Protecting Great Barrier Reef resilience through effective management of crown-of-thorns starfish outbreaks.

Resilience-based management is essential to protect ecosystems in the Anthropocene. Unlike large-scale climate threats to Great Barrier Reef (GBR) corals, outbreaks of coral-eating crown-of-thorns starfish (COTS; Acanthaster cf. solaris) can be directly managed through targeted culling. Here, we evaluate the outcomes of a decade of strategic COTS management in suppressing outbreaks and protecting corals during the 4th COTS outbreak wave at reef and regional scales (sectors). We compare COTS density and coral cover dynamics during the 3rd and 4th outbreak waves. During the 4th outbreak wave, sectors that received limited to no culling had sustained COTS outbreaks causing significant coral losses. In contrast, in sectors that received timely and sufficient cull effort, coral cover increased substantially, and outbreaks were suppressed with COTS densities up to six-fold lower than in the 3rd outbreak wave. In the Townsville sector for example, despite exposure to comparable disturbance regimes during the 4th outbreak wave, effective outbreak suppression coincided with relative increases in sector-wide coral cover (44%), versus significant coral cover declines (37%) during the 3rd outbreak wave. Importantly, these estimated increases span entire sectors, not just reefs with active COTS control. Outbreaking reefs with higher levels of culling had net increases in coral cover, while the rate of coral loss was more than halved on reefs with lower levels of cull effort. Our results also indicate that outbreak wave progression to adjoining sectors has been delayed, probably via suppression of COTS larval supply. Our findings provide compelling evidence that proactive, targeted, and sustained COTS management can effectively suppress COTS outbreaks and deliver coral growth and recovery benefits at reef and sector-wide scales. The clear coral protection outcomes demonstrate the value of targeted manual culling as both a scalable intervention to mitigate COTS outbreaks, and a potent resilience-based management tool to "buy time" for coral reefs, protecting reef ecosystem functions and biodiversity as the climate changes.

Contrasting size and fate of juvenile crown-of-thorns starfish linked to ontogenetic diet shifts.

Population dynamics of organisms are shaped by the variation in phenotypic traits, often expressed even among individuals from the same cohort. For example, individual variation in the timing of ontogenetic shifts in diet and/or habitat greatly influences subsequent growth and survival of some organisms, with critical effects on population dynamics. Few studies of natural systems have, however, demonstrated that marked phenotypic variation in growth rates or body size among individuals within a modelled cohort is linked to dietary shifts and food availability. Population irruptions of the crown-of-thorns starfish are one of the foremost contributors to the global degradation of coral reefs, but causes of irruptions have been debated for decades. Here we demonstrate, based on extensive field sampling of juvenile starfish (n = 3532), that marked variation in body size among juvenile starfish is linked to an ontogenetic diet shift from coralline algae to coral. This transition in diet leads to exponential growth in juveniles and is essential for individuals to reach maturity. Because smaller individuals experience higher mortality and growth is stunted on an algal diet, the ontogenetic shift to corallivory enhances individual fitness and replenishment success. Our findings suggest that the availability of coral prey facilitates early ontogenetic diet shifts and may be fundamental in initiating population irruptions.

Contributions of pre- versus post-settlement processes to fluctuating abundance of crown-of-thorns starfishes (Acanthaster spp.).

Numerous hypotheses have been put forward to account for population outbreaks of crown-of-thorns starfishes (CoTS, Acanthaster spp.), which place specific importance on either pre- or post-settlement mechanisms. The purpose of this review is to specifically assess the contributions of pre- versus post-settlement processes in the population dynamics of CoTS. Given the immense reproductive potential of CoTS (>100 million eggs per female), persistent high densities would appear inevitable unless there were significant constraints on larval development, settlement success, and/or early post-settlement growth and survival. In terms of population constraints, pre- and post-settlement processes are both important and have additive effects to suppress densities of juvenile and adult CoTS within reef ecosystems. It is difficult, however, to assess the relative contributions of pre- versus post-settlement processes to population outbreaks, especially given limited data on settlement rates, as well as early post-settlement growth and mortality. Prioritising this research is important to resolve potential effects of anthropogenic activities (e.g., fishing) and habitat degradation on changing population dynamics of CoTS, and will also improve management effectiveness.