Long-term study reveals top-down effect of crabs on a California salt marsh

Abstract

Consumers can structure plant communities and may function as keystone species or ecosystem engineers. In salt marshes, the prevailing paradigm has shifted in recent decades from nearly complete focus on bottom-up processes to inclusion of top-down effects. Although the number of studies investigating top-down control continues to climb, few experiments span multiple years, so temporal variability in or long-term impacts of consumers have not been well characterized. In addition, while top-down control has been found to be common in Western and Eastern Atlantic and Western Pacific salt marshes, our study is one of the first to experimentally consider top-down control of salt marsh plants in the Eastern Pacific. We conducted a five-year field experiment along eroding creekbank edges of a California salt marsh in which we manipulated densities of the shore crab, Pachygrapsus crassipes, and tracked marsh responses over time. Our results demonstrate that, through both consumption and engineering activities, this superabundant crab is regulating marsh vegetation and soil structure. Experimentally reducing crab abundance enhanced vegetation biomass and sediment bulk density. Moreover, root biomass and bulk density—factors known to increase marsh resilience to erosion and sea-level rise—decreased linearly with increasing burrow density. Our long-term study uniquely revealed that burrows can persist for years after crab abundances are reduced and that plant responses from grazer exclusions gradually strengthen over time, likely due to the relatively slow growth of woody perennial foundational plants. Since shore crabs are abundant throughout the marsh in most major estuaries within the range of the species (from Baja California, Mexico to Oregon, USA), we hypothesize that this species is exerting significant, yet underappreciated top-down control and modifying the sediment properties of many West Coast salt marshes.