Humanity is altering natural systems, removing habitat, causing extinctions and altering of natural cycles. This era is so dominated by anthropogenic processes that has come to known as the ‘Anthropocene’. Coastal and marine ecosystems are among the many systems experiencing degradation, due to continuous resource consumption and waste disposal that feature prominently within environmental agendas. These changes are occurring against a backdrop of climate change. The effects of climate change are already apparent in marine systems at local and global scales. Coastal ecosystems are in close proximity to human developments and deliver a range of important ecosystem services that support human well-being (e.g. global fisheries, sea defences, and regulates nutrient cycling and the climate). With 44% of the world’s population living within 100 km of the coast, human activities are very likely to influence the functioning of coastal ecosystems that underpin these ecosystem services. This thesis provides insights on the biodiversity of a very wide spread coastal ecosystem: mudflats. The macrofaunal assemblages inhabiting mudflats are globally and regionally important and mediate a number of ecological functions e.g. secondary production, nutrient and carbon cycling, and the creation of biogenic habitat. The biological traits of macrofauna are defined as the morphological, physiological and behavioural attributes of an organism that interact with the environment and as such they can describe the potential of an organism to contribute to these functional roles. This thesis aims to understand the biodiversity of macrofaunal assemblages inhabiting subtropical mudflats, differences in species composition on the functioning, functional redundancy and the resilience of mudflat functions in the presence of increasing human activities and climate change impacts. Mudflats are widespread but are subjected to continuous alterations by human activities. In chapter 2, a systematic database search (WoS and SCOPUS) was conducted to identify published literature focusing on mudflat macrofaunal assemblages. The searches produced 277 papers addressing benthic macrofaunal assemblages and more than 50% of the published literature were from temperate regions, 33% from the subtropics, 10% from tropical mudflats and data from polar systems were rare. Of the 277 papers, only 68 papers contained sufficient methodological detail and original data and so were deemed relevant for inclusion in the analysis. In total 163 datasets on macrofaunal assemblages were extracted for the analysis including 448 taxa from 4 climatic zones and 10 biogeographic region. In the lower latitudes (tropical), traits such as smaller body sizes, shorter life spans and taxa that have exoskeletons were abundant while direct developmental modes are more prevalent at high latitudes (temperate). Despite these differences, the degree of similarity in biological trait composition of macrofaunal assemblages was higher relative to the taxonomic composition across both climatic and biogeographic spatial scales. This suggest that mudflats globally deliver similar functions such as carbon and nutrient cycling, secondary production etc. The functional redundancy observed across regional and climatic boundaries suggest that ecosystem functioning and service delivery may be expected to show some resilience to perturbations. However, increased human activities may continue to pose a threat to mudflat functioning. Subtropical mudflats support a diverse macrofaunal assemblage consisting of a mixture of tropical and temperate species. Chapter 3 presents the most spatially and taxonomically comprehensive study to date of intertidal mudflat macrofauna along the SE Queensland coast. The effects of climate change were apparent among the studied macrofaunal assemblages. The results presented in chapter 3 showed that for the 24 mudflats sampled in SE Queensland, eight species have moved 146 km distance southwards since 1950s. The species substitutions (arrival of tropical range shifters) observed to date, do not appear to have altered the ecological functioning of these mudflats significantly, as indexed by biological trait composition, with ecosystem service delivery being maintained through functional redundancy. However, simple models of further species turnover suggest that the limits of this ‘buffer’ may soon be exceeded with consequences for a range of ecosystem services. The World’s coastlines have become heavily modified over the last century, and the biodiversity and health of adjacent natural intertidal habitats have declined under increasing pressure from urbanisation. Similarly, the coastal urbanization activities in SE Queensland regions have shown an effect on the macrobenthic assemblages because the species composition of sub-catchments containing 4-12% industrial activity differed significantly from the species composition observed in sub-catchments with 53-90% residential coverage and sub-catchments with a mix (54-70%) of nature reserves and low intensity urban/rural activities. However, we did not observe a clear impact of urbanisation in structuring macrofaunal assemblages. It is likely that the impacts of urbanisation will vary depending on the nature of industry, rather than ‘urbanisation’ as a whole. Although changes in species composition were observed at varying degrees of urbanisation, the trait composition of the macrofaunal assemblages did not vary between the four sub-catchment land uses suggesting conservation in functioning and ecological redundancy in subtropical mudflat ecosystems. The manipulation of biological traits in experimental studies is an important tool for understanding changes in ecological functioning under anthropogenic pressure, and exploring the relationships between biodiversity and functioning. In Chapter 5 I have experimentally investigated how the density manipulation of two common large taxa Macrophthalmus setosus (Ms) (sentinel crab) and Pyrazus ebeninus (Pe) (Hercules mudwhelk), that exhibit functionally distinct traits would influence the ecological assemblage and its functioning, e.g. the primary productivity, sediment oxygenation, carbon cycling and nutrient cycling. Theoretical biodiversity ecosystem functioning (BEF) models were used to explain the biodiversity functioning relationships observed in subtropical mudflats. A change in the species and trait composition of the resident macrofaunal assemblage was apparent when the densities of P. ebeninus was increased. The BEF models did show a degree of redundancy for some ecological functions (e.g. sediment oxygenation, carbon cycling and nutrient cycling) suggesting that initially subtropical mudflats may show some resilience to environmental perturbations.

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