The Impacts of Predation and Habitat Degradation on Coral Reef Sponge Assemblages in SE Sulawesi, Indonesia
Coral reefs across the globe are in decline due to multiple threats including overexploitation, pollution, coastal development, climate change and ocean acidification. Much research has focused on the effects of these threats on hard corals while their impacts on other important benthic invertebrate groups have been overlooked. Sponges are a diverse and abundant component of coral reef communities in the Indo-Pacific that play important functional roles on reefs including nutrient cycling, linking primary and secondary production, reef bioerosion and spatial competition. Consequently, changes in the abundance and distribution patterns of sponges can affect overall reef ecosystem function. Understanding the factors that control sponge distribution patterns is therefore essential for the successful prediction and mitigation of the effects of current threats to reef systems. Sponge distributions are known to be affected by a number of abiotic factors such as wave action, sedimentation and water flow, but the role of biological factors such as predation and competition is less clear. The primary aim of my thesis was to determine the effects of predation on the distribution and abundance of sponge assemblages in the Wakatobi Marine National Park (WMNP), SE Sulawesi, Indonesia. My first objective (chapter 2) was to identify the major spongivores in the Wakatobi using surveys and behavioural observations of key invertebrate (nudibranchs and starfish) and vertebrate taxa (fish). I then adopted a statistical modelling approach (chapter 3) to examine associations between sponges and a suite of abiotic and biological factors, including spongivore abundance, across nine sites in the WMNP. The results of this analysis showed that although sponge assemblage composition was weakly associated with spongivore abundance, sedimentation is more likely to have a greater impact on sponge abundance and distribution patterns. I found that degraded sites were characterized by low diversity sponge assemblages dominated by a single sediment tolerant species Lamellodysidea herbacea. In order to explore the relationship between sponges and spongivore abundance further, I used an experimental approach (chapter 4), establishing a caging experiment to examine the effect of excluding predators on reef slope sponge assemblages. The caging experiment did not reveal any effects of predator exclusion on sponge abundance or diversity. In the last data chapter (chapter 5), I focus on sub-lethal predation effects on sponges and examine the extent and impact of partial predation on the Indo-Pacific giant barrel sponge Xestospongia testudinaria. Video observations of fish predation and measurements of barrel sponge regeneration rates were used to model the impacts of predation on barrel sponges. In summary, a combination of observational, modelling and experimental approaches were used in order to determine the impact of fish predation on Indo-Pacific sponge assemblages. Spongivory does not appear to have a major influence on the abundance and distribution of reef sponges but is an important trophic link in reef ecosystems. Ecologically important sponge species, such as the giant barrel sponge X. testudinaria, are exposed to intensive partial predation and future changes in predation intensity could have consequences for the fitness of these species. Finally, my work suggests that changes on reefs such as increases in sedimentation could produce a shift from coral dominated to lower diversity sponge dominated communities.