DOI: 10.5061/dryad.t76hdr824
title: Weak interactions between strong interactors in an old-field ecosystem: Control of nitrogen cycling by coupled herbivores and detritivores

description: 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.
 We present the results of a three-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 microorganisms. Then, we used a
 dynamical systems 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. 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 systems model made the same prediction across a
 broad section of parameter space (e.g., feeding rates, death rates, etc)
 and after longer term simulations. 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.
description: We conducted a three-year
 experiment in an old-field ecosystem at Yale-Myers Research Forest in
 Northeastern Connecticut (41°56'48.1"N,
 72°07'13.5"W).  The experiment was conducted under field
 conditions that were expected to generate strong interactions between
 grasshoppers and earthworms. The<i> </i>experiment deployed 60
 mesocosms that transcended the aboveground-belowground interface. We
 filled the belowground portions of each mesocosm with the homogenized soil
 up to the surrounding soil surface. The entirety of year 1 (Fall 2015 to
 Fall 2016) of the experiment was devoted to establishing baseline plant
 and microbial communities. Subsequent years examined the effects of
 grasshopper and earthworm manipulations on nitrogen cycling. We
 established the plant community to guarantee functional diversity before
 allowing natural colonization from the surrounding field to increase
 species diversity.  Treatments were a full 2 x 2
 factorial combination of i) earthworm addition versus removal, and ii)
 grasshopper presence versus absence. Beginning in fall 2016, we
 electroshocked all the mesocosms every fall (October-December) and spring
 (March-May) to remove earthworms, survey their abundance, and add them at
 a constant density to the earthworm addition mesocosms. In early July 2017
 and 2018, we collected third-instar <i>M. femurrubrum</i>
 grasshoppers from a nearby field using sweep nets and added five of them
 to each of the grasshopper only and grasshopper & earthworm
 treatments. Caterpillars and moths were regularly removed from the
 treatment mesocosms to exclude non-focal herbivore effects. 
 We measured plant biomass,
 plant community composition, soil nitrogen mineralization, and microbial
 biomass. Each fall from 2016 to 2018 we measured aboveground plant biomass
 after plant senescence by collecting and weighing the plant litter after
 drying at 60°C to a constant mass.We estimated the percent cover of each
 plant species in the spring, summer, and fall. We used two methods to
 measure soil nitrogen mineralization: ion exchange membranes placed in the
 field soil and laboratory soil incubations to measure microbial
 mineralization potential. Microbial activity was measured as
 substrate-induced respiration (SIR) in the laboratory using yeast extract
 as a substrate.   Our statistical analyses were
 designed to resolve the individual effects of grasshoppers and earthworms
 on key ecosystem properties and test for any significant interactions
 between them. We used linear mixed effects models for univariate response
 variables (e.g., plant biomass), a redundancy analysis to explore
 responses at the level of plant community composition, and a structural
 equation model (SEM) to understand the relationships between the dependent
 variables in our linear mixed effects models. In each of these three
 analyses, we compared models with and without grasshopper
 <i>×</i>  earthworm interactions to test for their significance. The code also contains a theoretical ecosystem model describing the above ecosystem and experiment.
description: The descriptions of all the data columns is included as a Schema
 file in the data set.