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

Abstract

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. K E Y W O R D Sabove-ground-below-ground interactions, brown food chain, earthworm, ecosystem model, grasshoppers, green food chain, nitrogen mineralization, old field

Freshwater mussels promote functional redundancy in sediment microbial communities under different nutrient regimes

“…While biogeochemical cycling is primarily carried out by microbial heterotrophs, animals can modify nutrient cycling both directly and indirectly. For example, terrestrial herbivores can alter nutrient dynamics in soil directly, through excretion or egestion, or indirectly through consumptive effects in which they alter plant‐microbe interactions (Buchkowski & Schmitz, 2022; Sitters et al, 2017). Invasive marine worms can influence nitrogen fluxes in the sediment by aerating sediments by burrowing which impacts respiratory pathways available to microbial heterotrophs (Tait et al, 2020).…”