Deconstructing responses of sandy beach arthropodofauna to shoreline erosion: looking for the proper spatial scale to monitor biodiversity.

Abstract: The suitability of the small spatial scale of sandy beaches was tested as unit for monitoring biodiversity. The study targeted a coastal stretch in central Mediterranean, characterised by a patchy landscape and erosion trend. Beach arthropod communities, biodiversity and behavioural adaptation were considered in the context of ongoing changes and threats to beach habitats. Nine beach units were selected as part of three main coast sectors, each with a different exposure. Beach units were sampled for arthropod diversity, which was assessed at different levels of organisation: diversity indices, taxa abundance and behavioural adaptation. These features were used as response variables in models considering beach physical factors. Sand grain size, swash extent and beach slope resulted the driving forces for diversity and abundance. Behavioural tests indicated a local adaptation to the shoreline direction, with mean angles of orientation seawards. Data related to biodiversity assessment were applied to the estimate of Conservation and Recreation Indices (CI and RI). Coast sectors subject to different erosion rates scored differently, pointing to a potential conflict in management of those beaches subject to physical erosion. Overall, data depicted beach sectors, each one including one or more beach units, as the most suitable dimension for deconstruction to the small-spatial scale. The selection of the scale for deconstruction, applicable to geomorphological, biological and managerial contexts, would be of paramount importance to guide decision-making and compare coastal stretches also in other geographic contexts.