The number and variety of artificial structures is increasing in shallow marine habitats of urbanised coasts and estuaries. Despite this trend, very little is known about their ecology and the extent at which they can resemble natural habitats. In particular, the changes in intertidal assemblages caused by the replacement of rocky shores by artificial structures have not previously been documented.

Recruitment of organisms has been proposed to explain patterns of distribution and abundance of marine benthic organisms. Hence, in Sydney Harbour, experiments were done to compare the colonization of space between rocky shores and seawalls. The results of these experiments showed that recruitment can play a pivotal role in determining different assemblages on natural and artificial structures, although there were inconsistencies among locations. Differences in recruitment between rocky shores and seawalls were not due to the characteristics of the substratum at a small scale.

Since herbivores can affect the development of intertidal assemblages, an experiment was done to test the hypothesis that the effects of grazers would differ between natural and artificial structures. Indeed, grazing by herbivores affected the process of re-colonization of cleared space on rocky shores and seawalls, from the early stages. The effects of grazers, however, differed between structures, since limpets were able to control the colonization of the brown encrusting alga, Ralfsia verrucosa, on rocky shores, but not on seawalls.

Physical (e.g. wave-action and topography) and biological (e.g. density of animals, resident organisms) factors have been documented to affect patterns of movement of gastropods. Therefore, an experiment was done to test the hypothesis that patterns of movement of a species of limpet (Cellana tramoserica, which can play an important role in structuring intertidal assemblages), would differ between rocky shores and seawalls. The results of this experiment showed that short-term movements (over 1 and 14 days) did not vary between structures. In contrast the distance displaced over a longer period of time (3 months) was greater on seawalls than on rocky shores.

Finally, since the complexity of habitat can determine patterns of colonisation of space, by providing refuges from predators and physical factors, I tested the hypothesis that the effects of crevices (which occur more frequently on seawalls than on rocky shores) on the colonisation of space would differ between heights on the shore. Assemblages developing on artificial panels with crevices were different from those developing on panels without crevices at mid-shore levels, but not at low-shore levels. This suggests that crevices between adjacent blocks on seawalls may be important in structuring assemblages and that their effects could vary between heights on the shore. This could be related to the harshness of the environment and could depend on life-histories of organisms at different heights on the shore.

Overall, this study showed that mid-shore assemblages differed between rocky shores and artificial hard surfaces and that processes such as recruitment, grazing and movement of animals probably account for these differences. Furthermore, it suggested that differences in the structure of habitat between natural and artificial structures may underlie differences in such ecological processes.

In conclusion, this thesis stressed the importance of comparing assemblages between natural structures and the artificial counterparts that are increasingly replacing them and of identifying the biological and physical processes responsible for the observed differences in assemblages. This knowledge is necessary in order to evaluate the effects of loss and fragmentation of natural habitats on assemblages and populations of organisms and also to aid the design of artificial structures to mimic more closely the natural habitats they are replacing.