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., CC0, Link

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 Altieri, Mark D Bertness, Tyler Coverdale, Nicholas 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.

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July 14, 2020 8:21 am

I doubt a few mm of sea level rise has much to do with this issue, or many others for that matter. Over fishing is the real problem, with some nation states much worse than others for not taking seriously all of the negative effects that this can bring. And destroying ocean reefs with irresponsible fishing technique such as what the Chines are doing wherever they can, especially the South China Sea they have seized including the EEZ from all the other ASEAN countries, and are systematically destroying it with their insatiable requirement for food. This destruction leads to ecosystem collapse.

While I don’t have much specific knowledge of this particular issue, the one thing I have noticed in my global travels is that wherever you have very healthy Mangrove forests in the intertidal zones, the rest of the ecosystem seems to do much better in the entire local area. So preserving and promoting Mangroves are an important issue that we should really support the preservation of, including assisting their creation where possible and ensuring we aren’t damaging this sensitive habitat. Some things really do matter. A lot.

“Mangroves are defined as assemblages of salt tolerant trees and shrubs that grow in the intertidal regions of the tropical and subtropical coastlines. They grow luxuriantly in the places where freshwater mixes with seawater and where sediment is composed of accumulated deposits of mud.”

Tom in Florida
July 14, 2020 8:21 am

Apparently there is no grant money for “over fishing predators is the cause…”

Justin Burch
Reply to  Tom in Florida
July 14, 2020 9:11 am

That was my thought. I have seen this over and over again. A scientist can’t get grants so they invoke climate change the money starts rolling in. We have turned nour scientists into whores and the grant system is what is pimping them out.

Curious George
July 14, 2020 8:51 am

Climate must not change! Nothing must change! Evolution must stop.

Bill Powers
Reply to  Curious George
July 14, 2020 10:16 am

10+ We must stop that climate change and spare no cost.

But first we need for the Scientists (actually Bureaucrats) to arrive at a consensus as to what it needs to be stabilized to, since it is out of whack now, don’t you see.

That is going to require sacrifice on your part Curious. Much higher cost of living and much lower quality of life. will be demanded of us and they will keep us apprised of how we are doing as we work toward reaching stabilization.

I don’t suppose we can expect to achieve it in our lifetimes. And worse, we should expect it is likely to become an elusive goal (as the funding continues to pour into slimate industrial complex) because of all those dastardly non believers. Those cursed deniers will need to be locked-up.

Reply to  Curious George
July 14, 2020 5:32 pm

You left out and if it does change it’s your fault and you must pay.

Charles Higley
Reply to  Curious George
July 14, 2020 8:16 pm

I am a marine biologist and am ashamed that a marine biologist would begin to think that any slight change in a salt marsh would cause ecological havoc.

Years ago, there was an equine encephalitis scare in New England and the idiots in power had drainage ditches dug across all salt marshes, even out on the Massachusetts islands. Belatedly, a biologist figured out that the mosquito species that carried the disease was only found in or near logs in freshwater swamps. Oops, big mistake, a bit of over-reaction.

The damage to the salt marshes from the ditches was enormous, as the trenching machines threw the dug up material on either side of the ditch, creating dams on both sides such that the tide went in and out of the ditch but the water could not drain past the dams. Despite the horrendous increase in mosquitoes and the creation of standing water that was not ever part of a salt marsh experience, the organisms of the marsh survived just fine during the 20+ years it took for the marshes to recover from this egregious damage.

Such systems are much more robust than our experts today would like to think. We are not the saviors of nature. Nature is just fine and can handle what comes along

Reply to  Charles Higley
July 16, 2020 6:09 am

Cores taken in marshes tend to show stripes of sand in the layers of peat, spaced at irregular intervals but averaging every century or so. These layers of sand likely are due to the dunes on the seaward side of the marsh being pummeled by the enormous tides and waves associated with the biggest hurricanes. Such storms can fill old inlets and create new ones, and greatly upset the “fragile” ecosystem. Critters dependent on brackish water can be killed by water too salty rushing in from the sea, or floods too fresh pouring in after feet of rain fall on the land, or both. This is no fun if you make your living harvesting oysters, but the marshes do recover, which is shown by the accumulation of peat which buries the evidence of prior storms.

The “balance” of nature holds some wild swings, and any life that is too “fragile” either finds some niche elsewhere, or becomes extinct without any help from man.

In like manner, the funding of science holds some wild swings, and any scientist who is too “fragile” either finds some niche elsewhere, or does research without any help from man.

Reminds me of an old saying: “Never write poems without a patron, and never patronize a poet.”

July 14, 2020 8:57 am

Here’s a study which reports that eCO2 is making salt marsh vegetation grow faster & trap more sediment, thus helping salt marshes resist encroachment from rising sea-level:

Katherine M. Ratliff et al. Spatial response of coastal marshes to increased atmospheric CO2, PNAS (2015). DOI:10.1073/pnas.1516286112


Gary Pearse
July 14, 2020 9:00 am

How does a few cm of sealevel rise “soften” satuated soils? I never met the crab before but I have some knowledge of sedimentation of fine clays and call BS on this contrived explanation. Also, my new friend Sesarma the crab certainly has some serious looking digging tools to hand!

The over-fishing in the first edition and climate change in the second, ticked the ‘Its-man-what done it’ box and the ‘Goldilocks status quo ecology’ box that is de rigeur in fake climate studies. I have a prediction that also goes along with the modus of fake climate science. Christine is turning out to be a one-trick pony. We will be sick of the cute little crab, estuary mud and marsh grass by the end of her career and after several million bucks spent on salary and grants. Witness over 20yrs of Hockey Stick wrought outa tree rings, upside down, contaminated Kiljander lake-mud proxy and a secret novel ersatz statistical shuffle. Or Wadhams’s never-ending end of Arctic Ice, or …..

Justin Burch
Reply to  Gary Pearse
July 14, 2020 9:13 am

Correction that should be a few mm not cm which makes it even more preposterous.

Gary Pearse
Reply to  Justin Burch
July 14, 2020 4:31 pm

Justin: You are right for 12 months but, hey, years I’m sure the scale is in this article.

July 14, 2020 9:11 am

They sound like another foundation that will produce reports supporting any position the paymasters want supported.

July 14, 2020 9:36 am

It makes for a good living at the beach though. Don’t forget those drinks with the little parasols in them like Mike Mann wrote about.

July 14, 2020 9:36 am

There are no significant sea level rises. There may some places where the land is sinking. This is due to humans taking too many predators.
Why do these idiots lie?

July 14, 2020 9:47 am

“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.”

A cyclical sea level rise that has been relatively constant since the LIA…

Considering that others blame climate change for sea level rise, means that these authors did not investigate causes.

Next cause?

“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.”

Hmmm. Overfishing is what done it?
No no no! Salt marsh ecosystems are controlled by physical processes…
No no no! Marsh vegetation died off because of cows, cattle and deer…

A) The striped bass they refer to is a tightly controlled fishery with licenses, seasons and officials willing to shut down the fishery if participants catch too many fish.
• i) The researchers fail to mention that striped bass are not southern fish. i.e. Maine through North Carolina have abundant striped bass. Florida? hardly any at all.

• ii) Striped bass are just one fish that eat crabs. Any fish, i.e. every fish large enough to eat crustaceans happily gorges on every crab they spot. Quite a few fish evolved dentures specifically to easily crush crustacea and mollusca for easier digestion. That includes barnacles.

• iii) There is a problem with the main claim “That softening has allowed the burrowing crab species Sesarma reticulatum to thrive, feeding on the cordgrass that holds the marshes together.”.
That is, the vegetation they mention are shallow water plants.
All fish small enough to catch these crabs are under sized and not eligible for harvest. With the exception of general bait.

• iiii) What is not mentioned is that these crabs, Sesarma reticulatum spend significant time above water on land. Making seagulls, pelicans and whatever as predators of Sesarma reticulatum.

• iiiii) All real science/biology descriptions of sesarma reticulatum list their dietary preferences as; “Biology Glossary (e.g. epibenthic)
Feeds on crustaceans and primarily on salt marsh plant Spartina sp. (Ref. 111570)
That is; a significant predator of sesarma reticulatum is sesarma reticulatum.

Any one giving odds that these researchers failed to include cannibalism as a probable cause? Or predation by birds?

Robert of Texas
July 14, 2020 10:12 am

Post Neo-Science:

“Your pet hypothesis has to be modified or replaced until you hit upon one that provides enough money and press to make you feel good about yourself. Don’t worry about what you have already stated – no one will remember. Don’t worry about science – science is irrelevant.”

HD Hoese
July 14, 2020 10:16 am

Lots to digest, lots of references, may have missed it, but didn’t see this. Also, somewhere I seem to recall something about disease. Smaller droughts were associated with salt marsh die-off, exacerbated by snail grazing according to Silliman, B. R.,et al., 2005. Drought, snails, and large-scale die-off of southern U. S. salt marshes. Science. 310:1803-1806.

Sesarma are cute crabs, mostly you see them higher in intertidal than fiddlers, scurry around piers, pilings. NOAA funds lots of marsh studies, easier to work in than open waters. Give them credit, at least their model was three dimensional.

Robert of Ottawa
July 14, 2020 2:17 pm

I always knew it was the vegans that destroyed the climate, what with their farts and claws.

July 14, 2020 6:28 pm

So Crotty and his friends figured out where the money is and changed accordingly. They must be a bit slow. Many others found our decades before.

July 15, 2020 6:58 pm

The crabs are coming! We’re doomed!

July 15, 2020 7:00 pm
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