The evolutionary ecology of predator-driven elemental cycling: A stoichiometrically explicit approach

Background/Question/Methods

Elemental cycling is a critical ecosystem process viewed as fundamentally bottom-up regulated. This view is inconsistent with emerging empirical evidence that predators can regulate elemental cycling from the top-down through consumptive (Predation) and more importantly stress-inducing non-consumptive (Risk) effects on their prey. Yet there is insufficient theory to predict how such top-down effects should propagate throughout ecosystems. We developed a general theoretical model of ecosystem functioning based on empirical understanding that predators influence elemental cycling through a fundamental evolutionary ecological mechanism of adaptive physiological plasticity of herbivore species in response to predation risk stress. Our stoichiometric model couples N and C cycling to quantify how predators cause shifts in elemental stocks and fluxes, primary and secondary production and trophic transfer efficiencies among soil, plant, herbivore and predator trophic compartments. We conducted a modeling experiment that systematically compared the ecosystem effects of predator presence and their mechanism of effect (Risk, Predation, Risk & Predation) with ecosystem effects when they are absent (No Predation). 

Results/Conclusions

Risk, Predation, and Predation & Risk treatments had higher soil N than the No Predation treatment. Risk and Risk & Predation treatments had higher N stocks than plants in No Predation and Predation treatments. Herbivores under Risk had less N than herbivores in No Predation treatments, consistent with empirical evidence that they should release N under stress. Predation and Risk & Predation reversed the Risk effect on herbivores. Predators in the Risk & Predation treatment had less N than Predators in the Predator treatment. Treatment effects on C stocks mirrored N stocks. Risk, Predation and Risk & Predation caused higher primary productivity than No Predation. Risk and Predation increased secondary productivity above the No Predation treatment; Risk & Predation greatly exacerbated the productivity enhancing effect. Ecological efficiency of primary producers was higher under Risk, Predation, and Risk & Predation than under No Predation. Secondary producer efficiency was similar between No Predation and Risk conditions and was less than efficiency in Predation and Risk & Predation. The modeling experiment clearly revealed that predators can have important effects on ecosystem properties and functions relative to when they are absent.  Moreover, the outcomes are predictably related to the degree of risk imposed on herbivore prey, suggesting trophic control of ecosystems emanates from the “middle-out” rather than from top-down or bottom-up when predator effects permeate ecosystems.