Crabs killing Northeast saltmarshes, study confirms
A marathon summer of
field work by Mark Bertness, professor of ecology and evolutionary biology, and
a squadron of students may finally help settle the heated debate about what's
killing the coastal saltmarshes of southern New England and Long Island. The
group's work has yielded two new papers that offer clear evidence of the cause.
In one paper, published
March 20 in the journal PLoS
ONE, they provide the results of numerous measurements at 14 sites around
Rhode Island's Narragansett Bay. They sought correlations between the extent of
marsh death and evidence of any of several popular hypotheses. The winner by
far was runaway herbivory of cordgrass by the Sesarma crab.
In the other study,
published in the journal Ecology
Letters, they directly tested that hypothesis with experiments on Cape
Cod. The results were that wherever they protected Sesarma from the pressure of
predators, the crabs ruthlessly mowed the grasses down.
A summer of salt, crabs, and mud
Bertness' team included
lab manager Caitlin Brisson, seniors Matt Bevil and Sinead Crotty, and junior Elena Suglia. They divided their time between Rhode Island and Cape Cod from
May to August, often working from 3 a.m. into the early evening. Brown alumnus
Tyler Coverdale is a co-author on the Ecology
Letters paper.
In Narragansett Bay
they ran several tests during the summer at sites where die-off ranged from
less than 5 percent to 98 percent. They determined the extent and progress of
marsh death by examining aerial images of the sites from 1997, 2003, 2008, and
2012.
To gather data about
whether tides were eroding the marshes away, the researchers made chalk blocks
and placed them at each site so they could watch how quickly they dissolved. To
determine whether local growing conditions were poor for the grasses, they
planted healthy grass in each site and then protected it from all herbivores,
including the crabs. To assess whether any site had too much nitrogen, they
took leaves of the grasses at each site back to the lab, ground them up and
analyzed their chemistry.
In the second week in
July, they measured herbivory at each site by walking a 20-meter line at each
site and then stopping every two meters to measure signs of crab damage on 100
cordgrass stalks. Later in July they measured Sesarma populations (based on how
many they could trap). In August they tethered crabs to make them more vulnerable
to predators and measured how much predation there was.
They even measured how
hard the marsh soil was at each site.
When fall came, they
analyzed all the data. The number crunching revealed that differences in
herbivory at each site explained 73 percent of the variation in die-off from at
each site.
The only other
significant factor was the hardness of the soil. Why would that matter?
"Substrate hardness influences crab herbivory by limiting crab burrowing
in hard and soft substrates, leading to a peak in herbivory in medium hardness
substrates where burrows can be easily constructed and maintained," they
wrote in PLoS ONE.
Experiment on the Cape
Meanwhile on Cape Cod,
the group did many of the same things (chalk blocks, nitrogen measurements,
tests for growing conditions), but they also implemented a direct test of the
Sesarma herbivory hypothesis. They erected cages on some plots that could
protect the crabs from predators.
As controls for the experiment they put cages
on some patches that didn't exclude predators and left other patches marked
only with marker posts.
Sure enough, the team
reported in Ecology Letters that "excluding predators for a
single growing season rapidly led to a more than 100-percent increase in
Sesarma herbivory, a more than 60-percent decrease in aboveground cordgrass
biomass, a more than 95-percent increase in Sesarma substrate disturbance, and
a more than 150-percent increase in unvegetated bare space in comparison to
control plots."
There are still
experiments the team would like to do to further understand the ecosystem
dynamics that lead to the marsh die-off, but next summer the Bertness lab will
also focus on determining what the marsh loss to excess herbivory could mean.
"We're moving on
as well in Narragansett Bay and in Cape Cod to look at sea level rise and
die-off under the assumption that these are the mechanisms that are causing
it," Crotty said. "Combining impacts that we should see in the
future, what does die-off mean for the future of marshes?"
It's a problem that,
properly understood, must now be managed.
Story
Source:
The above story is
based on materials provided by Brown University. Note: Materials may be edited for
content and length.
Journal
References:
1.
Mark D. Bertness, Caitlin P. Brisson, Matthew C. Bevil, Sinead
M. Crotty. Herbivory
Drives the Spread of Salt Marsh Die-Off. PLoS ONE, 2014; 9 (3): e92916
DOI:10.1371/journal.pone.0092916
2.
Mark D. Bertness, Caitlin P. Brisson, Tyler C. Coverdale, Matt
C. Bevil, Sinead M. Crotty, Elena R. Suglia. Experimental
predator removal causes rapid salt marsh die-off. Ecology Letters, 2014; DOI: 10.1111/ele.12287
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"Crabs killing Northeast saltmarshes, study confirms." Science Daily, 28 April 2014.
<www.sciencedaily.com/releases/2014/04/140428094309.htm>.