8. Resilience

Coral reefs are among Earth’s most diverse, complex, productive, and geologically persistent ecosystems, ²⁰²⁹ but the stability of reef communities varies greatly across temporal and spatial scales.²⁰³⁰ Species composition appears to be more stable over centuries and millennia than it is over days to decades.^2030,2031 At these shorter ecological timescales, reef communities often represent transient states, shaped by the dynamic interplay of ecological interactions and physical disturbances.²⁰³² Disturbance can be a key enabler of high biodiversity in coral reef communities, but the relationship between disturbance and coral reef communities is more complex than is often recognised.²⁰³³ There is increasing evidence that human-driven environmental changes are pushing coral reefs towards states that are less biologically and functionally diverse, including in the context of past changes over thousands and millions of years.^2031,2034 

Coral reef ecosystem functioning is underpinned by interactions among numerous organisms and four key ecological processes: habitat creation and loss, photosynthetically derived biomass production, trophic structuring and nutrient cycling.⁸⁰⁵ Habitat is created by calcium carbonate production and removed by bioerosion. Biomass from photosynthesis is produced through primary production and assimilated through herbivory. Trophic structure is developed through different levels of carnivory and maintained by growth of prey. Nutrients are introduced, retained and re-integrated through the system. Chapter 3 includes descriptions of the Reef’s ecological and chemical processes. Together these processes combine with reef-specific biodiversity and community composition to define a reef’s identity (Figure 8.1) and, correspondingly, its thresholds, scales, cycles, feedback processes, and domains of attraction.²⁰¹⁹ How these relationships are maintained are key aspects of the resilience of coral reef habitats. Domains of attraction is a term for describing conditions conducive to return of a system to a reference state after perturbation and is relevant to robustness.²⁰¹⁶

Management

The multiple well-established and effective management tools available to protect the Region’s coral reef habitats can be grouped under three approaches: environmental regulation and legislation; engagement with reef users; and integration of evidence-based knowledge into resilience-based management approaches (Section 7.4). 

Relevant legislation includes the Great Barrier Reef Marine Park Act 1975 (Cth) (Marine Park Act) as the primary legislation related to the Marine Park, and the Great Barrier Reef Marine Park Regulations 2019 (Cth), the Great Barrier Reef Marine Park Zoning Plan 2003 (Cth) and the complementary Queensland zoning plan, and several plans of management.^{1175,1176,1178,2035} Special management areas are designed to manage access and use of a specific area in addition to zoning. The Reef Authority develops and implements policies that give effect to the agency’s responsibilities, functions and powers outlined in the Marine Park Act and is responsible for granting permissions for activities in the Marine Park.

The Reef Authority, Queensland Government agencies and other Australian Government agencies work together to conduct management activities in the Region. Conducting compliance actions is a key feature of this work. Such actions include educating Reef users and assisting self-regulation; aircraft, vessel, and land-based surveillance; tracking remote vessels; and auditing of activities that require a permit. On-ground interventions, such as the effective Crown-of-thorns Starfish Control Program (Box 8.1), can contribute to the resilience of coral-reef habitats in the Region.

Traditional Owner agreements, such as through the accreditation of a Traditional Use of Marine Resources Agreement, are an example of a pathway to co-management to enhance the effectiveness of the Marine Park management. More broadly, engagement with Reef users often focuses on promoting stewardship, both directly by supporting partners and stakeholders (Section 8.5.1) and indirectly, through education and by encouraging partners and stakeholders to engage with and promote stewardship actions themselves.

Effective adaptive management decisions should be based on the best available evidence-based science, traditional knowledge, and information from the wider community through citizen science programs such as Eye on the Reef. In addition to foundational management programs, new initiatives are being explored to support resilience in coral reef habitats through the Reef Restoration and Adaptation Program (Box 5.4). Tools to support decision-making are being developed within the Reef 2050 Integrated Monitoring and Reporting Program (Box 10.1). An adaptive management approach must also support continual improvement, including evolution of policy and strategic management documents, such as the Great Barrier Reef Blueprint for Climate Resilience and Adaptation (Blueprint 2030).¹⁹⁸⁴

Only the strongest and fastest possible actions to decrease global greenhouse gas emissions will reduce the risks from and limit the impacts of climate change on coral reef habitats. Effective global action on climate change will greatly increase the effectiveness and positive impact of existing management actions in the Catchment and the Region.

Evidence for recovery or decline

Over the past decade (to the end of 2023), the largest ecological impact on the Reef has been four mass coral bleaching events linked to global warming and the El Niño–Southern Oscillation, two of which occurred in the past 5 years.^311,2037 A sixth widespread bleaching event was detected in early 2024.¹⁶⁷ The Reef has continued to be affected by cumulative pressures arising from the chronic effects of declining water quality ¹⁷³⁴ and major large-scale disturbances, such as cyclones and periods of elevated sea temperatures (Figure 8.3), and crown-of-thorns starfish outbreaks.^{1888,2038,2039}

The existence of large-scale variability in the exposure of the Region’s coral reef habitats to stress and their ability to support recovery after acute disturbances,¹⁶¹ highlights the existence of systemic resilience.²⁰⁴⁰ Reassembly of the coral community, even in novel compositions, is the basis of a dynamic and resilient system.²⁰¹⁹ However, the strength of the system’s resilience is less certain — how much more disturbance can the Reef’s ecosystem absorb without significantly shifting its identity away from being a coral reef system. The recovery window between disturbances is expected to shrink in the near future because of the intensification and increased frequency of anthropogenic impacts.^573,2041

In recent years, recovery of coral reef habitats, reported as increases in hard coral cover, has been detected on some inshore reefs ¹⁸⁵ and documented for many offshore reefs across the Region.¹⁶¹ Coral cover on inshore reefs remains relatively low where chronic environmental pressures are likely exacerbating the effects of acute disturbances and limiting the potential for recovery of coral reef communities.^158,185,2028 Coral cover on many shallow offshore reef slopes has reached its highest levels since the beginning of monitoring.¹⁶¹ Increases in coral cover are driven primarily by fast-growing species, such as branching and tabular Acropora,¹⁶¹ which are also sensitive to disturbances.

The use of coral cover as the primary indicator of resilience is not ideal ^174,573 as it misses crucial information about recruitment, colony size, biodiversity and community composition.^{174,2045,2046} These components underpin the function and resilience of coral reef habitats. Increases in coral cover may mask ‘recovery debts’ when the ecosystem fails to reassemble to a similar configuration of species and functions.¹⁷⁴ The extent and size of this deficit may be determined by how the functions of resistant or rapidly recovering species compare to the original set of functions before impact and degradation.²⁰⁴⁵

Lizard Island (Jiigurru) in the northern part of the Reef, 270 kilometres northeast of Cairns, has felt the effects of several large-scale disturbances in the last decade. These include cyclones Ita (2014) and Nathan (2015)¹⁶⁴³ and thermally induced coral bleaching in 2016, 2017 and 2020.^{311,1604,2039,2047}  Ocean acidification also poses an ongoing threat.²⁰⁴⁸ The response of the coral reef habitats at Lizard Island to these disturbances demonstrates the complexity of resilience and recovery processes at a local scale.  

In April 2014, cyclone Ita crossed the island from the north as a category 4 cyclone. Reefs on the north of the island were severely affected, and coral cover declined by 85 per cent at North Reef, though there was little detectable change elsewhere around Lizard Island.¹⁶⁴³ The fish community showed similar substantial site-specific abundance changes and species-level turnover.³⁷²

In March 2015, cyclone Nathan crossed Lizard Island as a category 4 cyclone and affected reefs through waves generated from the south. In contrast to cyclone Ita, south-facing reefs suffered major declines in coral cover by up to 90 per cent at Trimodal Reef between South and Palfrey islands.^1643,1700 No further change in coral cover was detected at North Reef. Loss of coral cover was accompanied by reduction in abundances of branching coral (Acropora, Pocillopora, and Stylophora), but massive ²⁰⁴⁹ corals (such as Porites) were relatively unaffected.¹⁷⁰⁰ Some coral species also experienced sublethal effects such as a reduced reproductive capacity, probably due to energy being diverted towards processes such as injury repair.¹⁷⁰⁰ Some individuals of large surgeon fish were affected as their coral shelter was lost in cyclone-affected areas.²⁰⁵⁰ At the island scale, the decline in cover was particularly notable as it approximately halved. However, impacts from back-to-back cyclones were patchy compared to the more uniform loss of cover and species diversity following the subsequent bleaching events in 2016 and 2017.

In April 2016, the largest mass bleaching event on record affected the Reef. Estimates of bleaching and loss of coral cover on northern reefs were more than 50 per cent in many locations ³¹¹ and around 80 per cent for Lizard Island.^1643,2039 Coral communities experienced bleaching again in 2017. This sequence of disturbances drastically changed the assemblage structure of reefs around the island compared to data from earlier in the 2000s and the mid-1990s.¹⁶⁴³ For example, loss of coral cover was accompanied by declines in genus and species richness with the total number of species across surveyed sites decreasing from 76 in 2011 to 49 in 2017. Only 28 of the coral species recorded in 2011 were recorded again in 2017.¹⁶⁴³ 

Coral bleaching was widespread across surveyed sites; however, bleaching severity in adult colonies was often species-specific and habitat-dependent.^2051,2052 In general, Acropora corals showed a high susceptibility to thermal stress, including mortality of branching species,¹⁶¹⁷ whereas Porites was often observed to have a degree of resistance to thermal bleaching. Reef-building stony corals Porites, Pocillopora, Stylophora and corals in the Merulinidae family experienced the least significant declines in abundance after these disturbances.^1643,2052 

In 2017, the dominant benthic taxa at Lizard Island were soft corals and species of massive Porites and Merulinidae. Overall, juvenile abundance did not vary significantly after 2016 bleaching; however, susceptibility to bleaching was also taxon-dependent.^53,2052 There were notable increases in the relative coverage of algal turfs and other main functional groups of algae following the decrease in coral cover and species diversity from the 2016 bleaching event.⁸⁷⁴ 

Sites surveyed in 2020 showed a shift in benthic community composition towards a higher relative cover of crustose coralline algae and macroalgae, rather than returning to a higher coral cover state. In 2020, coral cover on some reefs still accounted for less than 20 per cent of total cover.¹⁶⁴³ 

By 2021, significant increases (up to 600 per cent) in coral cover were detected in semi-exposed reefs, such as North Reef and Trimodal Reef compared to 2018. These increases were primarily associated with recruitment of Acropora.⁸⁷⁴ In some sites, Acropora coral recruits were able to grow to coral cover levels of 25 per cent (equivalent to the average coral cover on coral reefs globally²⁰⁵³) within just 2 years (from 2020 to 2022).¹⁷⁴ Acropora corals appear to be responding with a pronounced boom-and-bust pattern, while massive Porites colonies, which were relatively unaffected by repeated bleaching events, exhibit a precarious degree of resilience, increasing in area but with an underlying recruitment deficit.¹⁰⁷²

A recent study showed that more than 75 per cent of the coral species recorded at sites around Lizard Island showed persistence through time despite the local disturbance history over the last 44 years. At site-level, coral species diversity has been in a state of flux over the 2011–2020 period. Significant declines in species richness were notable from 2011 to 2017 followed by  significant recoveries from 2017 to 2020.¹⁶⁴⁴ While 284 of the previously recorded species appear to have persisted, 28 species that were recorded before 2011 have not been recorded in subsequent years and may be at risk of local extinction.¹⁶⁴⁴ An additional 31 species are under threat of local range reduction as they have not been recorded at Lizard Island or at nearby reefs since 2015.

The 2016 bleaching event also led to changes in fish assemblages. Decreases in fish abundance were primarily driven by declines in coral-associated damselfishes. However, fish abundance increased at several sites, suggesting substantial spatial movements.¹⁶⁵⁰ Some changes in fish community composition across habitats seemed to be uncorrelated with coral loss.^1617,1650 For example, some populations of obligate coral-dependent fishes persisted and continued to recruit ³⁷⁰ despite a significant loss of coral cover and the local depletion of habitat-providing Acropora in 2017.¹⁶⁴³ Reef assemblages that were previously distinct from each other showed increasing functional and taxonomic similarities post-disturbance. There were shifts towards predominance of small-bodied, algal-farming habitat generalists.¹⁶¹⁷ Following disturbances and subsequent reassembly of fish communities, coral settlement and recruitment success were lower where the new fish assemblages contained more herbivorous fish but a lower diversity of foraging traits.²⁰⁵⁴

The resilience of coral reef habitats has been demonstrated at many locations on the Reef through the ability to begin recovery during periods free from intense acute disturbances. However, increases in coral cover do not necessarily represent reassembly of the coral community nor recovery of a reef’s full diversity and function. The evidence from Lizard Island strongly suggests that the four disturbances have changed coral reef community assemblages profoundly. These changes have also directly affected key reef functions, such as ecosystem primary production and net calcification.⁷⁵¹ Acropora corals appear to have made a remarkable recovery at Lizard Island since 2017.²⁷⁷ However, the sensitivity of these corals to bleaching means that these reefs also experience some of the most severe declines during marine heatwaves.¹⁶⁰⁴ 

Deficits in the performance and functioning of assemblages recovering from cumulative disturbances suggest that the return of coral cover alone cannot ensure the reassembly of reef trait diversity, and that shortening intervals between disturbances can limit recovery among functionally important species.²⁰⁴⁵ Therefore, it is important to consider other components of the ecosystems that will likely play a role in making benthic habitats more conducive for coral settlement and recruitment. The variation in fish biodiversity is likely to contribute to spatially uneven patterns of coral recruitment and reef recovery because of the ability of reef fish to shape the reef benthos.²⁰⁵⁴

Coral cover variability over the last decade has included both the lowest and highest levels recorded and may suggest the Reef has entered a period of instability in coral cover succession dynamics.¹⁶² In terms of resilience, a distinction can be made between assemblages that regain their original composition despite recurrent disturbances and those that maintain functional traits despite shifts in species composition (potential adaption).²⁰⁴⁵ The functioning of future coral reefs will be intimately linked to corals and the entire benthic and fish community. Given the increasing scale, frequency and intensity of disturbances on coral reefs, it is important to evaluate the persistence in space and time of corals, benthic organisms and fish species to further understand the impact that modern climate disturbances may be having on biodiversity, reef function and resilience.¹⁶⁴⁴ Indicators to support this evaluation are under development under the Reef 2050 Long-Term Sustainability Plan (Reef 2050 Plan).^2055,2056