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Top-down and bottom-up controls on southern New England salt marsh crab populations
Citation:
Raposa, K., R. Mckinney, C. Wigand, J. Hollister, C. Lovall, K. Szura, J. Gurak, J. McNamee, C. Raithel, AND E. Watson. Top-down and bottom-up controls on southern New England salt marsh crab populations. PeerJ. PeerJ Inc., Corte Madera, CA, 6:e4876, (2018).
Impact/Purpose:
The paper provides results of research that may aid in informing decisions regarding management of southern New England salt marshes that are changing rapidly in response to sea-level rise. Coincident with these changes are reports of expanding burrowing crab distributions and vegetation loss due to purple marsh crabs. Research has identified sea-level rise and reduced predation pressure as drivers of increasing crab populations. This study uses holistic, community-level assessments to provide additional insight into the relative impacts from multiple crab species. The study also sought to identify and evaluate additional factors that can potentially alter crab populations. To accomplish these objectives, we sampled crabs and environmental parameters in four Rhode Island salt marshes in 2014. We also compiled existing monitoring data to quantify trends in crab abundance, creek bank vegetation loss, and multiple factors that can potentially affect crabs. Our study documented continued Uca dominance of RI salt marshes and found associations between crabs and sea-level rise-induced changes in vegetation and soils. We were able to predict that marsh crab abundance will continue to expand with ongoing sea-level rise, at least until inundation thresholds for crab survival are exceeded. This information can help inform southern New England coastal resource managers who are addressing the fate of salt marshes under sea-level rise scenarios.
Description:
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.
