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1. Mesopredator release moderates trophic control of plant biomass in a Georgia salt marsh

   https://doi.org/10.1002/ecy.4452  

   Morton, Joseph P; Hensel, Marc_J S; DeLaMater, David S; Angelini, Christine; Atkins, Rebecca L; Prince, Kimberly D; Williams, Sydney L; Boyd, Anjali D; Parsons, Jennifer; Resetarits, Emlyn J; et al (December 2024, Ecology)

   Abstract Predators regulate communities through top‐down control in many ecosystems. Because most studies of top‐down control last less than a year and focus on only a subset of the community, they may miss predator effects that manifest at longer timescales or across whole food webs. In southeastern US salt marshes, short‐term and small‐scale experiments indicate that nektonic predators (e.g., blue crab, fish, terrapins) facilitate the foundational grass,Spartina alterniflora, by consuming herbivorous snails and crabs. To test both how nekton affect marsh processes when the entire animal community is present, and how prior results scale over time, we conducted a 3‐year nekton exclusion experiment in a Georgia salt marsh using replicated 19.6 m²plots. Our nekton exclusions increased densities of plant‐grazing snails and juvenile deposit‐feeding fiddler crab and, in Year 2, reduced predation on tethered juvenile snails, indicating that nektonic predators control these key macroinvertebrates. However, in Year 3, densities of mesopredatory benthic mud crabs increased threefold in nekton exclusions, erasing the tethered snails' predation refuge. Nekton exclusion had no effect onSpartinabiomass, likely because the observed mesopredator release suppressed grazing snail densities and elevated densities of fiddler crabs, whose burrowing alleviates soil stresses. Structural equation modeling supported the hypotheses that nektonic predators and mesopredators control invertebrate communities, with nektonic predators having stronger total effects onSpartinathan mud crabs by controlling densities of species that both suppress (grazers) and facilitate (fiddler crabs) plant growth. These findings highlight that salt marshes can be resilient to multiyear reductions in nektonic predators if mesopredators are present and that multiple pathways of trophic control manifest in different ways over time to mediate community dynamics. These results highlight that larger scale and longer‐term experiments can illuminate community dynamics not previously understood, even in well‐studied ecosystems such as salt marshes. 

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2. Long‐term data reveal that grazer density mediates climatic stress in salt marshes

   https://doi.org/10.1002/ecy.4323  

   Smith, Carter S; Zhang, Y Stacy; Hensel, Marc_J S; Pennings, Steven C; Silliman, Brian R (July 2024, Ecology)

   Abstract Understanding how climate and local stressors interact is paramount for predicting future ecosystem structure. The effects of multiple stressors are often examined in small‐scale and short‐term field experiments, limiting understanding of the spatial and temporal generality of the findings. Using a 22‐year observational dataset of plant and grazer abundance in a southeastern US salt marsh, we analyzed how changes in drought and grazer density combined to affect plant biomass. We found: (1) increased drought severity and higher snail density both correlated with lower plant biomass; (2) drought and snail effects interacted additively; and, (3) snail effects had a threshold, with additive top‐down effects only occurring when snails were present at high densities. These results suggest that the emergence of multiple stressor effects can be density dependent, and they validate short‐term experimental evidence that consumers can augment environmental stress. These findings have important implications for predicting future ecosystem structure and managing natural ecosystems. 

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3. Components of the CO ₂ Exchange in a Southeastern USA Salt Marsh

   https://doi.org/10.34133/olar.0077  

   Morris, James T; Whiting, Gary J (December 2024, Ocean-Land-Atmosphere Research)

   Different CO₂exchange pathways were monitored for a year in short- and tall-formSpartina alternifloragrasses in a southeastern USA salt marsh at North Inlet, South Carolina. The tall form of grass growing close to a creek under favorable conditions reached a higher standing biomass than the short form of grass growing in the interior marsh. However, the photosynthetic parameters of both forms of grass were equivalent. The tall canopy had greater net canopy production, 973 versus 571 g C m⁻²year⁻¹, canopy growth, 700 versus 131 g C m⁻²year⁻¹, and canopy respiration, 792 versus 225 g C m⁻²year⁻¹, but lower sediment respiration, 251 versus 392 g C m⁻²year⁻¹. In a single growing season, tall-canopy biomass increased to intercept all the available solar radiation, which limits gross photosynthesis. Total respiration increased during the growing season in proportion to live biomass to a level that limited net production. Theoretically, the difference between net canopy production and canopy growth is carbon allocated to belowground growth and respiration. However, the computation of belowground production by this method was unrealistically low. This is important because carbon sequestration is proportional to belowground production and accounts for most of the vertical elevation gain of the marsh surface. Based on the allometry of standing live biomass, alternative estimates of belowground production were 927 and 193 g C m⁻²year⁻¹in creekbank and interior marshes, which would yield gains in surface elevation of 0.2 and 0.04 cm/year, respectively. 

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4. Reducing disease and producing food: Effects of 13 agrochemicals on snail biomass and human schistosomes

   https://doi.org/10.1111/1365-2664.14087  

   Haggerty, Christopher J. E.; Halstead, Neal T.; Civitello, David J.; Rohr, Jason R. (December 2021, Journal of Applied Ecology)

   Abstract Agricultural expansion is predicted to increase agrochemical use two to fivefold by 2050 to meet food demand. Experimental evidence suggests that agrochemical pollution could increase snails that transmit schistosomiasis, a disease impacting 250 million people, yet most agrochemicals remain unexamined.Here we experimentally created >100 natural wetland communities to quantify the relative effects of fertilizer, six insecticides (chlorpyrifos, terbufos, malathion, λ‐cyhalothrin, permethrin and esfenvalerate), and six herbicides (acetochlor, alachlor, metolachlor, atrazine, propazine and simazine) on two snail genera responsible for 90% of global schistosomiasis cases.We identified four of six insecticides (terbufos, permethrin, chlorpyrifos and esfenvalerate) as high risk for increasing snail biomass by reducing snail predators. Hence, malathion and λ‐cyhalothrin might be useful for improving food production without increasing schistosomiasis. This top‐down effect of insecticides on predators was so strong that the effects of herbicides on schistosomiasis risk were masked in the presence of predators because there were so few snails. In the absence of snail predators, herbicide effects on snails were generally negative by reducing submerged vegetationHydrilla verticillata. The exception was that atrazine and acetochlor significantly increased the biomass of infected snails and total snails respectively.Like insecticides, fertilizer had strong positive effects on snail populations. Fertilizer increased both snail habitat (submerged vegetation) and snail food (periphyton), but of these two pathways, the increases in snail habitat resulted in greater snail population growth. Total snail biomass was positively associated with both infected snail biomass and parasite production and thus human infection risk.Synthesis and applications. Our findings suggest that fertilizers and insecticides generally have consistently higher chances of increasing human schistosomiasis than herbicides in natural communities. Furthermore, our results highlight the need to identify other low risk insecticides, which might help reduce crop pests without increasing snails and thus risk of schistosomiasis. 

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5. 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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