Alternative explanatory models of the species-area relationship: an experimental test on honeycomb worm (Sabellaria alveolata) reef patchesBiogenic Reefs

Iacopo Bertocci¹, Annalisa Storari^1,2, Sara Ometto¹, Maria Flavia Gravina³, Joachim Langeneck⁴, Lisandro Benedetti-Cecchi¹

¹University of Pisa, Italy - ²Stazione Zoologica Anton Dohrn, Italy - ³University of Rome “Tor Vergata“, Italy - ⁴Consorzio Nazionale Interuniversitario per le Science del Mare (CoNISMa), Italy

A fundamental property of biodiversity patterns is the species-area relationship (SAR), by which the number of species generally increases as the available or sampled area increases. Nonetheless, tests of alternative explanatory models of SAR are lacking for a number of natural systems, and emphasis on species richness has largely neglected the issue of how assemblage composition may differ among patches of different size. We tested alternative models of the SAR on intertidal reef patches of “small”, “medium”, and “large” size built by the honeycomb worm Sabellaria alveolata on the northern Latium coast (Italy). Model 1 is based on the random placement hypothesis, by which core samples collected from S. alveolata patches of different size would host comparable species richness and composition of infaunal assemblages per space unit. Alternatively, spatially variable processes could drive infaunal assemblages to differ among (i) small, medium and large patches, and (ii) bioconstructed cores located at different distances within medium and large patches (Model 2). In June 2023, four cores were collected 10 cm apart within small, medium and large S. alveolata patches. Four other cores were collected 50 cm apart in medium and large patches. Preliminary analyses indicate a lower infaunal richness × cm-3 in large compared to both medium and small patches, in contrast to Model 1 prediction. Consistently with Model 2, assemblages differed depending on patch size and the sampling distance within medium and large patches. Thus, scale-dependent processes, such as patterns of recruitment, predation and exposure to disturbance possibly varying with the perimeter-to-area ratio, would structure assemblages associated with S. alveolata patches. If confirmed by the final analyses and analogous evidence elsewhere, our findings would have key implications for the conservation of such biogenic habitats under current climate change and increasing human pressures. Likely, to maintain patterns of associated biodiversity, conservation strategies should prioritize not just large and continuous reefs, but also mosaics of patches of varying size.