Biodiversity surveys and stable isotope analyses reveal key differences in intertidal assemblages between tropical seawalls and rocky shores

As coastal cities around the world expand, and sea levels and the frequency of storms rise, natural shorelines are steadily being replaced by artificial defences such as seawalls. A growing number of studies have documented the assemblages that inhabit these novel environments, and some have contrasted them against those found in their natural analogues: rocky shores. Most of this work has, however, been conducted in temperate regions, and there is limited research on seawalls in the tropics. To address this, we conducted monthly surveys of adjacent seawall and rocky shores at multiple sites around Singapore for 1 yr. Our results concur with previous temperate studies — artificial seawalls support a lower diversity but share a substantial number of species with rocky shores. Multivariate analyses reveal that assemblage differences were largely driven by species that were found in both habitats (e.g. detritivore Ligia exotica, grazer Monodonta labio and carnivorous whelk Drupella margariticola) but occurred in different abundances. We also conducted (for the first time on seawalls) stable isotope analyses to elucidate the diets of the common species found in both habitats. Turf algae, which were found to be present in significantly lower abundances on seawalls, could possibly contribute substantially to the diets of many dominant herbivores. Future seawall enhancement efforts in the tropics could therefore look into whether enhancing turf algae will improve biodiversity.