Revisiting sandy beach ecological hypotheses to identify disturbance indicators: Supralittoral arthropod responses to urbanization and morphodynamics

The supralittoral mobile arthropod fauna on sandy beaches remains understudied, limiting knowledge of how environmental and anthropogenic drivers shape this biodiversity and the identification of reliable bioindicators. We quantified population-level responses of ten supralittoral taxa across 30 beaches in southwestern Atlantic, testing how beach morphodynamics and coastal urbanization interactively influence their abundance. Arthropods were sampled with nocturnal pitfalls, and urbanization was indexed by the Human Modification Metric (HMc). Abundance responses were taxon-specific: half the taxa were more abundant on dissipative beaches, while the remainder favored on reflective ones. At least half declined with increasing urbanization, with negative HMc effects contingent on morphodynamics, usually intensifying on dissipative beaches. These taxon-specific patterns highlight the need for local validation of candidate indicators and for morphodynamic-specific metrics in biomonitoring. Notably, the Coleoptera-Diptera, Coleoptera-Hemiptera and Amphipoda-Hemiptera ratios were sensitive to urbanization, supporting their application in monitoring protocols to guide coastal biomonitoring.