A potentially critical consequence of these changes is a shift in the importance and identity of keystone species-those species whose ecological effects are large relative to their abundance and/or biomass ( 5, 6) and whose functional roles, like all other species, are context-dependent and, hence, sensitive to physical and biotic conditions ( 5, 7– 9). Thus, sea-level rise is creating conditions within which Sesarma functions as a keystone species that is driving dynamic, landscape-scale changes in salt-marsh geomorphic evolution, spatial organization, and species interactions.Ĭlimate change and other human disturbances are widely redefining species distributions and environmental conditions, thereby modifying the organization of our world’s oceans, forests, grasslands, wetlands, tundras, and reefs ( 1– 4). Field experiments further demonstrate that Sesarma-grazed creekheads, through their removal of vegetation that otherwise obstructs predator access, enhance the vulnerability of macrobenthic invertebrates to predation and strongly reduce secondary production across adjacent marsh platforms. Aerial-image analyses reveal that resulting “ Sesarma-grazed” creekheads increased in prevalence from 10 ± 2% to 29 ± 5% over the past <25 y and, by tripling creek-incision rates relative to nongrazed creekheads, have increased marsh-landscape drainage density by 8 to 35% across the region. Together, these bio-physical changes provoke Sesarma to aggregate in high-density grazing and burrowing fronts at the heads of tidal creeks (hereafter, creekheads). In these increasingly submerged areas, cordgrass decreases belowground root:rhizome ratios, causing substrate hardness to decrease to within the optimal range for Sesarma burrowing. Our analyses indicate that sea-level rise in recent decades has widely outpaced marsh vertical accretion, increasing tidal submergence of marsh surfaces, particularly where creeks exhibit morphologies that are unable to efficiently drain adjacent marsh platforms. Here, we reveal that a historically innocuous grazer-the marsh crab Sesarma reticulatum-is rapidly reshaping the geomorphic evolution and ecological organization of southeastern US salt marshes now burdened by rising sea levels. However, global change is pervasively altering environmental conditions, potentially elevating new species to keystone roles. Keystone species have large ecological effects relative to their abundance and have been identified in many ecosystems.

Importance of tidal pools in evolution