Mudflat geomorphology determines invasive macroalgal effect on invertebrate prey and shorebird predators.

Impacts of invasive species are often context specific due to varying ecological interactions. Physical structure of environments hosting invaders is also potentially important but has received limited attention. An invasive macroalga, Agarophyton vermiculophyllum, has spread across the northern hemisphere with mixed positive, neutral and negative effects on resident species. Agarophyton colonizes mudflats that vary in topography due to interactions of sediments with hydrodynamic forces. We tested the hypothesis that mudflat geomorphology moderates the effect of Agarophyton on shorebirds and invertebrates. We surveyed 30 mudflats in the Virginia Coast Reserve quantifying elevation and topography. Invertebrate and bird abundances were also quantified. Mudflat geomorphology ranged from smooth to hummocky and was correlated with invertebrate and shorebird abundance and interactions based on piecewise structural equation models. After accounting for geomorphology, Agarophyton had little effect on invertebrate abundance. Shorebird numbers were differentially influenced by mudflat topography, with positive correlations to invertebrates (worms) on smooth mudflats, and to macroalgae on hummocky mudflats. These differences are likely to be due to sediment properties in interaction with structural changes induced by Agarophyton mats that affect prey accessibility for birds. Even on apparently simple mudflats, geomorphic structure emerged as important, modifying invasive species impacts and differentially influencing consumers.