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

Citation

Buchkowski, Robert; Schmitz, Oswald (2021), Weak interactions between strong interactors in an old-field ecosystem: Control of nitrogen cycling by coupled herbivores and detritivores, Dryad, Dataset, https://doi.org/10.5061/dryad.t76hdr824

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 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.
  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 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.
  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.

Methods

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 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 M. femurrubrum 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 ×  earthworm interactions to test for their significance.

The code also contains a theoretical ecosystem model describing the above ecosystem and experiment.

Usage Notes

The descriptions of all the data columns is included as a Schema file in the data set.