Claim: Climate Change is Helping Herbivorous Crabs To Wreck Salt Marshes

Sesarma-reticulatum / purple marsh crab. By Eric A. Lazo-Wasem – Gall L (2019). Invertebrate Zoology Division, Yale Peabody Museum. Yale University Peabody Museum. Occurrence dataset accessed via on 2019-06-22.

Guest essay by Eric Worrall

According to a study, higher sea levels / more inundation of salt marshes has softened the soil, allowing burrowing crabs to munch their way through cord grass which holds salt marshes together.

But a 2012 study by the same group blamed overfishing of predators for the rise in the crab population, and dismissed climate change as a factor.

Burrowing crabs reshaping salt marshes, with climate change to blame

Given higher sea levels and softer soil in the wake of a shifting climate, Sesarma crabs, which have already decimated salt marshes in the Northeast, are now rising to prominence in southeastern marshes, a new study finds.

PROVIDENCE, R.I. [Brown University] – A new study reveals how climate change has enabled a voracious crab species to dramatically alter salt marsh ecosystems across the southeastern U.S.

The study, published in Proceedings of the National Academy of Sciences, shows that soils beneath salt marshes from South Carolina to Florida have been softened by higher sea levels and increased tidal inundation. That softening has allowed the burrowing crab species Sesarma reticulatum to thrive, feeding on the cordgrass that holds the marshes together.

“What we’ve found is an example of how sea level rise can activate a keystone species that’s now dramatically remodeling these salt marshes,” said Mark Bertness, a professor emeritus of ecology and evolutionary biology at Brown University and a coauthor of the research. “That’s a big deal because sea level rise is a pervasive global phenomenon, and this is a largely unexpected consequence. We need to start thinking about how global climate change could activate new keystone species in other ecosystems.”

Research on Sesarma crabs and their impact on salt marshes has a long history in Bertness’s lab at Brown. In 2011, Bertness and his students discovered that Sesarma, voracious grazers of cordgrass roots and leaves, were behind sudden die-offs of marshes on Cape Cod. In that case, overfishing had suddenly pulled predator species like striped bass out of the water, giving the crabs free reign to decimate the marshes. One of the undergraduate co-authors on that earlier research was Christini Angelini, now an associate professor at the University of Florida and a senior author on this new paper.

Read more:

The abstract of the study 2020 Bertness Labs study;

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

Sinéad M. Crotty, Collin Ortals, Thomas M. Pettengill, Luming Shi, Maitane Olabarrieta, Matthew A. Joyce, Andrew H. Altieri, Elise Morrison, Thomas S. Bianchi, Christopher Craft, Mark D. Bertness, and Christine Angelini

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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The following is the abstract of a 2012 study by the same lead author, which blames overfishing;

A trophic cascade triggers collapse of a salt-marsh ecosystem with intensive recreational fishing

Article (PDF Available)inEcology 93(6):1402-10 · June 2012 with 894 Reads DOI: 10.2307/23213769 · Source:  PubMedCite this publication

Andrew H AltieriMark D BertnessTyler CoverdaleNicholas Carl Herrmann

Overexploitation of predators has been linked to the collapse of a growing number of shallow-water marine ecosystems. However, salt-marsh ecosystems are often viewed and managed as systems controlled by physical processes, despite recent evidence for herbivore-driven die-off of marsh vegetation. Here we use field observations, experiments, and historical records at 14 sites to examine whether the recently reported die-off of northwestern Atlantic salt marshes is associated with the cascading effects of predator dynamics and intensive recreational fishing activity. We found that the localized depletion of top predators at sites accessible to recreational anglers has triggered the proliferation of herbivorous crabs, which in turn results in runaway consumption of marsh vegetation. This suggests that overfishing may be a general mechanism underlying the consumer-driven die-off of salt marshes spreading throughout the western Atlantic. Our findings support the emerging realization that consumers play a dominant role in regulating marine plant communities and can lead to ecosystem collapse when their impacts are amplified by human activities, including recreational fishing.

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The body of the 2012 study appears to dismiss climate change as a factor;

… the discontinuous distribution of marshes with die-off interspersed with healthy, vegetated marshes suggests that local interactions within fished marshes, rather than regional-scale physical forcing by a factor such as sediment starvation or climate effects, is driving salt- marsh die-off. …

I guess everyone has a right to change their mind. The mirage news article discusses the process by which the group came to appreciate the dramatic impact of a few mm / year of sea level rise.

Eric Worrall / 34 mins ago July 14, 2020

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