Explore the vast range of titles available.

Coral reef ecosystems have suffered an unprecedented loss of habitat-forming hard corals in recent decades. While marine conservation has historically focused on passive habitat protection, demand for and interest in active restoration has been growing in recent decades. However, a disconnect between coral restoration practitioners, coral reef managers and scientists has resulted in a disjointed field where it is difficult to gain an overview of existing knowledge. To address this, we aimed to synthesise the available knowledge in a comprehensive global review of coral restoration methods, incorporating data from the peer-reviewed scientific literature, complemented with grey literature and through a survey of coral restoration practitioners. We found that coral restoration case studies are dominated by short-term projects, with 60% of all projects reporting less than 18 months of monitoring of the restored sites. Similarly, most projects are relatively small in spatial scale, with a median size of restored area of 100 m2. A diverse range of species are represented in the dataset, with 229 different species from 72 coral genera. Overall, coral restoration projects focused primarily on fast-growing branching corals (59% of studies), and report survival between 60 and 70%. To date, the relatively young field of coral restoration has been plagued by similar 'growing pains' as ecological restoration in other ecosystems. These include 1) a lack of clear and achievable objectives, 2) a lack of appropriate and standardised monitoring and reporting and, 3) poorly designed projects in relation to stated objectives. Mitigating these will be crucial to successfully scale up projects, and to retain public trust in restoration as a tool for resilience based management. Finally, while it is clear that practitioners have developed effective methods to successfully grow corals at small scales, it is critical not to view restoration as a replacement for meaningful action on climate change.

Ecosystem reconfigurations arising from climate-driven changes in species distributions are expected to have profound ecological, social, and economic implications. Here we reveal a rapid climate-driven regime shift of Australian temperate reef communities, which lost their defining kelp forests and became dominated by persistent seaweed turfs. After decades of ocean warming, extreme marine heat waves forced a 100-kilometer range contraction of extensive kelp forests and saw temperate species replaced by seaweeds, invertebrates, corals, and fishes characteristic of subtropical and tropical waters. This community-wide tropicalization fundamentally altered key ecological processes, suppressing the recovery of kelp forests.

Globally, many coral reefs are degraded and demonstrate reduced coral cover and increased macroalgal abundance. While negative correlations between macroalgae and coral recruitment have commonly been documented, the mechanisms by which macroalgae affects recruitment have received little attention. Here we examined the effect of macroalgae on larval settlement and the growth and survival of coral recruits, in a field experiment over nine months. Exclusion treatments were used to manipulate herbivory and macroalgal biomass, while settlement tiles measured coral settlement and survival. After nine months the volume of macroalgae was up to 40 times greater in the caged treatments than in controls and the settlement of coral larvae on the undersides of tiles within caged plots was 93% lower than in the uncaged treatments. The growth and survival of coral recruits was also severely reduced in the presence of macroalgae: survival was 79% lower in caged treatments and corals were up to 58% smaller with 75% fewer polyps. These data indicate that macroalgae has an additive effect on coral recruitment by reducing larval settlement and increasing recruit mortality. This research demonstrates that macroalgae can not only inhibit coral recruitment, but also potentially maintain dominance through a positive feedback system.

Studies of pre-spawning crown-of-thorns starfish Acanthaster (COTS) collected from the Great Barrier Reef showed average female gonad mass of 16 % (±8 s.d.) and for males 12 % (±6 s.d.). In females up to 34 % of the body mass could be devoted to gonad. Based on these data, we also derived relationships between diameter gonad weight and reproductive output for both male and female starfish. Due to the large average size of individuals in this study (39.2 ± 0.3 cm diameter), the potential oocyte production of females was between 29 and 38 million eggs per season for average size starfish. The highest oocyte production was estimated to be >100 million oocytes, which is the highest ever recorded for an individual female starfish. These relationships imply that the largest Acanthaster reported may have fecundities greater than 200 million eggs per season. The gonad mass of male starfish is similarly high, and in combination with measured concentrations of sperm exuded from the gonopore (5.2 × 10 10  ml −1 ) the sperm output of an average sized male is estimated to be 1.1 × 10 13  sperm. This high level of sperm production may be a key factor allowing this species to sustain itself and even initiate outbreaks at low population densities. We suggest that management targets for maintaining COTS at pre-outbreak thresholds should take account of starfish size as well as starfish density, especially given extreme reproductive potential of large starfish in pre-outbreak populations.

Research on the coral-eating crown-of-thorns starfish (CoTS) has waxed and waned over the last few decades, mostly in response to population outbreaks at specific locations. This review considers advances in our understanding of the biology and ecology of CoTS based on the resurgence of research interest, which culminated in this current special issue on the Biology, Ecology and Management of Crown-of-Thorns Starfish. More specifically, this review considers progress in addressing 41 specific research questions posed in a seminal review by P. Moran 30 years ago, as well as exploring new directions for CoTS research. Despite the plethora of research on CoTS (>1200 research articles), there are persistent knowledge gaps that constrain effective management of outbreaks. Although directly addressing some of these questions will be extremely difficult, there have been considerable advances in understanding the biology of CoTS, if not the proximate and ultimate cause(s) of outbreaks. Moving forward, researchers need to embrace new technologies and opportunities to advance our understanding of CoTS biology and behavior, focusing on key questions that will improve effectiveness of management in reducing the frequency and likelihood of outbreaks, if not preventing them altogether.

Despite the significance of marine habitat-forming organisms, little is known about their large-scale distribution and abundance in deeper waters, where they are difficult to access. Such information is necessary to develop sound conservation and management strategies. Kelps are main habitat-formers in temperate reefs worldwide; however, these habitats are highly sensitive to environmental change. The kelp Ecklonia radiate is the major habitat-forming organism on subtidal reefs in temperate Australia. Here, we provide large-scale ecological data encompassing the latitudinal distribution along the continent of these kelp forests, which is a necessary first step towards quantitative inferences about the effects of climatic change and other stressors on these valuable habitats. We used the Autonomous Underwater Vehicle (AUV) facility of Australia's Integrated Marine Observing System (IMOS) to survey 157,000 m2 of seabed, of which ca 13,000 m2 were used to quantify kelp covers at multiple spatial scales (10-100 m to 100-1,000 km) and depths (15-60 m) across several regions ca 2-6° latitude apart along the East and West coast of Australia. We investigated the large-scale geographic variation in distribution and abundance of deep-water kelp (>15 m depth) and their relationships with physical variables. Kelp cover generally increased with latitude despite great variability at smaller spatial scales. Maximum depth of kelp occurrence was 40-50 m. Kelp latitudinal distribution along the continent was most strongly related to water temperature and substratum availability. This extensive survey data, coupled with ongoing AUV missions, will allow for the detection of long-term shifts in the distribution and abundance of habitat-forming kelp and the organisms they support on a continental scale, and provide information necessary for successful implementation and management of conservation reserves.

Recent increases in the frequency of Extreme Climate Events (ECEs) such as heatwaves and floods have been attributed to climate change, and could have pronounced ecosystem and evolutionary impacts because they provide little opportunity for organisms to acclimate or adapt. Here we synthesize information on a series of ECEs in Australia from 2011-2017 that led to well-documented, abrupt and extensive mortality of key marine habitat-forming organisms – corals, kelps, seagrasses and mangroves – along nearly more than 45% of the continental coastline of Australia. Coral bleaching occurred across much of northern Australia due to marine heatwaves affecting different regions in 2011, 2013, 2016 and 2017, while seagrass was impacted by anomalously high rainfall events in 2011 on both east and west tropical coasts. A marine heatwave off western Australia during the 2011 La Niña extended into temperate and subtropical regions, causing widespread mortality of kelp forests and seagrass communities at their northern distribution limits. Mangrove forests experienced high mortality during the 2016 El Niño across coastal areas of northern and north-western Australia due to severe water stress driven by drought and anomalously low mean sea levels. This series of ECEs reflects a variety of different events – marine heatwaves, intense rainfall from tropical storms, and drought. Their repeated occurrence and wide extent are consistent with projections of increased frequency and intensity of ECEs, and have broad implications elsewhere because similar trends are predicted globally. The unprecedented and widespread nature of these ECE impacts has likely produced substantial ecosystem-wide repercussions. Predictions from ecosystem models suggest that the widespread mortality of habitat-forming taxa will have long-term and in some cases irreversible consequences, especially if they continue to become more frequent or severe. The abrupt ecological changes that are caused by ECEs could have greater long-term impacts than slower warming that leads to gradual reorganisation and possible evolution and adaptation. ECEs are an emerging threat to marine ecosystems, and will require better seasonal prediction and mitigation strategies.

The short spined crown-of-thorns starfish Acanthaster brevispinus inhabits deeper water soft bottom habitats, in contrast to the more infamous Indo-Pacific Acanthaster species complex of which population outbreaks have been responsible for widespread coral mortality throughout the Indo-West Pacific region. Acanthaster brevispinus has not previously been regarded as a threat to corals or coral reefs. Here, we report A. brevispinus occurring on mesophotic off-reef or inter-reef habitats in 20–70-m depths dominated by solitary corals off both the western and eastern coasts of Australia. On Ningaloo Reef, A. brevispinus were found on mushroom coral (Cycloseris distorta) beds using an underwater towed camera and further exploration using a remotely operated vehicle (ROV) confirmed predation by A. brevispinus on C. distorta. On the southern Great Barrier Reef, A. brevispinus in large numbers were found in habitat dominated by the dendrophylliid Heteropsammia cf. cochlea. Predation on H. cf. cochlea was also directly observed. This is the first confirmed report of predation on hard corals by A. brevispinus, and while there are yet to be any records of population outbreaks of this species, such outbreaks could have a significant effect on mesophotic solitary coral assemblages.

The crown-of-thorns starfish Acanthaster planci (COTS) has contributed greatly to declines in coral cover on Australia's Great Barrier Reef, and remains one of the major acute disturbances on Indo-Pacific coral reefs. Despite uncertainty about the underlying causes of outbreaks and the management responses that might address them, few studies have critically and directly compared competing hypotheses. This study uses qualitative modelling to compare hypotheses relating to outbreak initiation, explicitly considering the potential role of positive feedbacks, elevated nutrients, and removal of starfish predators by fishing. When nutrients and fishing are considered in isolation, the models indicate that a range of alternative hypotheses are capable of explaining outbreak initiation with similar levels of certainty. The models also suggest that outbreaks may be caused by multiple factors operating simultaneously, rather than by single proximal causes. As the complexity and realism of the models increased, the certainty of outcomes decreased, but key areas that require further research to improve the structure of the models were identified. Nutrient additions were likely to result in outbreaks only when COTS larvae alone benefitted from nutrients. Similarly, the effects of fishing on the decline of corals depended on the complexity of interactions among several categories of fishes. Our work suggests that management approaches which seek to be robust to model structure uncertainty should allow for multiple potential causes of outbreaks. Monitoring programs can provide tests of alternative potential causes of outbreaks if they specifically monitor all key taxa at reefs that are exposed to appropriate combinations of potential causal factors.

There is currently very limited information around the spatial patterns of coral recruitment at mesophotic depths globally. This study investigated depth-related differences in coral recruitment patterns from shallow (~ 3 m) to mesophotic depths (~ 40 m) in the Indian Ocean. A new method is described for assessing coral recruitment, which allows for the improved study of recruitment patterns on deep reefs globally, as the method does not require SCUBA diving. This method allows for comparisons with other studies as there appears to be no influence on the density, composition, or settlement orientation of recruits relative to the most commonly used methods. Using this method, we investigated coral recruitment at Ningaloo Reef, Western Australia, finding the abundance of coral recruits varied significantly with depth and was highest at 25 m. The size of coral recruits changed significantly with depth, with larger recruits observed in shallower areas (3 and 8 m) than in deep areas. Distinct changes in settlement densities on tile surfaces occurred with increasing depth, with a shift to upper tile surfaces between 8 and 25 m, where the proportion of recruits increased from 10.72 to 87.69%, respectively. Overall counts of recruits were low, with minimal recruitment at the deepest sites and moderate but significant correlations between recruit numbers and hard coral cover were observed. This suggests that variations in larval supply, potentially coupled with larval behaviour and local-scale influences, limit exchange of larvae between depths and locations. This is consistent with genetic studies that show limited exchange between shallow and mesophotic reefs and points to a limited potential for mesophotic reefs to act as a source of larvae for impacted shallow reefs.