Urbanization outweighs morphodynamics in shaping macrobenthic biodiversity on tropical island sandy beaches

Ecological understanding of sandy beach ecosystems has advanced considerably in recent decades, with studies demonstrating the roles of morphodynamic factors (e.g., waves, sediment, slope) and anthropogenic activities in shaping biodiversity. Yet, knowledge gaps persist for tropical island beaches, ecosystems of disproportionate ecological, cultural, and economic importance but constrained by distinctive geomorphological and oceanographic conditions. We investigated how natural drivers and human disturbance interact to structure macrobenthic diversity across 16 beaches in the U.S. Virgin Islands (USVI), spanning a gradient from minimally disturbed to highly urbanized sites. Given the limited natural variability of local beaches, we hypothesized that morphodynamic characteristics would exert a minor influence compared to anthropogenic pressures, and that adjacent seagrass meadows would enhance macrobenthic richness and abundance on neighboring beaches through food subsidy. Our results support the first hypothesis: urbanization was the dominant predictor of macrobenthic biodiversity, with significant declines in both richness and abundance along the urbanization gradient, while morphodynamic effects were comparatively weak. Macrobenthic assemblages were dominated by annelids and arthropods, with urbanization effects most pronounced in submerged zones. Contrary to expectations, proximity to seagrass meadows was not significantly associated with higher richness or abundance, likely due to the influx of macroalgal wrack on local sandy beaches. These findings provide a critical baseline for conservation and management of tropical island sandy beaches and suggest that their limited morphodynamic variability diminishes its ecological role, while anthropogenic disturbance emerges as the primary driver of biodiversity patterns.