Direct and indirect ecosystem responses to vehicle compaction of soft sediments

Change in an ecological community after a disturbance may be a direct impact or may be indirectly mediated through the response of a highly influential species. In northeastern Pacific tidal flats, a native bioturbator, ghost shrimp Neotrypaea californiensis, engineers soft-sediment habitat and interacts antagonistically with bivalve shellfish. Vehicle compaction has been used in pest control of ghost shrimp, but this disturbance lacks quantitative evidence of its efficacy and environmental impacts. Through three large (∼10 ha) experiments in Grays Harbor, Washington, USA, we tested the direct and indirect impacts of compaction by a tracked vehicle (MarshMaster) on ghost shrimp density, sediment conditions, and infauna. We also examined how oyster survival and waterbird usage of tidal flats responded post-compaction. Compaction pushed <20 % of ghost shrimp to the surface, where they were vulnerable to predation and damage, yet did not significantly reduce subsurface densities within 1-2 days. Rather, declines in shrimp density and shifts to smaller size classes appeared at later sample timepoints and were more pronounced with more compaction passes. All compaction experiments resulted in firmer sediment for at least a year, even in the experiment where shrimp densities were unaffected by a single compaction pass. Where compaction briefly reduced shrimp densities below 50 m^-2, sediment increased in mud and organic content and infauna increased in abundance, suggesting that these changes were mediated through reduced bioturbation rather than a direct impact of compaction. Similarly, multivariate responses of infauna appeared only in the experiments where compaction reduced shrimp densities. Habitat use by waterbirds was more influenced by tidal stage than by compaction; statistically, only dunlin (Calidris alpina) foraged more on compacted than on reference beds. Finally, although survival of outplanted oyster seed improved with compaction at one site, it remained too low (∼34-40 % yr^-1) for viable farming. This implementation of vehicle compaction, with disturbance from as many as five passes or spaced at annual intervals, provided insufficient pest control. Nevertheless, reduced densities of ghost shrimp were associated with follow-on effects on sediment content and the infaunal community.