Eco-engineering marine urban infrastructure: the role of habitat complexity in predator mitigation

<p dir="ltr">The urbanisation of coastlines is occurring on large scales worldwide. Compared to natural habitats, built structures are more homogeneous, less topographically complex, and are composed of different materials with varying substrate colour. As a consequence, artificial structures often support different communities and decreased biodiversity compared to natural habitats. There is increasing interest in eco-engineering artificial structures to improve their ecological value. Marine eco-engineering typically focuses on the enhancement of habitat complexity, i.e., the number of structural elements per unit area, which is broadly considered a key determinant of biodiversity. Through the provision of microhabitats, habitat complexity can ameliorate the effects of biotic and abiotic stressors, such as predation and thermal stress. Retrofitting topographic features like pits, crevices, and depressions, as well as habitat-forming species like oysters, to artificial structures has, however, not always led to increased biodiversity. This may be because habitat complexity is not always effective at mitigating key limiting stressors to the establishment of biodiversity. Further research is needed to understand how best to integrate topographic features and material attributes to successfully implement eco-engineering interventions. </p><p dir="ltr">In this thesis, I studied the extent to which relationships between habitat complexity and biodiversity on artificial structures are driven by predator mitigation and, hence, how the effects of habitat complexity vary across predation gradients. I conducted field experiments in the Philippines that manipulated habitat complexity and predator access to substrates inside and outside marine protected areas (MPAs). I found that habitat complexity had the greatest impact when fish predators could access the panels, and this impact diminished when fish access was restricted. Additionally, the effects of complexity and predator exclusion were most significant in areas with the highest fish observations. These findings suggest that creating protective spaces through eco-engineering will be most effective in environments with strong top-down control on fouling communities. </p><p dir="ltr">In another field experiment, I looked at how substrate brightness, habitat complexity, and predator exclusion interact to influence the survival of the oyster, <i>Saccostrea glomerata</i>, and the colonisation of associated invertebrates and algae. Darker substrates generally had negative effects, and the positive effects of complexity were more pronounced when predators could access the panels. The impacts of predator mitigation and microhabitat characteristics varied between sites depending on the prevailing stressors, emphasising the need for site-specific knowledge to achieve successful eco-engineering initiatives.</p><p dir="ltr">Lastly, I investigated the effects of different types of eco-engineered habitat complexity on oyster survivorship and the species density and abundances of mobile and sessile taxa across a tidal elevation gradient of a Sydney Harbour site. Habitat complexity generally enhanced oyster survival by providing protective microhabitats from finfish predators, but the magnitude of this effect depended on the type of habitat complexity provided. Colonisation of associated benthic communities by contrast was driven strongly by the presence of water-retaining microhabitats and did not respond to all forms of habitat complexity. These results highlight the significant role of predation in limiting oyster survivorship and the varying effects of complexity on different taxa. </p><p dir="ltr">Overall, these findings emphasise the need to identify target species and key stressors prior to the implementation of eco-engineering interventions. For instance, identifying dominant predators in areas with strong top-down control and addressing thermal stress or desiccation in sites exposed to high daytime temperatures. The spatially and temporally variable results underscore the importance of bespoke rather than one-size-fits-all approaches. Customising strategies to site-specific goals and environmental conditions will help to ensure that interventions are successful and not simply a form of greenwashing.</p>