Weak interactions between strong interactors in an old-field ecosystem: Control of nitrogen cycling by coupled herbivores and detritivores

1. Interactions between herbivores and detritivores are common in greenhouse and laboratory experiments. Such interactions are thought to cause feedbacks in real ecosystems where the combined actions of these animals create either high or low nutrient cycling rates. There is limited evidence from factorial field experiments to support these expectations. 2. We present the results of a 3-year experiment wherein we factorially manipulated grasshopper herbivores and earthworm detritivores in an old-field ecosystem and tested for significant interaction effects on plants, nitrogen mineralization and micro-organisms. Then, we used a dynamical system model built and parameterized for the study ecosystem to test the theoretical strength of these interactions. We predicted that grasshoppers and earthworms would have a positive interaction effect on plant growth and nitrogen cycling by driving plant community change. 3. We found neither evidence for interaction effects on any of the variables we measured nor a consistent change in the composition of the plant community even though the individual effects of grasshoppers and earthworms were as expected. Our dynamical system model made the same prediction across a broad section of parameter space (e.g. feeding rates, death rates, etc) and after longer term simulations. 4. Our results suggest that interactions between herbivores and detritivores are only likely in situ when animals have exceptionally high individual effects on ecosystems and where the exogenous forces driving plant community change and soil biogeochemical fluxes are weak. A free Plain Language Summary can be found within the Supporting Information of this article.

Automated summary

Interactions between herbivores and detritivores may only occur in situ when animals have exceptionally high individual effects on ecosystems and when external forces driving plant community change and soil biogeochemical fluxes are weak.