| Summary: | Coral reefs are among the world’s most threatened ecosystems. Recent declines in coral reef health are largely attributed to pervasive anthropogenic stressors such as coastal development, overfishing and global change. While the status of tropical reefs is typically assessed through the extent of live coral cover, these ecosystems also support a vast diversity of non-coral invertebrates, the importance of which is often overlooked. This thesis examines the effects of coastal development (e.g. declining water quality, eutrophication), overfishing and global change (e.g. ocean acidification) on tropical Echinodermata in two case studies that present contrasting anthropogenic effects in this taxon. These case studies focus on two phenomena; 1. the expanding population outbreaks of the boom-and-bust crown-of-thorns starfish, Acanthaster cf. planci (herein: A. planci) (Asteroidea), and 2. the contracting populations of the recently listed Vulnerable sea cucumber, Stichopus herrmanni (curryfish) (Holothuroidea), a model species representing overfished tropical bêche-de-mer holothuroids. Predation on corals by high-density populations of A. planci is a major cause of coral reef decline, but the factors behind population outbreaks remain unclear. The ‘enhanced nutrients’ hypothesis posits that increased phytoplankton levels resulting from anthropogenic eutrophication enhance larval survival, with flow-on effects to benthic life stages, leading to population outbreaks. However, outbreaks also occur in regions unaffected by anthropogenic eutrophication, indicating that the larvae of A. planci are resilient to the naturally oligotrophic waters of coral reef ecosystems; the ‘larval resilience’ hypothesis. This thesis addresses the dichotomy between the ‘enhanced nutrients’ and ‘larval resilience’ hypotheses. In targeted experiments, the current paradigm that A. planci outbreaks are linked to terrestrial runoff driving enhanced food for larvae – a premise that has driven management actions for over 30 years – is addressed. ...
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