Understanding the drivers of biodiversity on the rocky coast

Urbanisation and climate change are pervasive stressors to natural ecosystems and have been linked to decreased biodiversity and ecological function. At the coastal fringe, intertidal rocky shores are threatened by urbanisation along shorelines and sea level rise, with the result that complex intertidal habitats are becoming rare and often replaced with simpler concrete structures. Communities that remain also experience altered light climates due to shading by engineered structures. To effectively manage and conserve urban intertidal ecosystems, we require a greater understanding of the drivers of diversity at micro- to macro scales. I begin by investigating the drivers of biodiversity on the small scale, where I was able to identify relationships between rock pool physical characteristics and associated biota around Sydney Harbour. Maximum width and depth, volume and height on shore were important drivers of biodiversity, but effects varied among organisms (i.e. sessile vs. mobile taxa) and between inner and outer locations of the lower estuary. I also found that the structure within rock pools can influence species abundances. Microhabitats such as overhangs were important to support grazers and rare species. I then investigated the effect of another potential driver of biodiversity common in urban intertidal areas: shading by engineered structures. Using a manipulative experiment, I found that the effect of varying levels of shading can influence taxa in different ways. Whereas high light intensities promoted greater algal cover, low light intensities supported higher abundances of mobile taxa. Finally, I worked at a larger scale and used predictive modelling to assess potential habitat loss of intertidal rocky shores due to sea level rise. I found that rocky shore communities in the Sydney area are likely to lose substantial habitat and may be considered ‘near threatened’ (under one category of the IUCN communities listing criteria) under the predicted 11.2mm/year sea level rise (i.e. scenario RCP8.5 (IPCC)).My research highlights the need to manage intertidal rocky shores at multiple scales. While conserving natural habitat should be set as priority, management strategies that incorporate features which support diversity, as identified in this thesis, can help mitigate biodiversity loss and help protect this important ecosystem