Functional and structural responses to marine urbanisation

Abstract

Urban areas have broad ecological footprints with complex impacts on natural systems. In coastal
\nareas, growing populations are advancing their urban footprint into the ocean through the
\nconstruction of seawalls and other built infrastructure.While we have some understanding of how
\nurbanisation might drive functional change in terrestrial ecosystems, coastal systems have been largely
\noverlooked. This study is one of the first to directly assess how changes in diversity relate to changes in
\necosystem properties and functions (e.g. productivity, filtration rates) of artificial and natural habitats
\nin one of the largest urbanised estuaries in the world, Sydney Harbour. We complemented our
\nsurveys with an extensive literature search.We found large and important differences in the
\ncommunity structure and function between artificial and natural coastal habitats. However,
\ndifferences in diversity and abundance of organisms do not necessarily match observed functional
\nchanges. The abundance and composition of important functional groups differed among habitats
\nwith rocky shores having 40% and 70% more grazers than seawalls or pilings, respectively. In
\ncontrast, scavengers were approximately 8 times more abundant on seawalls than on pilings or rocky
\nshores and algae were more diverse on natural rocky shores and seawalls than on pilings. Our results
\nconfirm previous findings in the literature. Oysters were more abundant on pilings than on rocky
\nshores, but were also smaller. Interestingly, these differences in oyster populations did not affect in
\nsitu filtration rates between habitats. Seawalls were the most invaded habitats while pilings supported
\ngreater secondary productivity than other habitats. This study highlights the complexity of the
\ndiversity-function relationship and responses to ocean sprawl in coastal systems. Importantly, we
\nshowed that functional properties should be considered independently from structural change if we
\nare to design and manage artificial habitats in ways to maximise the services provided by urban coastal
\nsystems and minimise their ecological impacts.