The process of energy and materials transfer governs ecosystem production, food chain length and species diversity.  Herbivores are pivotal in this process because their low plant assimilation efficiencies constrain transfer rates and ecosystem carbon: nitrogen balance.  Theory suggests that physiological stress due to predation risk could be an important determinant of trophic transfer efficiency and ecosystem nutrient budget. We tested this assertion in a series of field and laboratory experiments. We reared grasshopper herbivore nymphs with or without risk of spider predation and measured their metabolic rate, nutritional requirements, and excretion and body elemental composition. We also calculated how the risk of spider predation affects elemental composition of uneaten plant material entering the soil organic matter pool. 

Results/Conclusions

We show that shifting nutrient demand of herbivores facing predation risk causes carbohydrate carbon to become an important nutrient to fuel stress-heightened respiration. This had cascading effects on C:N balance of herbivore body tissue, fecal material and uneaten plant material. Fear from predation risk thus represents an endogenous mechanism regulating materials transfer via chronic exacerbation of herbivore physiological stress and shift in what becomes a limiting nutrient.