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Sesarma were known to inhabit southern marshes in Florida and the Carolinas, but their populations hadn’t boomed like those further north. One potential reason for that was differing soil substrates. While working several years ago as an undergraduate researcher in Bertness’ lab, Sinead Crotty, now project director at Yale’s Carbon Containment Lab, showed that ground hardness played a big role in where Sesarma are able to establish themselves. Her findings indicated that Sesarma had a much easier time building burrows and feeding on grass roots in the peaty New England soil compared to harder soil substrates often found in southern marshes.
But as sea levels continue to rise due to climate change, Crotty, Angelini and Bertness wondered if softening soils might be giving Sesarma more of a foothold in the South. Looking at aerial photos from nine locations across South Carolina and Florida, they found that the number of marsh creeks with evidence of Sesarma grazing increased by up to 240% from the late 1990s to the late 2010s. Meanwhile, surveys of sea level rise show that the ground in these areas is tidally submerged up to an hour longer per day now compared to the late 1990s.
“You’ve got the sea level rising, which softens the substrate that these crabs usually can’t burrow in,” Bertness said. “Now that it’s softer you’ve got an ideal habitat to support these huge communal Sesarma burrows.”
This new Sesarma activity is reshaping marshes, the researchers found. Elimination of grasses has increased the rate at which creeks form in the marshes, and increases the drainage density of marsh creeks by up to 35%.
Sesarma activity is also influencing interactions between predators and prey in the creeks. Clearing of grasses provides predators increased access to shellfish and other prey species. The research found that populations of mussels were dramatically lower in Sesarma-grazed creeks compared to creeks that weren’t grazed.
“As they drown, southeastern U.S. marshes are fracturing from grasslands to patches of marsh, with depleted populations of mussels, snails and other invertebrates,” Angelini said. “These dynamics reveal how quickly marshes may disappear with accelerating sea level rise and how long they will remain foraging grounds for commercially, recreationally and ecologically important species.”
The fact that Sesarma is now altering the geomorphology of the marshes, as well as the ecological interactions between other species, is evidence that it now qualifies as a keystone species in southern marshes. This is the first example, the researchers say, of activation of new keystone species as the result of anthropogenic climate change.
“This is going to be something for the textbooks,” Bertness said. “This is an underappreciated way in which climate change alters ecosystems.”
Other authors on the paper were Collin Ortals, Thomas Pettengill, Luming Shi, Maitane Olabarrieta, Matthew Joyce, Andrew Altieri, Elise Morrison, Thomas Bianchi and Christopher Craft. The research was supported by the National Science Foundation (1652628, 1546638, 1315138).