Hydrodynamic gradients affect facilitation in soft sediment communities

Abstract Positive species interactions, or facilitation, are fundamental drivers of ecological community assembly and stability. In terrestrial plant‐dominated systems, the effects of facilitation on biodiversity have been shown to intensify along environmental stress gradients; however, comparable examples are absent for marine soft‐sediment ecosystems (particularly those with animal facilitators), hindering progress in expanding ecological theory. Using a multi‐site field experiment, we investigated the role of an intertidal infaunal clam ( Austrovenus stutchburyi ) as a community‐level facilitator of macrofaunal species diversity across gradients of hydrodynamic forces acting on the seabed (wind waves and tidal currents). Strong density‐dependent influences of the clam on seabed stability were expected to be central to facilitation. At 21 intertidal sites across 4 estuaries in Aotearoa New Zealand, we removed adult clams from 9 m 2 plots (removal plots) and then, after 12 months, measured the difference in macrofaunal community composition, richness and abundance between removal and adjacent non‐removal plots. After 12 months, differences in macrofauna biodiversity between removal and non‐removal plots were observed, and at some sites, these differences were associated with non‐removal plots supporting up to 9× higher macrofauna abundance and up to 4× higher species richness than removal plots, indicating facilitation by Austrovenus as important for community composition. Facilitation was strongest at sites with high adult Austrovenus density (linear model R 2 = 0.52) but also proximity to strong tidal currents (polynomial model R 2 = 0.52), indicating that Austrovenus may extend macrofaunal communities into locations where they would otherwise be excluded by hydrodynamic forces. Synthesis : Our results provide evidence that the strength of facilitation in intertidal soft‐sediments changes across hydrodynamic stress gradients; however, there are likely to be interactions between indirect and direct effects that we discuss in the context of our study system. Our study extends ecological theory on facilitation to marine soft‐sediment systems and offers insight into the functional role of habitat‐forming infauna.