Top-down control by crabs along tidal creeks in a salt marsh: vegetation suppression and variable sediment deposition

Sediment deposition shapes the resilience of coastal wetlands by driving marsh accretion and helping marsh platforms keep pace with sea-level rise. Vegetation is widely recognized as an ecosystem engineer that promotes sediment trapping and substrate stabilization, yet the role of crab as a top-down biotic control on salt marsh geomorphic evolution remains poorly understood. Here, we hypothesized that crab activity alters creek-edge sedimentary processes by suppressing vegetation establishment and weakening vegetation-mediated sediment retention. A field exclusion experiment was conducted at the actively colonizing transition between unvegetated creek banks and Scirpus-dominated pioneer marsh in the Chongming Dongtan salt marsh, Yangtze River Estuary, comparing crab-exclusion and crab-access plots during the growing season. Here we show, through direct field measurement, that plant abundance in crab-exclusion plots was 2.2-fold higher than in crab-access plots, and vegetation cover also diverged markedly through time, indicating strong suppression of creek-edge vegetation establishment by crab herbivory. Direct treatment effects on short-term sediment deposition and grain-size structure were spatially variable and not statistically consistent across experimental groups. Model-based analyses showed that vegetation abundance was positively associated with net sediment deposition after accounting for treatment and spatial covariates, and pathway analysis estimated a statistically supported vegetation-mediated effect of crab herbivory on sedimentation. A typhoon during plant grow season provided opportunistic hydrodynamic context, with increased wave forcing coinciding with the lowest vegetation growth in crab-access plots. These findings show that crab activity strongly suppressed creek-edge vegetation establishment, and was associated with reduced vegetation-mediated sediment deposition. Our study highlights crabs as an important biotic control on vegetation structure that supports creek-edge sediment deposition. Over longer timescales, this vegetation-mediated pathway should be incorporated alongside hydrodynamics, sediment supply, sea-level rise, and vegetation growth in models of tidal-flat and salt marsh evolution.