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GCE IV - Key Finding in 2020

    The biogeomorphic and ecological effects of the marsh crab Sesarma reticulatum

    In recent decades, the headward erosion of salt marsh creeks associated with the activities of the herbivorous marsh crab, Sesarma reticulatum, has become increasingly common in salt marshes in the southeastern US. Sesarma aggregates in high densities at the heads of creeks, and GCE researchers have found that they excavate large amounts of soil, consume plants at the creekbank (increasing erodibility), and probably also enhance decomposition of soil organic matter through burrow networks that serve to increase oxygen penetration (Vu et al. 2017; Vu and Pennings 2018). As a result, grazed creeks have been shown to elongate onto the marsh platform at a rate of 1 to 4.5 m y-1. In a study in PNAS, Crotty et al. (2020) analyzed aerial imagery and found that Sesarma-grazed creekheads have increased in prevalence over the past 25 y, and in so doing have increased drainage density by 8 to 35% across the southeast. They postulate that this increase in grazed creeks is a consequence of sea level rise, which serves to soften the substrate and make it easier for crabs to burrow. They also show that the biomass of other marsh invertebrates is severely decreased adjacent to grazed creeks, likely due to increased predator access to the marsh platform in denuded areas (Fig. 2). In order to understand the response and recovery of areas affected by this perturbation, Wu et al. (in revision) measured a suite of variables along three transects: one that ran from the marsh platform, through the denuded creek head, and onto newly-formed creekbank; another where the enlarged creek head had passed through but was no longer in evidence; and a third that had never been affected and served as a control. There was a strong response to the headward-eroding creek in most variables that were assessed, with most effects persisting for 5-10 y and following multiple trajectories of recovery; some variables never recovered (i.e. a state change to tall Spartina). These studies demonstrate the far-reaching and long-lasting effects of Sesarma and the feedbacks between crabs, marsh drainage, and ecosystem function. Crotty et al. (2020) was also featured in Scientific American, which highlighted the intriguing idea put forth in the paper that crabs have emerged as keystone grazers in this system as a consequence of climate change.

    2020_Accomplishments_Fig2

    Fig. 2 Surveys of the border and platform zones in Sesarma-grazed (A) and ungrazed (B) marshes reveal differences in invertebrate community biomass and composition (C). Error bars represent standard errors of 56 replicate survey quadrats per marsh type and zone. Photo credit: C. Ortals. Source: Crotty et al. 2020.



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This material is based upon work supported by the National Science Foundation under grants OCE-9982133, OCE-0620959, OCE-1237140 and OCE-1832178. Any opinions, findings, conclusions, or recommendations expressed in the material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.