Crabs Are Behind Marsh Die-Off
Brett Smith for redOrbit.com – Your Universe Online
Some coastal salt marshes along the East Coast of the United States have been dying off over the past two decades and two new research papers point to overgrazing on marsh grasses by the Sesarma crab as a primary driver of the decline.
In the first paper, published in the open access journal PLOS ONE, researchers from Brown University tested several hypotheses that might explain marsh die-off around Rhode Island’s Narragansett Bay. These hypotheses include the effects of cordgrass herbivory – as well as physical forces acting on the marsh and nutrient pollution.
Study author Mark Bertness, professor of ecology and evolutionary biology at Brown, said his team did not find any evidence to support the popular theory that physical erosion is destroying the Rhode Island marsh ecosystem.
“These are very deeply embedded paradigms and dogma,” Bertness said in a recent statement. “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.”
In the PLOS ONE study, researchers conducted multiple summer-time tests at locations where die-off varied from under 5 percent to 98 percent. The study team established the scope and advancement of marsh death by looking at aerial images of the locations from 1997, 2003, 2008, and 2012.
To find out if tides were deteriorating the marshes, the scientists chiseled out blocks of chalk and positioned them at each location so they could observe how rapidly they washed away. To figure out if regional growing factors were inadequate for the grasses, the researchers planted vibrant grass in each site and then safeguarded it from all herbivores, including crabs. To determine if any site had an excessive amount of nitrogen, they sampled the grasses at each location and examined their chemical makeup.
During the follow up, the team assessed herbivory at each location by walking a 20-meter line, pausing every two meters to evaluate signs of crab herbivory on 100 cordgrass stalks. The team also assessed Sesarma populations by seeing how many they could trap. Near the end of the summer, the Brown researchers trapped crabs to cause them to be more susceptible to predators and assessed how much predation there was in each study site. They also measured how stiff the marsh soil was at each location.
The researchers found herbivory was behind 73 percent of the variation in die-off from at each site. The secondary biggest factor was the hardness of the soil.
“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,” the team wrote in their report.
In the second study, published in Ecology Letters, many of the same researchers conducted the same trials in the salt marshes around Cape Cod. However, this time they built cages on some spots to protect the crabs from predators.
In this study, the team reported 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.”