Who Is Eating Up All The Marshlands? Crabs, According To A New Research
Salt marshes along the New England coast are dying at an accelerated rate and a team of researchers have found the reason why: crabs. This theory has been published in two new studies carried out by researchers from Brown University who conducted extensive studies on the coastal saltmarshes from Long Island to Cape Cod. The studies present enough evidence that point to unchecked populations of crabs eating away the cordgrass.
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The experiments were conducted by Mark Bertness, professor of ecology and evolutionary biology, and a team of students. Of the two papers published, one was in the journal PLoS ONE where results of fieldwork conducted at 14 sites around Rhode Island's Narragansett Bay, were published. The team tried to find correlations between the extent of marsh death and other popular theories supposed to be responsible for their decline. But an overwhelming majority of evidence suggested runaway herbivory of cordgrass by the Sesarma crab. In the second study published in the journal Ecology Letters, the researchers directly tested the hypothesis of the crabs with experiments on Cape Cod. The results were that wherever they protected Sesarma from predators, the crabs had an insatiable appetite for the grass.
In both the experiments they concluded that populations of Sesarma, unchecked due to over-harvesting of their natural predators like striped bass and cod, were responsible for the declining marshes. Long-held beliefs that other physical factors, rather than disrupted food webs, are killing the marshes just aren't true, Bertness said. "These are very deeply embedded paradigms and dogma," Bertness said. "But the conservation implications of this are enormous, so at some point in time people have to set that aside and look at this objectively."
The experiments were conducted in Rhode Island and Cape Cod from May to August. 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 also determined the extent of marsh death by examining aerial images of the sites captured in 1997, 2003, 2008, and 2012. To test the other popular hypotheses held responsible for reduction in marshes the team conducted the following experiments: To check if tides were eroding marshland away the researchers made chalk blocks and placed them at each site so they could watch how quickly they dissolved. To check if 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 test if too much nitrogen was responsible, 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 measuring signs of crab damage every two meters 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. The hardness of the marsh soil on each site was also measured.In fall, all the data were analyzed. The number crunching revealed that differences in herbivory at each site explained 73 percent of the variation in die-off at each site.
The hardness of the marsh soil also mattered according to the team since, "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."
Experiments on Cape Cod also involved testing the popular hypotheses with similar techniques like chalk blocks and nitrogen tests and it also involved a direct test of the Sesarma herbivory hypothesis. This was done by erecting cages on some plots that could protect the crabs from predators. As controls for the experiment the team put cages on some patches that didn't exclude predators and left other patches marked only with marker posts.
Their hypothesis was right. As they have reported in Ecology Letters, "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." The team's next project is to understand the implications of marsh loss due to herbivory and other systems that may be responsible for marsh die-offs.
"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." Sinead Crotty, a team member said. "Combining impacts that we should see in the future, what does die-off mean for the future of marshes?"
This research can give an impetus to the conservation of these shrinking marshlands that provide valuable ecosystem services to coastal areas.
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