Ecology of seawalls in Sydney Harbour
Professor Gee Chapman
Built seawalls are one of the most extensive intertidal habitats in Sydney Harbour. They can be made of cement or sandstone, be sloping or vertical and have rough surfaces or be very smooth. Even sandstone walls, which are made of the same material as rocky shores around the Harbour, differ from nearby natural shores in some fundamentally important ways. They tend to have steeper slopes, which reduces the amount of intertidal area from high- to lowshore, from 10s of metres, to about 2 metres. In addition, they do not have particular microhabitats, such as rock-pools. Nevertheless, many intertidal animals and plants can be found living on seawalls.
With increasing urbanization and the continued alteration of our foreshores from natural to built habitats, natural habitats are being fragmented and replaced and areas of the estuary that did not originally have hard shores are now lined with walls. The consequences of this for local biodiversity depends, to some extent, on the use of seawalls as habitat by indigenous and introduced species, or by rare and common species.
The Centre therefore supports a number of research projects, including many student projects, that are examining the animals and plants that live on seawalls compared to natural shores. More importantly, we are attempting to identify those that do not live on walls, or are perhaps very sparse on walls compared to natural habitats. In addition, to these measures of ecological structure (the numbers and types of organisms present), we are also measuring ecological functions, e.g. recruitment, mortality. There may also be strong effects of walls on adjacent habitats. For example, boulder-fields have been formed at the base of seawalls, from disposal of the stones during earlier repairs to the walls.
Understanding which intertidal species can use seawalls as habitat, which reach much larger numbers on seawalls than they appear to naturally, how species interact in these different habitats and how processes, such as predation, dispersal, mortality, reproduction, etc. vary between walls and shores, will identify more clearly what impacts seawalls may be having on biodiversity. With this information, ways in which to manage these artificial habitats should become clearer.
Results will be posted as they become available