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1. Intraspecific diversity at two trophic levels influences plant–herbivore interactions

   https://doi.org/10.1002/ecs2.3121  

   Noto, Akana E. ; Hughes, A. Randall ( May 2020 , Ecosphere)

   Diversity within species can have community‐level effects similar in magnitude to those of species diversity. Intraspecific diversity in producers and consumers has separately been shown to affect trophic interactions, yet we have little understanding of how variation at these two levels could simultaneously affect trophic interactions. Salt marshes dominated bySpartina alternifloraare an ideal system in which to ask this question as this plant exhibits substantial genetically based trait variation. Further, herbivores can have sizable impacts onSpartina, but the impact of herbivore trait variation is not well understood. We conducted an experiment in a Massachusetts salt marsh to determine how herbivorous crab (Sesarma reticulatum) size diversity andSpartinagenotypic diversity affect the plant community. Herbivore effects on plant traits varied by herbivore size, with large crabs generally having stronger impacts on plants. At times, the effect of small crabs on plant traits depended on plant genotypic diversity. The effects of crab size diversity (i.e., small and large crabs combined) were most often predicted by the independent effects of each size class, though there were synergistic effects on stem density, flowering stems, and mean stem height. Finally, we tested whether herbivore size or size diversity could have reciprocal effects on plant genotypic diversity. Small‐ and mixed‐crab treatments promoted plant genotypic richness, whereas large crabs did not. Our results demonstrate that intraspecific diversity at multiple trophic levels can have simultaneous and sometimes interactive effects on species interactions, highlighting the importance of variation within species for understanding species interactions and community processes.

    

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2. Consumer trait variation influences tritrophic interactions in salt marsh communities

   https://doi.org/10.1002/ece3.1564  

   Hughes, Anne Randall ; Hanley, Torrance C. ; Orozco, Nohelia P. ; Zerebecki, Robyn A. ( June 2015 , Ecology and Evolution)

   The importance of intraspecific variation has emerged as a key question in community ecology, helping to bridge the gap between ecology and evolution. Although much of this work has focused on plant species, recent syntheses have highlighted the prevalence and potential importance of morphological, behavioral, and life history variation within animals for ecological and evolutionary processes. Many small‐bodied consumers live on the plant that they consume, often resulting in host plant‐associated trait variation within and across consumer species. Given the central position of consumer species within tritrophic food webs, such consumer trait variation may play a particularly important role in mediating trophic dynamics, including trophic cascades. In this study, we used a series of field surveys and laboratory experiments to document intraspecific trait variation in a key consumer species, the marsh periwinkleLittoraria irrorata, based on its host plant species (Spartina alternifloraorJuncus roemerianus) in a mixed species assemblage. We then conducted a 12‐week mesocosm experiment to examine the effects ofLittorariatrait variation on plant community structure and dynamics in a tritrophic salt marsh food web.Littorariafrom different host plant species varied across a suite of morphological and behavioral traits. These consumer trait differences interacted with plant community composition and predator presence to affect overall plant stem height, as well as differentially alter the density and biomass of the two key plant species in this system. Whether due to genetic differences or phenotypic plasticity, trait differences between consumer types had significant ecological consequences for the tritrophic marsh food web over seasonal time scales. By altering the cascading effects of the top predator on plant community structure and dynamics, consumer differences may generate a feedback over longer time scales, which in turn influences the degree of trait divergence in subsequent consumer populations.

    

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3. Resource availability and plant age drive defense against herbivory in salt marshes

   https://doi.org/10.1111/oik.09672  

   Wittyngham, Serina S. ; Carey, Joanna ; Johnson, David Samuel ( August 2023 , Oikos)

   The resource‐availability hypothesis (RAH) and the intraspecific RAH (RAH intra ), posit that resources, (i.e. nutrients) control plant antiherbivore defenses. Both hypotheses predict that in low‐resource environments, plant growth is slow, and constitutive defense is high. In high‐resource environments, however, the RAH predicts that plant growth is fast, and constitutive defense is low, whereas the RAH intra predicts that increased resources attract more herbivores, and this intensified grazing pressure leads to high constitutive defense. Salt marshes are nutrient‐limited ecosystems threatened by eutrophication and chronic herbivory, yet we know little about how these stressors shape saltmarsh plant antiherbivore defenses, which influence trophic interactions and ecosystem resilience. We manipulated resource availability via nutrient addition and herbivory via the marsh periwinkle Littoraria irrorata , on the saltmarsh foundation species Spartina alterniflora , in mesocosms. Because plant age can also influence trait variation, we measured traits in both original and clonally‐grown new stems. Feeding assays then evaluated how treatments and plant age affected subsequent Littoraria consumption of Spartina . Nutrient addition stimulated growth, while decreasing defensive traits (e.g. fiber and silica content), following the RAH. Herbivory enhanced belowground production and increased stem diameter, yet did not induce defensive traits, contrary to our expectations. Herbivory plus nutrients increased Spartina biomass and reduced phenolics, a defensive trait, further supporting the RAH. Regardless of treatment, clonally‐grown new stems had greater variation in measured traits. Despite altered traits, however, treatments and plant age did not affect Littoraria consumption. Our results support the RAH and part of the RAH intra and suggest: 1) nutrient availability is a primary driver of plant trait change and 2) plant age controls the magnitude of trait variation in Spartina . Further, our findings indicate that eutrophic conditions may not always increase top‒down control by herbivores, and in some instances can enhance saltmarsh resilience against sea‐level rise via stimulated Spartina biomass production. 

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4. Short‐ and long‐term effects of nutrient enrichment on salt marsh plant production and microbial community structure

   https://doi.org/10.1111/1365-2745.13756  

   Hanley, Torrance C. ; Bowen, Jennifer L. ; Kearns, Patrick J. ; Hughes, A. Randall ( August 2021 , Journal of Ecology)

   Nutrient enrichment impacts ecosystems globally. Population history, especially past resource environments, of numerically dominant plant species may affect their responses to subsequent changes in nutrient availability. Eutrophication can also alter plant–microbe interactions via direct effects on associated microbial communities or indirect effects on dominant species’ biomass production/allocation as a result of modified plant–soil interactions.

   We combined a greenhouse common garden and a field reciprocal transplant of a salt marsh foundation species (Spartina alterniflora) within a long‐term, whole‐ecosystem, nutrient‐enrichment study to determine whether enrichment affects plant production and microbial community structure differently depending on plant population history. For the greenhouse portion, we collected 20S. alternifloragenotypes—10 from an enriched creek that had received elevated nutrient inputs for 10 years and 10 from an unenriched reference creek—and reared them in a common garden for 1 year. For the field portion, we conducted a 2‐year, fully crossed reciprocal transplant experiment with two gardens each at the enriched and unenriched sites; we examined the effects of source site (i.e. population history), garden site and plant genotype.

   After 2 years, plants in enriched gardens had higher above‐ground biomass and altered below‐ground allocation compared to plants in unenriched gardens. However, performance also depended on plant population history: plants from the enriched site had decreased above‐ground and rhizome production compared to plants from the unenriched site, most notably in unenriched gardens. In addition, almost all above‐ and below‐ground traits varied depending on plant genotypic identity.

   Effects of nutrient enrichment on the associated microbial community were also pronounced. Following 1 year in common garden, microbial community structure varied by plant population history andS. alternifloragenotypic identity. However, at the end of the reciprocal transplant, microbial communities differed primarily between enriched and unenriched gardens.

   Synthesis. Nutrient enrichment can impact plant foundation species and associated soil microbes in the short term. Most importantly, nutrient enrichment can also have long‐lasting effects on plant populations and associated microbial communities that potentially compromise their ability to respond to changing resource conditions in the future.

    

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5. Intraspecific variation in host plant traits mediates taxonomic and functional composition of local insect herbivore communities

   https://doi.org/10.1111/een.12923  

   Tielens, Elske K. ; Gruner, Daniel S. ( August 2020 , Ecological Entomology)

   1. Host plant phenotypic traits affect the structure of the associated consumer community and mediate species interactions. Intraspecific variation in host traits is well documented, although a functional understanding of variable traits that drive herbivore community response is lacking. We address this gap by modelling the trait‐environment relationship using insect traits and host plant traits in a multilevel model.

   2. We compare herbivore assemblages from the canopy of the phenotypically variable treeMetrosideros polymorphaon Hawai‘i Island. Multiple distinct varieties ofM. polymorphafrequently co‐occur, with variation in morphological traits. Using this system, we identify host and insect traits that underlie patterns of herbivore abundance and quantify the strength of host‐insect trait interactions.

   3. This work examines plant‐insect interactions at a community scale, across 36 herbivore species in three orders. We find that co‐occurring trees of varying phenotype support distinct communities. Leaf traits, including specific leaf area, trichome presence, and leaf nutrients, explain 46% of variation in insect communities. We find that feeding guild and nymphal life history are correlated with host plant traits, and we show that model predictions are improved by including the host and insect trait interaction.

   4. This study demonstrates how insect herbivores traits influence community response to morphologically variable hosts. Environmental heterogeneity indirectly affected herbivore community structure via intraspecific variation in host plants, providing an important source of variation for maintaining diversity in the broader community.

    

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