In very diverse and complex habitats such as intertidal rocky shores, where there are dozens of species of animals and plants living together in a small area, the numbers of individuals and types of species that one finds are very variable.  Measuring biodiversity is difficult.  It is controlled by factors external to the habitat (e.g. the numbers that arrive from the plankton) and factors associated with the habitat itself (e.g. rates of predation or competition).  Intertidal rocky shores in NSW are among the most intensively studied in the world, so we know a lot about the most important processes maintaining biodiversity on these shores.  Some of the larger or more numerous species, or those that have been shown to be most important in determining patterns of diversity on natural shores, are absent from or found in abnormally small or large densities on seawalls.  Little to nothing known to date about how important ecological processes differ on seawalls compared to natural shores, either because the habitat or the mix of species is different.

Urbanisation affects the sustainability of biodiversity.  Emphasis has been generally on terrestrial habitats and the “charismatic” megafauna and flora, largely ignoring the very real problems for marine habitats.  Concern is mostly about large plants in seagrass beds and intertidal wetlands and an obsession with contamination or invasive species, ignoring other important effects of urbanization.  Alterations to shoreline in coastal urban centres have replaced or fragmented natural habitat.  They also add new artificial habitat causing large changes to natural assemblages, especially if some species cannot live on the new structures, or can do very well and reach abnormally large numbers.