Scared grasshoppers change soil chemistry

Grasshoppers who die frightened leave their mark in the Earth in a way that more mellow ones do not, US and Israeli researchers have discovered.

"Indeed this sounds a little bit weird," says lead author Dr Dror Hawlena in an audio interview posted on the journal Science's website.

Hawlena, a researcher at the Hebrew University of Jerusalem, along with colleagues at Yale University devised a test to measure the legacy of grasshoppers who were scared by spiders.

They placed cages in areas of natural vegetation and allowed some grasshoppers to be alone while others were placed in cages with a spider.

They glued the mouths of the spiders shut in order to make sure that the grasshoppers experienced pure fear, but were not actually killed by the predators.

Previous research has shown that grasshoppers, which normally consume nitrogen-rich grass, move to a diet of carbohydrate-rich grass to cope with stress.

In their experiment, which appears in the journal Science, Hawlena and colleagues found that this resulted in the carbon to nitrogen ratio of the frightened grasshoppers changing by about four percent.

The researchers then buried the carcasses of the grasshoppers in soil containing grass litter to measure the decomposition.

They found the grass decomposed between 60 per cent and 200 per cent faster in soil containing stress-free grasshoppers compared to the stressed grasshoppers, a result they say is "huge."

"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," says co-author Professor Oswald Schmitz, of the Yale School of Forestry & Environmental Studies.

"This shows that animals could potentially have huge effects on the global carbon balance because they're changing the way microbes respire organic matter."

"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."

Hawlena says the findings shed light not only on how predators and prey influence the makeup of the soil, but how stresses invoked by drought and extreme heat might have lasting effects on crops and growth cycles.