Recruitment determines differences between assemblages on shaded or unshaded seawalls

The distribution of intertidal organisms can often be determined by processes operating at the time of recruitment. Recruitment has been demonstrated to influence the composition of assemblages of natural habitats, but there is less evidence of its importance in artificial habitats. Coastal areas are becoming increasingly urbanised, with the replacement of many natural habitats by man-made structures. It is, therefore, important to test processes that influence the structure of assemblages in artificial habitats in order to evaluate whether processes act in a way that is predictable from knowledge of natural shores. Much of the foreshore of Sydney Harbour has been replaced by seawalls, many of which are built in association with wharves. It has been shown that features of wharves may cause small-scale differences between assemblages on adjacent sections of seawalls which are under or not under wharves. The present study tested the hypothesis that these patterns are determined by differences in recruitment, by monitoring recruitment to experimental clearings and settlement plates in both habitats. In general, algae and mobile invertebrates had greater cover or abundance on unshaded seawalls, while sessile invertebrates had greater cover on shaded seawalls. Not all taxa in these habitats recruited to experimental clearings or plates, but the abundance or cover of those that did resembled patterns observed in the surrounding established assemblage. This indicates that, at least for some organisms on intertidal seawalls, patterns of distribution are determined at the time of recruitment and are probably influenced by shading. Furthermore, patterns in the present study were similar to what was predicted from work done on natural shores, showing that studies from natural shores can be applied to these artificial habitats.