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1. 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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2. Sea-level rise and the emergence of a keystone grazer alter the geomorphic evolution and ecology of southeast US salt marshes

   https://doi.org/10.1073/pnas.1917869117  

   Crotty, Sinéad M. ; Ortals, Collin ; Pettengill, Thomas M. ; Shi, Luming ; Olabarrieta, Maitane ; Joyce, Matthew A. ; Altieri, Andrew H. ; Morrison, Elise ; Bianchi, Thomas S. ; Craft, Christopher ; et al ( July 2020 , Proceedings of the National Academy of Sciences)

   null (Ed.)

   Keystone species have large ecological effects relative to their abundance and have been identified in many ecosystems. However, global change is pervasively altering environmental conditions, potentially elevating new species to keystone roles. Here, we reveal that a historically innocuous grazer—the marsh crab Sesarma reticulatum —is rapidly reshaping the geomorphic evolution and ecological organization of southeastern US salt marshes now burdened by rising sea levels. Our analyses indicate that sea-level rise in recent decades has widely outpaced marsh vertical accretion, increasing tidal submergence of marsh surfaces, particularly where creeks exhibit morphologies that are unable to efficiently drain adjacent marsh platforms. In these increasingly submerged areas, cordgrass decreases belowground root:rhizome ratios, causing substrate hardness to decrease to within the optimal range for Sesarma burrowing. Together, these bio-physical changes provoke Sesarma to aggregate in high-density grazing and burrowing fronts at the heads of tidal creeks (hereafter, creekheads). Aerial-image analyses reveal that resulting “ Sesarma- grazed” creekheads increased in prevalence from 10 ± 2% to 29 ± 5% over the past <25 y and, by tripling creek-incision rates relative to nongrazed creekheads, have increased marsh-landscape drainage density by 8 to 35% across the region. Field experiments further demonstrate that Sesarma- grazed creekheads, through their removal of vegetation that otherwise obstructs predator access, enhance the vulnerability of macrobenthic invertebrates to predation and strongly reduce secondary production across adjacent marsh platforms. Thus, sea-level rise is creating conditions within which Sesarma functions as a keystone species that is driving dynamic, landscape-scale changes in salt-marsh geomorphic evolution, spatial organization, and species interactions. 

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3. 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)

   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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4. 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)

   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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5. Geographic variability in headward erosion of marsh tidal creeks: Ecological and physical causes

   https://doi.org/10.1002/esp.5747  

   Hughes, Zoe J. ; Farron, Sarah J. ; FitzGerald, Duncan M. ( November 2023 , Earth Surface Processes and Landforms)

   Expansion of drainage networks through the headward erosion of tidal creeks is an eco‐geomorphologic response of salt marshes to accelerated sea‐level rise (SLR). This response can counter the negative impacts of an elevation deficit by increasing drainage and encouraging plant health, thereby reducing potential for submergence and marsh platform loss. In the wetlands of Cape Romain, SC, intense bioturbation near creek heads by the common marsh crabSesarma reticulatumhas been found to facilitate sediment erosion and rapid creek growth. This keystone grazer has been recently observed to have increasing influence on landscape evolution throughout the southeast US coast. Here, we compare measurements taken at Sapelo Island, GA, with those previously collected at Cape Romain, to confirm that eco‐geomorphic feedbacks facilitating creek growth at each location are similar, and to compare these processes under differing background conditions. We use sediment cores, precise elevation measurements and historical imagery to compare substrate properties, elevation within the tidal frame, creek growth rates and drainage morphology at both sites. Our results show identical processes; however, the higher elevation of the marsh at Sapelo Island leads to shallower and shorter periods of tidal inundation, explaining the greater soil strength and lower belowground biomass compared with the marsh at Cape Romain. The smaller tidal range at the site in Cape Romain compared with Sapelo Island translates to a proportionally shallower depth of tidal creeks, which therefore requires less erosion to produce headward creek extension. These effects are likely to have contributed to slower growth rates of tidal creeks at Sapelo Island during the past several decades of SLR. Our findings highlight the similarities in process but differences in rates in how marshes are responding to climate‐related stress.

    

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