Researchers at the Hebrew University of Jerusalem and at Yale University have discovered that grasshoppers stressed by spiders affect the productivity of soil.
“How do grasshoppers who are being frightened by spiders affect our ecosystem? In no small measure,” the researchers said.
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.
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.
In the study, published in the journal Science, the team exposed grasshoppers to spiders in order to arouse the stress reaction. The researchers 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, they 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 % faster than in the samples in which the stressed grasshoppers 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 lead author Dr Dror Halwena of the Hebrew University of Jerusalem.
“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 concluded.
Bibliographic information: Hawlena D., Strickland M.S., Bradford M.A., Schmitz O.J. 2012. Fear of Predation Slows Plant-Litter Decomposition. Science vol. 336 no. 6087 pp. 1434-1438; doi: 10.1126/science.1220097