Hebrew University, Yale researchers show how grasshoppers 'stressed' by spiders affect the productivity of our soil.
How do grasshoppers who are being frightened by spiders affect our ecosystem? In no small measure, say researchers at the Hebrew University of Jerusalem and at Yale University in the US.
A grasshopper who is in fear of an attacker, such as a spider, will enter a situation of stress and will consume a greater quantity of carbohydrate-rich plants -- similar to humans under stress who might eat more sweets.
This type of reaction will, in turn, cause chemical changes in the grasshopper and in its excretions, affecting the ecosystem it inhabits. How does this happen?
When the scared grasshopper dies, its carcass, now containing less nitrogen as a result of its diet change, will have an effect on the microbes in the ground, which are responsible for breaking down animals and plants.
With less nitrogen available, the microbes will be decomposing the hard-to-break-down plant materials in the soil at a slower rate. Thus, the fear of predation may slow down degradation of complex organic materials to the simpler compounds required for plant growth.
Research on this biological-ecological phenomenon was carried out by Dr. Dror Halwena of the Department of Ecology, Evolution and Behavior at the Alexander Silberman Institute of Life Sciences at the Hebrew University of Jerusalem, in cooperation with researchers at Yale University in the United States. An article on their research appears in the current edition of the journal Science.
In their research, the scientists exposed grasshoppers to spiders in order to arouse the stress reaction. They also used a control group of non-stressed grasshoppers. The scared grasshoppers had a higher carbon-to-nitrogen ratio in their bodies than non-scared grasshoppers.
In further laboratory and field tests, the researchers tested the influence of remains of grasshoppers from the two groups on soil. After the microbes consumed the grasshopper remains, the researchers added plants to the surface. In the experiments, it was shown that the decomposition rate of the plants in the areas in which the stress-free grasshopper remains were introduced decomposed at a rate between 62 and 200 percent faster than in the samples in which the stressed grasshopper s were put.
In a further experiment, the researchers used "artificial grasshoppers" -- a mixture of sugar, protein, and chitin (the organic compound found in the grasshopper external skeleton) -- in varying quantities. Here, too, they found that even a small amount of nitrogen (found in the protein) added to the soil increases significantly the functioning of the microbes responsible for breaking down the organic matter in plants.
"We are dealing here with an absolutely new kind of mechanism whereby every small chemical change in a creature can regulate the natural cycle, thus in effect affecting the ecology in total, such as the amount of carbon dioxide released into the atmosphere (through decomposition) and field crop productivity. This has tremendous consequences for our ecological understanding of the living world," said Dr. Halwena.
"We are gaining a greater understanding of the necessity of conserving all of the component parts of the ecosystem in general and of predators in particular. We are losing predators in nature at a much faster rate than other species," Dr. Halwena commented.
He also viewed the research as a vehicle that will enable scientists to better predict changes in the biological system as a result of such human-induced phenomenon as overfishing, hunting or global warming that can undermine the entire ecosystem.
- Dror Hawlena, Michael S. Strickland, Mark A. Bradford, and Oswald J. Schmitz. Fear of Predation Slows Plant-Litter Decomposition. Science, 15 June 2012: 1434-1438 DOI: 10.1126/science.1220097
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