Stress and Ecosystems: Role of Predation Reconsidered in the Hunt for Stable Ecology

Ecosystems are losing predators faster than organisms in any other trophic level, and this may be of even more concern for the stability of nutrient cycles than previously thought. Recent research at the Yale School of Forestry and Environmental Studies from former postdoctoral associate Dr. Dror Hawlena and his colleagues demonstrates how predators considerably influence belowground community functions by instilling fear in their prey.

Stressed grasshoppers reared in the presence of spider predators show elevated metabolisms and consequently have diets high in energy-rich carbohydrates. In addition, stress hormones increase the conversion of proteins to glucose, a process that excretes nitrogen. These two reactions to stress increase the carbon–to-nitrogen ratio in grasshoppers’ bodies.

The grasshopper herbivore Melanoplus femurrubrum. Courtesy of Dr. Dror Hawlena.

Hawlena’s experiments tested the effects of this body chemistry variation on the decomposition of plant litter. He reared grasshoppers in the field with and without the risk of spider predation. “You can manipulate the risk of predation without actually killing the prey by gluing the mouthparts of spiders,” Hawlena explained. The grasshopper carcasses were then allowed to decompose in laboratory microcosms of their grassland habitat.

Plant litter was added to each of the microcosms, and the researchers measured rates of carbon mineralization, the release of carbon dioxide during microbial decomposition. Notably, the mineralization of grass litter in soil where the stressed grasshoppers decomposed was 62 percent lower than the soil with the non-stressed grasshoppers. These results suggest a causative link between the changes in prey body chemistry and altered functioning of the soil microbial community. Hawlena hypothesizes that this is due to nitrogen’s role in priming microbes to break down plant litter. In the presence of lower levels of nitrogen, plant decomposition decreases.

Conducted over a three-year period, variations on the experiment included measuring decomposition rates in the field rather than in the laboratory microcosm, and using artificial grasshopper carcasses whose C:N ratios spanned greater variation than those observed in nature. Collectively, the experiments’ results suggest that predation exerts top-down, cascading effects on plant decomposition in both laboratory and field settings by fear-induced changes in prey body chemistry.

Graphs A & B above show the cumulative carbon mineralization of the decomposition of the grasshoppers and plant litter, respectively. Steps 1 and 2 show the decomposition of grasshoppers and plant litter and the corresponding respiration of carbon dioxide. Courtesy of Dr. Dror Hawlena.

Hawlena stresses the importance of these findings. “We show that if you add a tiny amount of animal you can change the way the microbial community is processing resources. Therefore even a small amount of animal detritus can be extremely influential in regulating the recycling rates of different nutrients.”

This research has major repercussions for how scientists think about ecosystems. The interconnections of every trophic level have been underestimated, especially in terms of top-down control and nutrient cycles. The biological, chemical, and physical stability of ecosystems is truly reliant on each member’s contributions. The predators, whose role in the ecosystem is more important than previously expected, are disappearing at alarming rates. Hawlena warns, “These results may be an important hint that we need to take seriously what we do to the predators.”