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Despite escalating anthropogenic alteration of food webs, how the carbon cycle in ecosystems is regulated by food web processes remains poorly understood. We quantitatively synthesize the effects of consumers (herbivores, omnivores and carnivores) on the carbon cycle of coastal wetland ecosystems, ‘blue carbon’ ecosystems that store the greatest amount of carbon per unit area among all ecosystems. Our results reveal that consumers strongly affect many processes of the carbon cycle. Herbivores, for example, generally reduce carbon absorption and carbon stocks (e.g. aboveground plant carbon by 53% and aboveground net primary production by 23%) but may promote some carbon emission processes (e.g. litter decomposition by 32%). The average strengths of these effects are comparable with, or even times higher than, changes driven by temperature, precipitation, nitrogen input, CO 2 concentration, and plant invasions. Furthermore, consumer effects appear to be stronger on aboveground than belowground carbon processes and vary markedly with trophic level, body size, thermal regulation strategy and feeding type. Despite important knowledge gaps, our results highlight the powerful impacts of consumers on the carbon cycle and call for the incorporation of consumer control into Earth system models that predict anthropogenic climate change and into management strategies of Earth's carbon stocks. This article is part of the theme issue ‘Integrative research perspectives on marine conservation’. 

Keystone species have large ecological effects relative to their abundance and have been identified in many ecosystems. However, global change is pervasively altering environmental conditions, potentially elevating new species to keystone roles. 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. 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. In these increasingly submerged areas, cordgrass decreases belowground root:rhizome ratios, causing substrate hardness to decrease to within the optimal range for Sesarma burrowing. Together, these bio-physical changes provoke Sesarma to aggregate in high-density grazing and burrowing fronts at the heads of tidal creeks (hereafter, creekheads). 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. 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. 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.