induced changes in biodiversity will affect life-supporting ecosystem processes such as nutrient cycling, primary productivity and carbon sequestration.
"This is a first demonstration that our innovative theory could actually link physiological processes in response to the fear of predation to biogeochemical processes," said Hawlena. "In the future I plan to continue exploring the mechanistic details of this theory across ecosystems, to develop the theory further and to expand its scope to other natural and human-induced environmental stressors."
The researchers reported their work in Science.
About the author
Liz Kalaugher is editor of environmentalresearchweb.