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1. A fiddler crab reduces plant growth in its expanded range

   https://doi.org/10.1002/ecy.4203  

   Martínez‐Soto, Kayla S.; Johnson, David S. (December 2023, Ecology)

   Abstract Species across the planet are shifting or expanding their ranges because of climate change. These are climate migrants. Although climate migrants are well documented, their impacts on recipient ecosystems are not. Climate migrants that are also ecosystem engineers (species that modify or create habitats) will likely have profound effects on ecosystems. The Atlantic marsh fiddler crab,Minuca pugnax, is a burrowing crab that recently expanded its range into the northeastern United States. In its historical range,M. pugnaxenhances the aboveground growth of the cordgrassSpartina alterniflora, a plant critical to marsh persistence. In a control‐impact study, however, we found thatSpartinaaboveground biomass was 40% lower whenM. pugnaxwas present. Thus, the positive effect ofM. pugnaxonSpartinaaboveground biomass flipped to a negative one in its expanded range.Spartinabelowground biomass was also 30% lower on average when crabs were present, a finding consistent with what is seen in the historical range. These impacts onSpartinaare likely due to burrowing byM. pugnax.Benthic microalgae was, on average, 45% lower when crabs were present. Fiddler crabs eat benthic microalgae, and these results suggest that fiddler crabs can control algal biomass via grazing. Because fiddler crabs reduced the biomass of foundational primary producers in its expanded range, our results imply thatM. pugnaxcan influence other saltmarsh functions such as carbon storage and accretion as they expand north. Most strikingly, our results suggest that as species expand or shift their range with climate change, not only can they have profound impacts in their new ranges but those impacts can be the inverse of what is seen in their historical ranges. 

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2. Utilizing airborne LiDAR data to quantify marsh edge morphology and the role of oyster reefs in mitigating marsh erosion

   https://doi.org/10.3354/meps13728  

   Hogan, S; Wiberg, PL; Reidenbach, MA (July 2021, Marine Ecology Progress Series)

   null (Ed.)

   Marsh habitats, experiencing accelerated change, require accurate monitoring techniques. We developed methods to quantify marsh edge morphology using airborne LiDAR data. We then applied these methods within the context of oyster reef restoration within the shallow coastal bays of Virginia, USA, by comparing retreat and morphology quantified at paired reef-lined and control marsh edges at 10 different marsh sites. Retreat metrics were analyzed between 2002 and 2015, utilizing a LiDAR derived edge for the year 2015 from points of maximum slope and aerial imagery pre-2015. Retreat was also compared before and after oyster reef restoration to determine if reefs slow erosion. We found that slope statistics from airborne LiDAR elevation data can accurately capture marsh edge morphology. Retreat rate, measured at edges typically found near the vegetation line, was not significantly different between reef-lined and control marshes and ranged from 0.14 to 0.79 m yr -1 . Both retreat rate (ρ = -0.90) and net movement (ρ = -0.88) were strongly correlated to marsh edge elevation. Exposed control marshes had significantly greater mean and maximum slope values compared to reef-lined marshes. The mean edge slope was 11.4° for exposed marshes and 6.0° for reef-lined marshes. We hypothesize that oyster reefs are causing an elongation of the marsh edge by reducing retreat at lower elevations of the marsh edge. Therefore, changes in marsh edge morphology may be a precursor to changes in marsh retreat rates over longer timescales and emphasizes the need for repeated LiDAR measurements to capture processes driving marsh edge dynamics. 

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3. Herbivore Fronts Shape Saltmarsh Plant Traits and Performance

   https://doi.org/10.1002/ece3.71360  

   Wittyngham, Serina_S; Johnson, David_S (April 2025, Ecology and Evolution)

   ABSTRACT Herbivore fronts can alter plant traits (chemical and/or morphological features) and performance via grazing. Yet, herbivore‐driven trait alterations are rarely considered when assessing how these fronts shape ecosystems, despite the critical role that plant performance plays in ecosystem functioning. We evaluated herbivore fronts created by the purple marsh crab,Sesarma reticulatum, as it consumes the cordgrass,Spartina alterniflora, in Virginian salt marshes.Sesarmafronts form at the head of tidal creeks and move inland, creating a denuded mudflat between the tall‐formSpartinalow marsh (trailing edge) and the short‐formSpartinahigh marsh (leading edge). We quantifiedSesarmafront migration rate, tested ifSesarmaherbivory altered geomorphic processes andSpartinatraits at the trailing and leading edges, and examined how these trait changes persisted through the final 8 weeks of the growing season.Sesarmafront migration in our region is two times slower than fronts in the Southeast United States, andSpartinaretreat rate at the leading edge is greater than the revegetation rate at the trailing edge.Sesarmafronts lowered elevation and decreased sediment shear strength at the trailing edge while having no impact on soil organic matter and bulk density at either edge. At the leading edge,Sesarmagrazing reducedSpartinagrowth traits and defensive ability, and trait changes persisted through the remaining growing season. At the trailing edge, however,Sesarmagrazing promoted belowground biomass production and had limited to no effect on growth or defensive traits. We show that herbivore fronts negatively impact saltmarsh plant traits at their leading edge, potentially contributing to front propagation. In contrast, plants at the trailing edge were more resistant to herbivore grazing and may enhance resilience through elevated belowground biomass production. Future work should consider herbivore‐driven plant trait alterations in the context of herbivore fronts to better predict ecosystem response and recovery. 

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4. 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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5. Influence of the Keystone Grazer, Sesarma reticulatum, on the Hydrology and Organic Matter Cycling in Salt Marshes of the Southeastern USA

   https://doi.org/10.1007/s12237-024-01336-9  

   Morrison, Elise S.; Bianchi, Thomas S.; Kenney, William F.; Brenner, Mark; Prince, Kimberly; Williams, Sydney; Ortals, Collin; Cordero, Orlando; Crotty, Sinéad M.; Angelini, Christine (March 2024, Estuaries and Coasts)

   Abstract In salt marshes of the Southeastern USA, purple marsh crabs (Sesarma reticulatum), hereafterSesarma, aggregate in grazing and burrowing fronts at the heads of tidal creeks, accelerating creek incision into marsh platforms. We explored the effects of this keystone grazer and sediment engineer on salt marsh sediment accumulation, hydrology, and carbon (C) and nitrogen (N) turnover using radionuclides (²¹⁰Pb and⁷Be), total hydrolyzable amino acids (THAA), and C and N stable isotopes (δ¹³C and δ¹⁵N) in sediment from pairedSesarma-grazed and un-grazed creeks.Sesarma-grazed-creek sediments exhibited greater bioturbation and tidal inundation compared to sediments in un-grazed creeks, as indicated by larger²¹⁰Pb and⁷Be inventories. Total organic carbon (TOC) to total nitrogen (TN) weight ratios (C:N) were higher and δ¹⁵N values were lower in grazed-creek sediments than in un-grazed-creek sediments, suggestingSesarmaremove and assimilate N in their tissues, and excrete N with lower δ¹⁵N values into sediments. In support of this inference, the percent total carbon (TC) and percent TOC declined by nearly half, percent TN decreased by ~ 80%, and the C:N ratio exhibited a ~ threefold increase betweenSesarmafore-gut and hind-gut contents. An estimated 91% ofSesarma’s diet was derived fromSpartina alterniflora,the region’s dominant salt marsh plant. We found that, asSesarmagrazing fronts progress across marsh landscapes, they enhance the decay ofSpartina-derived organic matter and prolong marsh tidal inundation. These findings highlight the need to better account for the effects of keystone grazers and sediment engineers, likeSesarma, in estimates of the stability and size of blue C stores in coastal wetlands. 

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