Fear of predation slows plant-litter decomposition

Background/Question/Methods

Traditional concepts of trophic pyramids of biomass in ecosystems highlight that inputs of plant-derived materials to soils are more important than inputs from other trophic levels, in regulating belowground ecosystem processes, because plant-inputs quantitatively dominate biomass inputs. Accordingly predators are presumed to regulate ecosystem processes mainly by altering the quality and quantity of plant-derived materials entering belowground systems, through control of herbivore density (i.e. trophic cascades) and/or by altering herbivore foraging behavior. Our work addresses the general question about whether food web structure (especially the existence of predators) influences ecosystem functioning via changes in the nutritional contents of prey. We reared grasshopper prey in field mesocosms with and without risk of predatory spider predation. We measured decomposition of grasshopper carcasses and that of subsequent addition of much larger plant litter biomass, in the lab and in the field. We complemented these experiments by conducting another experiment that tested whether changes in proportions of protein to carbohydrate (with known C:N ratios) could lead to similar legacy effects.

Results/Conclusions

Grasshopper herbivores stressed by spider predators have a higher body carbon-to-nitrogen ratio than grasshoppers growing without spiders. This change in elemental content does not slow grasshopper decomposition but perturbs belowground community function, decelerating subsequent decomposition of plant-litter inputs. This legacy effect of predation on belowground community function appears regulated by the amount of herbivore-protein entering soil. Fear induces this top-down mechanism, emphasizing the need to consider the integral role of predators in maintaining vital ecosystem functioning.