Sea wall 'eco-engineering' can help boost biodiversity
Slight modifications to sea defences - at little or no extra cost - can boost the level of biodiversity found in intertidal zones, a study has shown.
Researchers found that attaching artificial rock pools to the structures created habitats suitable for mobile creatures, such as starfish or crabs.
They added that they hoped the results would encourage future designs to incorporate "ecological engineering".
The findings have been published in the Marine Ecology Progress Series journal.
"When we looked into the economics, more than 80% of money that is spent to protect coastlines from climatic changes is spent building new sea walls, increasing the height, stability and length of existing ones," explained co-author Mark Browne, an ecologist based at the US-based National Center for Ecological Analysis and Synthesis.
"This is because we want to protect human lives and we also want to protect the important infrastructure," he told BBC News.
"But when we looked at how those seawalls were being built, they were being built according to engineering and financial criteria, not really on their capacity to support marine life."
Dr Browne, who was based at the University of Sydney for the research, said colleague and co-author Prof Gee Chapman had spent many years examining how plants and animals found in the intertidal area of shorelines were affected by engineering.
"The overwhelming evidence she had was that there were major differences between the types of organisms that you find on seawalls compared with those on natural shorelines," he said.
"One of the major differences was the absence of mobile organisms - such as limpets, starfish and crabs."
So the researchers set up an experiment to find out why this was the case and what could be done to mitigate the impact of the structures.
Previous studies had shown that when artificial shorelines replaced the natural ones, there was usually a change of species locally as sedimentary habitat was replaced with hard materials.
"Although many native species live on the hard substratum, they are not usually the same species that live in or on soft substratum," they wrote.
They suggested that the changes to the composition of organisms living on or near the structure may be the result of the steep sides of the sea defences, limiting the intertidal area available to species.
"Alternatively, the walls may lack important intertidal habitats microhabitats," they added. "The most obvious are rock pools."
In order to test the idea whether simple additions to otherwise featureless sea walls in Sydney Harbour would make the structures more biodiversity friendly, the researchers installed a number of large, concrete flowerpots to create artificial rock pools.
They observed: "The size of the pot, its height on the wall and its location affected the assemblages that developed, with greater abundances and diversity of organisms in shallower pots and those at mid-shore levels."
Dr Browne explained: "We have shown quite clearly that you are able to improve levels of biodiversity by more than 110% and the size of the pot and the location it is situated matters.
"If we are going to be spending more than US $144bn each year to build new [flood defences] or increase the height or stability of existing ones and 80% of those funds on coastal defences, we really need to be starting to think about how we can put these types of ecological engineering approaches into practice.
"The measures can be added to the work with little or no additional cost without diminishing the integrity of the structure."