Arthropodofauna richness and abundance across beach-dune systems with contrasting morphodynamics

Abstract The across-shore distribution of arthropods in two Uruguayan sandy beach-dune systems with contrasting morphodynamics was investigated. A deconstructive analysis was performed to describe faunal changes from the dunes to the shoreline. The Habitat Safety Hypothesis (HSH), which states that sandy beach supralittoral species are more abundant in the backshore of reflective beaches than in dissipative ones, was assessed. A hypothesis that predicts more diverse and abundant supralittoral and dune insects and arachnids in dissipative beaches was also tested. Sampling was performed simultaneously in both beaches with pitfall traps that were kept active for 24 h at three transects that included 17 sampling levels. The reflective beach presented significantly higher elevation, sand temperature, grain size and sorting, and lower sediment compaction and moisture than the dissipative one. Total abundance of arthropods was significantly higher in the reflective beach, supporting the HSH. However, the deconstructive analysis revealed different patterns in the across-shore distribution, diversity and abundance among taxa. Crustaceans, coleopterans and dipterans exhibited higher abundance in the backshore of the reflective beach, whereas acarines were more abundant in the dunes of the dissipative beach. Ants were similarly abundant in the dunes of both beach types. Species diversity did not differ between beaches and was higher in the dunes than in the backshore. The higher abundance observed in the backshore of the reflective beach supports the HSH, reinforcing the idea that the backshore of reflective beaches can be a safer zone not only for beach crustaceans inhabiting this beach fringe, but also for some insects (coleopterans and dipterans). Other arthropods (acarines) inhabiting the dunes were more abundant in the dissipative beach. The similar abundance of ants found in both beaches suggests that dunes offer suitable microhabitats for them, overriding the effects of beach morphodynamic factors.