No lines in the sand: Impacts of intense mechanized maintenance regimes on sandy beach ecosystems span the intertidal zone on urban coasts

As coastal population growth accelerates, intensive management practices increasingly alter urban shorelines, creating major conservation challenges. To evaluate key ecological impacts and identify indicators of coastal urbanization, we compared intertidal macroinvertebrate communities between urban beaches with intense maintenance regimes (sediment filling and grooming) and reference beaches lacking such maintenance in densely populated southern California. On urban beaches, intertidal communities were highly impacted with significantly reduced species richness, abundance, and biomass (effect sizes: 79%, 49%, 30%, respectively). Urban impacts affected macroinvertebrates across all intertidal zones, with greatest effects on upper intertidal wrack-associated taxa. On urban beaches altered intertidal communities were remarkably homogeneous across littoral cells in a biogeographically complex region. Functional diversity comparisons suggested degraded ecological functioning on urban beaches. No taxa flourished on urban beaches, but we identified several vulnerable indicator taxa. Our results suggest intense maintenance regimes on urban coasts are negatively impacting sandy beach ecosystems on a landscape scale. Beaches not subject to intense mechanized maintenance, can support high biodiversity, even near major urban centers.