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ABSTRACT Herbivore fronts can alter plant traits (chemical and/or morphological features) and performance via grazing. Yet, herbivore‐driven trait alterations are rarely considered when assessing how these fronts shape ecosystems, despite the critical role that plant performance plays in ecosystem functioning. We evaluated herbivore fronts created by the purple marsh crab,Sesarma reticulatum, as it consumes the cordgrass,Spartina alterniflora, in Virginian salt marshes.Sesarmafronts form at the head of tidal creeks and move inland, creating a denuded mudflat between the tall‐formSpartinalow marsh (trailing edge) and the short‐formSpartinahigh marsh (leading edge). We quantifiedSesarmafront migration rate, tested ifSesarmaherbivory altered geomorphic processes andSpartinatraits at the trailing and leading edges, and examined how these trait changes persisted through the final 8 weeks of the growing season.Sesarmafront migration in our region is two times slower than fronts in the Southeast United States, andSpartinaretreat rate at the leading edge is greater than the revegetation rate at the trailing edge.Sesarmafronts lowered elevation and decreased sediment shear strength at the trailing edge while having no impact on soil organic matter and bulk density at either edge. At the leading edge,Sesarmagrazing reducedSpartinagrowth traits and defensive ability, and trait changes persisted through the remaining growing season. At the trailing edge, however,Sesarmagrazing promoted belowground biomass production and had limited to no effect on growth or defensive traits. We show that herbivore fronts negatively impact saltmarsh plant traits at their leading edge, potentially contributing to front propagation. In contrast, plants at the trailing edge were more resistant to herbivore grazing and may enhance resilience through elevated belowground biomass production. Future work should consider herbivore‐driven plant trait alterations in the context of herbivore fronts to better predict ecosystem response and recovery. 

Abstract Consumers can directly (e.g., consumption) and indirectly (e.g., trophic cascades) influence carbon cycling in blue carbon ecosystems. Previous work found that large grazers have nuanced effects on carbon stocks, yet, small, bioturbating‐grazers, which remove plant biomass and alter sediment properties, remain an understudied driver of carbon cycling. We used field‐derived and remote sensing data to quantify how the purple marsh crab,Sesarma reticulatum, influenced carbon stocks, flux, and recovery in salt marshes.Sesarmacaused a 40%–70% loss in carbon stocks as fronts propagated inland (i.e., ungrazed to recovered transition), with front migration rates accelerating over time. Despite latitudinal differences, front migration rate had no effect on carbon stocks, flux, or time to replacement. When we includedSesarmadisturbance in carbon flux calculations, we found it may take 5–100 years for marshes to replace lost carbon, if at all. Combined, we show that small grazers cause a net loss in carbon stocks as they move through the landscape, and irrespective of migration rate, these grazer‐driven impacts persist for decades. This work showcases the significant role of consumers in carbon storage and flux, challenging the classic paradigm of plant–sediment feedbacks as the primary ecogeomorphic driver of carbon cycling in blue carbon ecosystems.