Rebuilding coral reefs: success (and failure) 16 years after low‐cost, low‐tech restoration

Ecosystem restoration: Healing blasts from the past

Blast fishing reduces coral reefs to fields of rubble. A new study of a project to restore blast-fished reefs reveals rapid recovery of reef carbonate budgets and reef health but highlights that further work is needed to restore coral biodiversity.

Shifting reef restoration focus from coral survivorship to biodiversity using Reef Carpets

To enhance the practice of farmed-coral transplantation, we conducted a trial of an approach called "Reef Carpets" (RC), which draws inspiration from the commercial turf-grass sod in land-based lawn gardening. Three 8.4m2 RCs were established on a sandy seabed, containing preselected combinations of branching corals (Acropora cf. variabilis, Pocillopora damicornis, Stylophora pistillata) with nursery recruited dwellers, and were monitored for 17-months. Corals within RCs grew, supported coral recruitment and offered ecological habitats for coral-associated organisms. While the unstable sediment underneath the RCs increased corals' partial mortalities, corals managed to grow and propagate. The extent of fish and gastropods corallivory varied among the coral species and planulation of Stylophora transplants was significantly higher than same-size natal-colonies. The RCs provided conducive environments for fish/invertebrate communities (183 taxa), and each coral species influenced specifically species-diversity and reef-associated communities. Even dead corals played crucial roles as habitats for reef biota, sustaining >80% of the RCs diversity; hence, they should not be considered automatically as indicators of failure. RCs scaled-up reef restoration and generated, in short periods, new reefs in denuded zones with enhanced biodiversity. Yet, RCs employment on soft-beds could be improved by using more structured artificial frameworks, requiring further research efforts.

Mapping the susceptibility of reefs to rubble accumulation across the Great Barrier Reef

Disturbance-induced rubble accumulations are described as "killing fields" on coral reefs as coral recruits suffer high post-settlement mortality, creating a bottleneck for reef recovery. The increasing frequency of coral bleaching events, that can generate rubble once coral dies, has heightened concerns that rubble beds will become more widespread and persistent. But we currently lack the tools to predict where rubble is most likely to accumulate. Here, we developed a modelling framework to identify areas that are likely to accumulate rubble on forereef slopes across the Great Barrier Reef. The algorithm uses new high-resolution bathymetric and geomorphic datasets from satellite remote sensing. We found that 47 km of reef slope (3% of the entire reef surveyed), primarily in the southern region, could potentially reach 50% rubble cover. Despite being statistically significant (p < 0.001), the effects of depth and aspect on rubble cover were minimal, with a 0.2% difference in rubble cover between deeper and shallower regions, as well as a maximum difference of 0.8% among slopes facing various directions. Therefore, we conclude that the effects of depth and aspect were insufficient to influence ecological processes such as larval recruitment and recovery in different coral communities. Maps of potential rubble accumulation can be used to prioritise surveys and potential restoration, particularly after major disturbances have occurred.

Coral reef benthic community changes in the Anthropocene: Biogeographic heterogeneity, overlooked configurations, and methodology

Non-random community changes are becoming more frequent in many ecosystems. In coral reefs, changes towards communities dominated by other than hard corals are increasing in frequency, with severe impacts on ecosystem functioning and provision of ecosystem services. Although new research suggests that a variety of alternative communities (i.e. not dominated by hard corals) exist, knowledge on the global diversity and functioning of alternative coral reef benthic communities, especially those not dominated by algae, remains scattered. In this systematic review and meta-analysis of 523 articles, we analyse the different coral reef benthic community changes reported to date and discuss the advantages and limitations of the methods used to study these changes. Furthermore, we used field cover data (1116 reefs from the ReefCheck database) to explore the biogeographic and latitudinal patterns in dominant benthic organisms. We found a mismatch between literature focus on coral-algal changes (over half of the studies analysed) and observed global natural patterns. We identified strong biogeographic patterns, with the largest and most biodiverse biogeographic regions (Western and Central Indo-Pacific) presenting previously overlooked soft-coral-dominated communities as the most abundant alternative community. Finally, we discuss the potential biases associated with methods that overlook ecologically important cryptobenthic communities and the potential of new technological advances in improving monitoring efforts. As coral reef communities inevitably and swiftly change under changing ocean conditions, there is an urgent need to better understand the distribution, dynamics as well as the ecological and societal impacts of these new communities.

Regime shifts on tropical coral reef ecosystems: future trajectories to animal-dominated states in response to anthropogenic stressors

Despite the global focus on the occurrence of regime shifts on shallow-water tropical coral reefs over the last two decades, most of this research continues to focus on changes to algal-dominated states. Here, we review recent reports (in approximately the last decade) of regime shifts to states dominated by animal groups other than zooxanthellate Scleractinian corals. We found that while there have been new reports of regime shifts to reefs dominated by Ascidacea, Porifera, Octocorallia, Zoantharia, Actiniaria and azooxanthellate Scleractinian corals, some of these changes occurred many decades ago, but have only just been reported in the literature. In most cases, these reports are over small to medium spatial scales (<4 × 104 m2 and 4 × 104 to 2 × 106 m2, respectively). Importantly, from the few studies where we were able to collect information on the persistence of the regime shifts, we determined that these non-scleractinian states are generally unstable, with further changes since the original regime shift. However, these changes were not generally back to coral dominance. While there has been some research to understand how sponge- and octocoral-dominated systems may function, there is still limited information on what ecosystem services have been disrupted or lost as a result of these shifts. Given that many coral reefs across the world are on the edge of tipping points due to increasing anthropogenic stress, we urgently need to understand the consequences of non-algal coral reef regime shifts.

Optimizing coral reef recovery with context-specific management actions at prioritized reefs

Assisting the natural recovery of coral reefs through local management actions is needed in response to increasing ecosystem disturbances in the Anthropocene. There is growing evidence that commonly used resilience-based passive management approaches may not be sufficient to maintain coral reef key functions. We synthesize and discuss advances in coral reef recovery research, and its application to coral reef conservation and restoration practices. We then present a framework to guide the decision-making of reef managers, scientists and other stakeholders, to best support reef recovery after a disturbance. The overall aim of this management framework is to catalyse reef recovery, to minimize recovery times, and to limit the need for ongoing management interventions into the future. Our framework includes two main stages: first, a prioritization method for assessment following a large-scale disturbance, which is based on a reef's social-ecological values, and on a classification of the likelihood of recovery or succession resulting in degraded, novel, hybrid or historical states. Second, a flow chart to assist with determining management actions for highly valued reefs. Potential actions are chosen based on the ecological attributes of the disturbed reef, defined during ecological assessments. Depending on the context, management actions may include (1) substrata rehabilitation actions to facilitate natural coral recruitment, (2) repopulating actions using active restoration techniques, (3) resilience-based management actions and (4) monitoring coral recruitment and growth to assess the effectiveness of management interventions. We illustrate the proposed decision framework with a case study of typhoon-damaged eastern outer reefs in Palau, Micronesia. The decisions made following this framework lead to the conclusion that some reefs may not return to their historical state for many decades. However, if motivation and funds are available, new management approaches can be explored to assist coral reefs at valued locations to return to a functional state providing key ecosystem services.

Synergies between local and climate-driven impacts on coral reefs in the Tropical Pacific: A review of issues and adaptation opportunities

Coral reefs in the tropical Pacific region are exposed to a range of anthropogenic local pressures. Climate change is exacerbating local impacts, causing unprecedented declines in coral reef habitats and bringing negative socio-economic consequences to Pacific communities who depend heavily on coral reefs for food, income and livelihoods. Continued increases in greenhouse gas emissions will drive future climate change, which will accelerate coral reef degradation. Traditional systems of resource governance in Pacific island nations provide a foundation to address local pressures and build reef resilience to climate change. Management and adaptation options should build on the regional diversity of governance systems and traditional knowledge to support community-based initiatives and cross-sectoral cooperation to address local pressures and minimize climate change impacts. Such an inclusive approach will offer enhanced opportunities to develop and implement transformative adaptation solutions, particularly in remote and regional areas where centralized management does not extend.

Relative roles of biological and physical processes influencing coral recruitment during the lag phase of reef community recovery

Following disturbances, corals recolonize space through the process of recruitment consisting of the three phases of propagule supply, settlement, and post-settlement survival. Yet, each phase is influenced by biophysical factors, leading to recruitment success variability through space. To resolve the relative contributions of biophysical factors on coral recruitment, the recovery of a 150 km long coral reefs in Palau was investigated after severe typhoon disturbances. Overall, we found that benthic organisms had a relatively weak interactive influence on larval settlement rates at the scale of individual tiles, with negative effects mainly exerted from high wave exposure for Acropora corals. In contrast, juvenile coral densities were well predicted by biophysical drivers, through both direct and indirect pathways. High densities of Acropora and Poritidae juveniles were directly explained by the availability of substrata free from space competitors. Juvenile Montipora were found in higher densities where coralline algae coverage was high, which occurred at reefs with high wave exposure, while high densities of juvenile Pocilloporidae occurred on structurally complex reefs with high biomass of bioeroder fish. Our findings demonstrate that strengths of biophysical interactions were taxon-specific and had cascading effects on coral recruitment, which need consideration for predicting reef recovery and conservation strategies.

Sunken Worlds: The Past and Future of Human-Made Reefs in Marine Conservation

Structures submerged in the sea by humans over millennia provide hard and longstanding evidence of anthropogenic influence in the marine environment. Many of these human-made reefs (HMRs) may provide opportunities for conservation despite having been created for different purposes such as fishing or tourism. In the middle of controversy around the costs and benefits of HMRs, a broad analysis of biodiversity and social values is necessary to assess conservation potential. This requires reframing HMRs as social–ecological systems, moving beyond comparisons with natural coral or rocky reefs to consider their roles as ecosystems in their own right; creating frameworks to track their type, number, size, units, location, characteristics, origins, social uses, and associated biodiversity locally and worldwide; and applying systematic assessment of conservation benefits in relation to stated conservation intentions. This integrative approach can catalyze learning, identify conservation opportunities, and inform positive management of HMRs into the future.

The Role of Maximum Shelf Depth versus Distance from Shore in Explaining a Diversity Gradient of Mushroom Corals (Fungiidae) off Jakarta

Many coral reef systems are shelf-based and consist of reefs that are arranged in rows parallel to the coastline. They usually show an increase in species richness in the offshore direction, coinciding with decreasing terrigenous impact and a deeper seafloor. These two conditions usually concur, which makes it less easy to distinguish how each of them influences coral diversity separately. Since reefs off Jakarta (in the Thousand Islands archipelago) are arranged in an 80 km long string perpendicular to the coastline in south-to-north direction, with a maximum shelf depth halfway along (instead of at the end of) the string, this archipelago is very suitable for studies on inshore–offshore gradients. In the present study, mushroom corals (Fungiidae; n = 31) were used to examine diversity patterns on 38 reef sites along such a gradient, involving species richness over their entire depth range from reef flat to reef base (2–30 m) and separately at shallow depths (2–6 m). Total species diversity was highest in the central part of the archipelago, with unique species occurring in deep habitats. Diversity at shallow depths was only slightly higher here than at reefs located more nearshore and offshore, which both had less clear water. Therefore, shelf depth and distance from the mainland can be considered separate determinants of coral diversity off Jakarta.