Frightened animals make bad fertilizer.
You are tense and wary, alert to every rustle and snapped twig. A predator is near, you can sense it. Your heart races; you sweat. Quietly, you reach for a doughnut.
Stress speeds up the metabolism of grasshoppers, making them seek out easily digested sugars and carbohydrates for a quick energy boost. This and other results, published in three journals in the past month, could have big implications — not just for prospective prey, but also for the ecosystems they live in.
In more relaxed conditions, many animals opt for high-protein foods that help them to grow and reproduce. But with a predator lurking, they need fuel to quickly feed their amped-up bodies — and to bolt, if needs be. Dror Hawlena, an ecologist at Yale University in New Haven, Connecticut, has been teasing out the ecological ramifications of this predation stress in meadows.
In cages placed on naturally growing vegetation, Hawlena added grasshoppers and, in some cases, spiders with their mouthparts glued shut, so that they could induce fear without killing the grasshoppers. Grasshoppers that were exposed to spiders switched from eating protein-rich grasses to munching on several species of sugary goldenrod plants.
Initially, this diet shift was thought to be related to how easy it is for grasshoppers to hide from spiders in the branched and flowering goldenrod. To separate out the possible effects refuge-seeking, Hawlena also studied grasshoppers and muzzled spiders in indoor terrariums. Instead of plants, the grasshoppers were fed with an artificial diet of high-sugar or protein-rich 'biscuits' — and he saw the same trend. Fearful grasshoppers went for the high-sugar cookies rather than the protein-rich bars1.
All that sugary food means that the stressed-out insects are ingesting foods richer in carbon and poorer in nitrogen than their calmer, protein-pumping cousins. Meanwhile, their bodies are breaking down proteins to make even more glucose. The result is a body that is made of significantly more carbon and less nitrogen — and thus makes poorer fertilizer when it dies and rots.
Hawlena thinks that the ecosystem is likely to be changed in two ways by frightened grasshoppers. First, they eat more goldenrod and less grass, changing the ratio of these species in the landscape. Second, the soil is receiving less nitrogen, potentially influencing what can grow there. In ongoing experiments, Hawlena is getting intriguing results by looking at the different kinds of soil bacteria that thrive on stressed or unstressed grasshopper corpses. He expects to see a similar story in the bodies of most other animals. The stressed-out living are likely to alter their diet, and the relaxed and happy dead are likely to make better fertilizer2.
Hawlena says that this phenomenon may help ecologists to understand previously unexplained ecosystem changes, and could move ecology closer towards being a fully predictive science.
The elk effect
But the relationship, if it exists, may not be clear cut. Like Hawlena's grasshoppers, the elk of Yellowstone National Park in Wyoming were thought to eat differently because of the threat of predation. Some researchers proposed that the return of wolves to the park would cause elk to begin avoiding certain 'risky' areas containing the predators. That in turn would allow aspen trees — munched into submission by the elk — to begin growing back in those areas. Even in small numbers, the theory went, wolves could have a huge effect on the landscape.
Alas not, according to a recent study by Matthew Kauffman of the US Geological Survey in Laramie, Wyoming, and his colleagues. Tree rings and fenced-off experimental areas revealed that aspen growth didn't track well with wolf presence or absence. The elk do change their behaviour in response to wolves and do avoid risky areas in general — just not often enough to change the picture for aspen. The effect, says Kauffman, doesn't "scale up"3.
This might be because elk that are near starvation — as many often are in the winter — are willing to take any risk to eat. "All it takes is one bull in really poor condition to wander into a risky habitat, to say 'to hell with wolves' and feed there for a few days," says Kauffman.
It remains to be seen whether the physiological effects of stress on grasshoppers scale up to plants, soil, bacteria and onwards, or whether the effect is too small and is swamped by all the other convoluted causal factors in ecosystems.
"It is very intriguing that [Hawlena and colleagues] can detect these effects. What will determine the importance ecologically is how strongly these effects translate from one level of biological organization to another," says Kauffman.
Hawlena, D. & Schmitz, O. J. Proc. Natl Acad. Sci. USA 107, 15503-15507 (2010).
Hawlena, D. & Schmitz, O. J. Am. Nat. advance online publication doi:10.1086/656495 (2010).
Kauffman, M. J., Brodie, J. F. & Jules, E. S. Ecology 91, 2742-2755 (2010).
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Marris, E. How stress shapes ecosystems. Nature (2010). https://doi.org/10.1038/news.2010.479