Role of recruitment in causing differences between intertidal assemblages on seawalls and rocky shores

Following progressive urbanisation of coastal areas, artificial structures are becoming common features of landscapes in shallow waters. Despite this, few studies have focused on the ecological role of these structures or have attempted to assess the extent to which they can act as surrogates for natural habitats. This study investigated whether colonisation of space can determine the occurrence of different intertidal assemblages on rocky shores and sandstone seawalls in Sydney Harbour (New South Wales, Australia). Areas were cleared on rocky shores and seawalls at 3 different locations to test hypotheses from 2 alternative models: (1) patterns of distribution and abundance of organisms on the 2 types of structure are the direct result of different patterns of recruitment and (2) early stages of development of assemblages are the same on the 2 types of structure, but later processes (post-recruitment) differ between structures, producing different older assemblages. Furthermore, the model that assemblages developing in clearings on each structure would converge toward mature assemblages found on the same type of structure was tested. Assemblages in clearings differed between seawalls and rocky shores from the early stages of succession and differences persisted through time. Although there was variability among locations, these assemblages tended to converge toward mature assemblages on the same type of structure. These results support the model that intrinsic differences (e.g. topography, weathering, shape and extent of surfaces) between seawalls and rocky shores could affect the recruitment of algae and invertebrates, leading to the establishment of distinct assemblages. This knowledge could improve our ability to design artificial structures that more closely mimic natural habitats, potentially mitigating some effects of loss and fragmentation of coastal habitats in urban areas.