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Changes in salt marsh detritivore identity influences on ecosystem multifunctionality

Citation

Hines, Jes (2021), Changes in salt marsh detritivore identity influences on ecosystem multifunctionality, Dryad, Dataset, https://doi.org/10.5061/dryad.sqv9s4n40

Abstract

Ecosystems world-wide experience changes in species composition in response to natural and anthropogenic changes in environmental conditions.  Research to date has greatly improved our understanding of how species affect focal ecosystem functions.  However, because measurements of multiple ecosystem functions have not been consistently justified for any given trophic group, it is unclear whether interpretations of research syntheses adequately reflect the contributions of consumers to ecosystems.  Using model communities assembled in experimental microcosms, we examined the relationship between four numerically dominant detritivore species and six ecosystem functions that underpin fundamental aspects of carbon and nitrogen cycling above- and below-ground.  We tested whether ecosystem responses to changes in detritivore identity depended upon species trait dissimilarity, food web compartment (aboveground, belowground, mixed), or number of responses considered (one to six). We found little influence of detritivore species identity on brown (i.e. soil-based) processes.  Only one of four detritivore species uniquely influenced decomposition, and detritivore species did not vary in their influence on soil nitrogen pools (NO3- and NH4+), or root biomass.  However, changes in detritivore identity influenced multiple aboveground ecosystem functions.  That is, by serving as prey, ecosystem engineers, and occasionally also as herbivores as well as detritivores, these species altered the strength of aboveground predator-herbivore interactions and plant-shoot biomass.  Yet, dissimilarity of detritivore functional traits was not associated with dissimilarity of ecosystem functioning.  These results serve as an important reminder that consumers influence ecosystem processes via multiple energy channels and that food web interactions set important context for consumer-mediated effects on multiple ecosystem functions.  Given that species are being lost, gained, and redistributed at unprecedented rates, we can anticipate that changes in species identity will have additional ecosystem consequences beyond those predicted by species’ primary functional role.

Methods

Experimental design: To determine the influence of changes in detritivore species on multiple ecosystem functions, we established 5 detritivore treatments in microcosms.  Species densities were chosen to fall within the range of naturally variable field densities, and also to establish communities with comparable detritivore biomasses: no detritivore, 50 Littorophiloscia vittata, 50 Orchestia grillus, 50 Melampus bidentatus, or 5 Littoraria irrorata) (Graça et al. 2000, Zimmer et al. 2002, 2004).  These microcosm densities correspond to 52 Pardosa m-2, 130 Littoraria m-2, and 1315 individuals m-2 for each the species stocked at 50 individuals per microcosm.  To tease apart the influences of detritivores that are realized via nutrient/decomposition pathways from influences on ecosystem functioning that are a consequence of their role as prey (Hines and Gessner 2012, Hines et al. 2015), the detritivore treatments were crossed with two levels of spider predation (4 Pardosa littoralis present or absent).  The full experimental was a randomized complete block, 5 ⅹ 2 factorial design experiment, which consisted of 10 treatments that were replicated in each of 9 spatial blocks, resulting in 90 microcosms. The experiment was located in a common garden habitat, at the Smithsonian Environmental Research Center in Edgewater, MD.    

Each microcosm consisted of a sand-filled pot (22 cm diameter x 21 cm deep) with five transplants of small Spartina plants, 25 g S. alterniflora leaf litter, and 30 adult Prokelisia herbivores (25 females and 5 males, corresponding to 825 individuals m-2).  Potted plants were enclosed in clear plastic tube cages (21.5 cm diameter x 30 cm tall), capped with a mesh top, and pots were placed in plastic pans (30 cm diameter x 10 cm deep) filled with water.  Treatments were established in a sequential fashion that approximated the phenology of field interactions.  Detritivore species, which remain in meadow habitats year-round, were added on 7 June 2007 and were allowed to settle and feed before herbivores emerged.  Prokelisia herbivores, which overwinter as nymphs in leaf litter and emerge as adults by the end of June, were added to microcosms on 25-27 June 2007 and allowed to settle for 4 days, before 4 juvenile Pardosa spiders were added to half of the microcosms.

The experiment was terminated on day 60, when visual counts indicated that the second generation adult planthoppers began a slight natural decline (Denno and Roderick 2003).  Because detritivores and spiders were often hidden in the leaf litter, it was not possible to obtain a visual assessment of their densities throughout the course of the experiment.  Therefore, final invertebrate densities were obtained by collecting all animals from microcosms using a vacuum sampler fitted with mesh bags (Shop-vac®, Williamsport, PA, USA).  Arthropods were killed using ethyl acetate, transferred into plastic bags, before they were counted in the laboratory. 

To assess plant growth and litter mass loss, S. alterniflora litter, root, and shoot biomass was harvested, washed and sieved through 1.7 mm sieves, freeze-dried, and weighed to the nearest 0.01 g.  Because damage imposed by phloem feeders can be difficult to detect, we did not distinguish between loss of plant biomass caused by detritivore grazing and phloem feeding herbivores.  Instead, herbivore abundance was used as a proxy for herbivory by strict herbivores, which is in line with rationale for energy flux to herbivores (Barnes et al. 2018, Gauzens et al. 2019), but could overlook the potential for non-linear density dependent feeding rates (Hines et al. 2016) or non-consumptive effects of predators (Peckarsky et al. 2008).  That is, herbivory was considered as ability of the ecosystem to support two generations of herbivore populations during the course of the experiment, and is a proxy related to secondary production of herbivores.  Pardosa predators molted to adults but, due to longer generation times, they did not reproduce during the time course of the experiment.  Nonetheless, for consistency, we also consider persistence of predators as a measure of predation potential.  Soil inorganic soil nitrogen (NH4+ and NO3-) was extracted using 2.0 M KCL (Mulvaney 1996). 

Statistical analysis:  We used a mixed model ANOVA (lme4: Bates et al. 2014) to assess the influence two fixed factors (i.e. detritivore identity and predator presence) on six individual functions as well as ecosystem multifunctionality.  For all analyses, the microcosm was considered as the experimental unit in which response variables were reported and spatial block was included as a random factor.  To calculate average ecosystem multifunctionality, we used the averaging approach (Byrnes et al. 2014), and included the average of all possible reported response variables in each microcosm (shoot biomass, root biomass, litter decomposition rate k, nutrient concentration [NO3-], herbivore abundance, predator abundance).  Predator abundance was not included in the calculation when predators were absent.  Because standardized nutrient concentrations of NH4+ and NO3- were similar, and neither molecule was sensitive to treatments in this experiment, we chose to report NO3- in the average ecosystem multifunctionality metric and text.

We used another mixed model analysis to consider how two additional factors (i.e. food web compartment, and number of functions included in the average multifunctionality metric) influenced the average multifunctionality responses in the original 2 ⅹ 5 factorial experimental.  In this analysis, we calculated average multifunctionality for each microcosm using all viable combinations of 1, 2, 3, 4, 5, and 6 standardized functions that filled the criteria of being from aboveground compartment, belowground compartment, or mixed compartment multifunctionality metric that included a combination of at least one aboveground and one belowground function.  The experimental design was necessarily unbalanced as some combinations of covariate calculations were not possible (e.g. mixed aboveground-belowground compartment with only one function, or aboveground compartment with three functions when predators were absent); For this analysis, we were interested in how the number of functions and/ or the focal compartment influenced variation in multi-functionality results, rather than the specific combination of functions included in calculations for each microcosm. Therefore, we included the identity of function combinations in the model as a random factor.  Spatial block was also included in the model as a random factor.  All data met assumptions of homogeneity of variance and normality of residuals.           

Literature Cited

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Funding

EPA-STAR fellowship, Award: FP-91648701-1

Garden Club of America wetland fellowship

Deutsche Stiftung Friedensforschung, Award: FZT-118

EPA-STAR fellowship, Award: FP-91648701-1

Garden Club of America wetland fellowship