Abstract
Southern New England salt marsh vegetation and habitats are changing rapidly in response to sea-level rise. At the same time, fiddler crab (Uca spp.) distributions have expanded and purple marsh crab (Sesarma reticulatum) grazing on creekbank vegetation has increased. Sea-level rise and reduced predation pressure drive these changing crab populations but most studies focus on one species; there is a need for community-level assessments of impacts from multiple crab species. There is also a need to identify additional factors that can affect crab populations. We sampled crabs and environmental parameters in four Rhode Island salt marshes in 2014 and compiled existing data to quantify trends in crab abundance and multiple factors that potentially affect crabs. Crab communities were dominated by fiddler and green crabs (Carcinus maenas); S. reticulatum was much less abundant. Burrow sizes suggest that Uca is responsible for most burrows. On the marsh platform, burrows and Carcinus abundance were negatively correlated with elevation, soil moisture, and soil percent organic matter and positively correlated with soil bulk density. Uca abundance was negatively correlated with Spartina patens cover and height and positively correlated with Spartina alterniflora cover and soil shear strength. Creekbank burrow density increased dramatically between 1998 and 2016. During the same time, fishing effort and the abundance of birds that prey on crabs decreased, and water levels increased. Unlike in other southern New England marshes where recreational overfishing is hypothesized to drive increasing marsh crab abundance, we propose that changes in crab abundance were likely unrelated to recreational finfish over-harvest; instead, they better track sea-level rise and changing abundances of alternate predators, such as birds. We predict that marsh crab abundance will continue to expand with ongoing sea-level rise, at least until inundation thresholds for crab survival are exceeded.
Highlights
Southern New England salt marshes are undergoing rapid and dramatic change
Vegetated habitats were typical of southern New England salt marshes (e.g., Warren & Niering, 1993), but the relative amount of dominant species in each marsh followed a predictable pattern in relation to overall mean site elevation: as site elevation increased, the percent composition of flood-tolerant S. alterniflora decreased, and the composition of less flood-tolerant high marsh species (e.g., S. patens, Distichlis spicata) increased
Our study provides new insight into the complex relationships between bottom-up and top-down stressors, crab populations, and ongoing marsh vegetation loss
Summary
Southern New England salt marshes are undergoing rapid and dramatic change. Some conspicuous examples include the replacement of high marsh with low marsh vegetation, increasing areas of high marsh vegetation dieback, shallow pond formation, edge erosion, channel expansion, and soil weakening (Cole Ekberg, Ferguson & Raposa, 2015; Raposa et al, 2017a; Watson et al, 2017a). Sea-level rise is driving many of these changes, but other factors such as elevated nutrient levels, latent effects from hydrologic alterations, and plant fungal infections have been identified as potential stressors (Deegan et al, 2012; Coverdale et al, 2013a; Elmer et al, 2013). Another emerging co-stressor, is impacts from crab populations (e.g., Holdredge, Bertness & Altieri, 2009). Observations of expanding Uca and Sesarma populations have proliferated across the region within the last decade, and studies are confirming negative impacts to marshes across the region associated with excessive crab burrowing and grazing (Holdredge, Bertness & Altieri, 2009; Coverdale, Bertness & Altieri, 2013b; Schultz, Anisfeld & Hill, 2016)
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