Unexplained dieback of salt marsh plants, particularly along salt marsh creek edges and the low tide line, has been an increasing issue along the New England coast since the 1990s. The dieback is characterized by the disappearance of salt marsh cordgrass (Spartina alterniflora), leaving behind large swaths of exposed soil, filled with tunnels. The disappearance of salt marsh cordgrass, both above and below ground, leaves marsh soils exposed and vulnerable to erosion. Given projected climate change impacts and projected changes in sea level, salt marshes are already at risk and soil erosion from dieback makes these ecosystems even more threatened!
Salt marsh dieback had been observed on Cape Cod salt marshes about 10 years prior to showing up on Nantucket. Research elsewhere documented that the primary cause of the dieback is an increased population of the native purple marsh crab (Sesarma reticulata), likely caused by a decrease in native predators. The purple marsh crab eats the salt marsh cordgrass, roots to shoots. In 2015 we trapped crabs at Foundation owned marshes across the island and documented marshes with higher purple marsh crab populations containing extensive salt marsh dieback.
In 2019, we initiated a research project at Medouie Creek to explore the feasibility of mitigating salt marsh dieback by trapping and removing purple marsh crabs - Foundation staff acted as predators. Within just one year of trapping, we removed over 500 crabs and saw the smooth cordgrass begin to recolonize the dieback areas for the first time in over 5 years. In 2020 we out-planted greenhouse-grown smooth cordgrass and continued in trap crabs in 2020 and 2021. This simple management technique appears to be effective at controlling purple marsh crab populations and reverse salt marsh dieback impacts.