From temperate to tropical seas : drivers of variation in reef-associated epifaunal invertebrate communities

Globally, shallow reef ecosystems are undergoing dramatic changes, largely due to unprecedented climate change and exacerbated by local anthropogenic drivers. These changes are generally mediated by habitat transformation, which has critical implications for ecological communities and food web dynamics, often beginning at basal trophic levels with flow-on effects throughout food webs. Small mobile invertebrates ('epifauna') inhabiting the surfaces of structurally diverse reef habitats are prolific and ubiquitous on reefs worldwide, representing an essential basal trophic group that fuels much of shallow reef food webs. This thesis examines the interactive influences of local and broad-scale drivers on epifaunal community composition, size structure and productivity on shallow reefs along a steep eastern Australian climatic gradient of 28.6¬∞ with mean annual temperature range of ~13¬∞C. The overarching aim of this research is to understand how epifaunal communities vary across different reef habitat types, locations and latitudes, and environmental and ecological gradients, to better understand the broader ecological implications of ocean warming and local anthropogenic impacts on reef food webs. Habitat is identified as the most important correlate of variation in epifaunal assemblages regardless of latitude or the metric used to quantify assemblages. Macroalgae, live coral and turfing algae represent three habitat extremes in terms of the taxonomic composition of epifaunal assemblages they host, with assemblage variation shaped by structural differences among habitats. The three habitat extremes also apply to the size structure and daily productivity of epifaunal assemblages, both important ecological properties with regards to the availability of this trophic group at a critical basal level in shallow reef food webs. Despite distinct assemblage-habitat associations and the variation in dominant habitats across the latitudinal range studied ‚- from cool-temperate macroalgae-dominated reefs to tropical coral reefs ‚- daily community productivity of epifauna was largely invariant among sampling locations. On subtropical to tropical reefs, dramatically different epifaunal assemblages were evident on live versus dead coral habitat, with dead coral supporting density, biomass, and daily productivity of epifauna 1 ‚- 2 orders of magnitude higher than live coral. These distinctions between broadly classified coral habitats were consistent among four heterogenous subtropical and tropical reef locations. Epifaunal communities apparently represent an important avenue for ecological change associated with coral mortality through mass bleaching events. Overall, my research strongly implies that habitat is the dominant driver of variation in reef-associated epifaunal assemblages. Broad-scale ocean warming and local anthropogenic stressors will likely influence changes to epifaunal assemblages on shallow reefs almost exclusively via transformation of habitats. The consistent trends across large biogeographic scales also suggest that accurate prediction of the basal food web resource provided by epifaunal invertebrates is possible when information on habitat distribution is available.