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Purple crabs are destroying Cape Cod salt marshes, but there may be a way to fight back

Eve Zuckoff
"This purple crab problem is very much like other ecological catastrophes that we've seen where one organism in a food web gets super abundant or out of control from loss of predation pressure, or change in habitat," said plant ecologist Stephen Smith. "And then the whole community gets out of whack because of that one key component."

In the early 2000s, scientists began finding what appeared to be bald spots in dense, grassy salt marshes up and down the East Coast. These patches have since multiplied, and they now threaten to destroy coastal landscapes. 

Eventually, the culprit was identified: persistent, ravenous, purple crabs.  

Walking through a salt marsh in Wellfleet on a sunny afternoon, Stephen Smith, a plant ecologist from Cape Cod National Seashore, relayed just how much damage the crustaceans have done, and what experts did to find a solution.   

What follows is what he told me, lightly edited for clarity.

IZ.081221: Sesarma reticulatum
E. Lazo-Wasem, 2016/Digital Image: Yale Peabody Museum of Natural History; photo by E. Lazo-Wasem, 2016
"I would say 15 years ago people would have said, 'Oh, that [purple crab] problem, that's kind of a Cape Cod thing.' But it's it's happening in a lot of places all the way down to Georgia."
"Purple crabs are here on Cape Cod, and actually in many places up and down the East Coast, now in numbers that are roughly ten times normal."

Salt marshes buffer our coastline from erosion and they play a role in filtering out pollutants from groundwater before they make it to the nearshore waters. They're a really excellent habitat for many different kinds of organisms. And probably the most important function is that they keep building up these layers of highly organic material: peat. And essentially what that is, is thousands of years of taking carbon dioxide out of the atmosphere and turning it into plant biomass through photosynthesis and then burying it in these marsh areas where it does not interact with the atmosphere anymore, just sort of stays un-decomposed for a long, long, long time. So in that way, our coastal marshes ameliorate carbon emissions.

But two of the major problems facing marshes up and down the East Coast, and particularly here, are—well, one is sea level rise, and in this region, it's about four times-ish faster than what marshes can build up vertically to keep up with it. So sea level rise is sort of drowning our marshes in place slowly.

And what's acting on them more quickly is a species of herbivorous, nocturnal crab called Sesarma Reticulum, purple marsh crab. They're here on Cape Cod and actually in many places up and down the East Coast now in numbers that are roughly ten times normal. But Sesarma crabs, purple marsh crabs, they're nocturnal. So you don't ever see them, really. And they're the ones that eat this grass that comprises the actual structure of the salt marsh.

Eve Zuckoff
"Normally along salt marsh creek banks, the vegetation is really, really thick and luxurious," Smith said. "These crabs love the creek bank habitat. And so you can see they've removed all the vegetation along that edge. And then when they you lose the edges of these creeks, they just collapse into the channel. And so ~3000 years of carbon and peat build-up, all that good stuff ... is just falling into the tidal channels during one winter storm."

After a while, the plants just can't regrow anymore. You know, you cut these plants once or twice, and they'll just regrow from the root reserves that they use to re-sprout. But after a while, this repeated mowing through grazing causes them to disappear. And so in these marshes you normally shouldn't see any bare area.

He points to a bare patch.

It's hard to see, but all of this ground should be vegetated marsh.

Once you lose the plant cover, a lot of the organic matter that had been built up for many thousands of years just oxidizes away, so the ground surface sinks. And so therein lies the big problem: when these crabs remove the vegetation and the soil goes away, our marshes are collapsing and decreasing in elevation very, very rapidly in the face of sea level, which is rising quite rapidly.

"There's a marsh in Provincetown that just recently suffered an invasion of these crabs, or they started to proliferate sort of, in 2014 or 2015," Smith said. "So we were able to see a before-and-after. And that marsh is going to be completely gone in ten years."

So I've been working on this problem for a long, long time. And so we've tried to think of what we could do.

Now, you can't trap these crabs because, first of all, there are millions and millions of them in these marshes, and you can't bait them because they just eat vegetation, so there's nothing really to attract them into a trap. So we've tried to look at ways to recover vegetation where it's been lost in these bare areas.

And we thought we could maybe add sediment to give them an elevation boost and help them contend with sea level rise, but add the kind of substrate that the Sesarma crabs do not like. In these natural marshes, they will not inhabit really sandy soils because they can't support those elaborate burrow structures. So in the last couple of years we did these very, very small-scale experiments. We put in these meter-square plots, and to half of them we added about 20cm of sand. And then to their paired controls, nothing was added.

Now, everywhere where we added 20cm of sand, the vegetation completely recovered.

Eve Zuckoff
"You could totally recover this marsh if you had the ability to just drop a layer of sand on this whole marsh," Smith said. "It would, in very short order, recover. But that's that's the the trick, to scale up from a meter squared."

Even though marsh crabs could just run up onto these plots full of sand and eat away, they just avoid them. They do not live in them. So we're getting an elevation boost with a substrate that is not suitable for this species of crab to live in.

What we wanted to do here is basically prove a concept: is there a way to recover vegetation in the face of these two major stressors to buy more life for our marshes? And these are some of the clearest results I've had in my career here over 20 years.

There's cause for optimism. It's just translating it to a landscape scale.

Smith said his team hoping to work with the state to expand these experiments, save salt marshes, and defeat those darn purple crabs.   

Eve Zuckoff covers the environment and human impacts of climate change for CAI.
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