Coral Reef Ecology Lecture 15 - Constraints to Coral Reef Recovery

Coral Reef Ecology Lecture 15 - Constraints to Coral Reef Recovery

  • Lecture Outline

    • Coral recovery vs community reassembly

    • Coral recovery rates

      • Causes of variation

      • Major constraints

    • Further Reading

      • Emslie MJ, Ceccarelli DM, Logan M, et al. (2024) Changing dynamics of Great Barrier Reef hard coral cover in the Anthropocene. Coral Reefs. 2024 43(3):747-62.

      • Emslie, M.J., Ceccarelli, D.M., Logan, M. et al. (2025). Anthropogenic climate change causes substantial loss of coral on the northern Great Barrier Reef during the 2024 bleaching event. Coral Reefs – online only.

      • Gilmour JP, Smith LD, Heyward AJ, Baird AH, Pratchett MS (2013) Recovery of an isolated coral reef system following severe disturbance. Science 340(6128):69-71.

      • Graham NA, Nash KL, Kool JT (2011) Coral reef recovery dynamics in a changing world. Coral Reefs 30:283-294.

Population Resilience

  • Definition of Ecological Resilience

    • Ecological resilience refers to the capacity of a population, community, or ecosystem to withstand disturbances without changes in fundamental structure or function (sensu Holling 1973).

    • Forms of Resilience

      • Resistance: Characterized as having no apparent effects from disturbances.

      • Recovery: Characterized by the ability to return to the pre-disturbance state.

  • Population Dynamics

    • Capacity Observations:

      • Resilience for populations is observable when there is no change in carrying capacity (equilibrium abundance).

      • Resistance accounts for changes in population size due to disturbance.

      • Recovery indicates the tendency for the population to bounce back to its pre-disturbance numbers.

      • Resilience can only be directly assessed through population size monitoring devoid of ongoing disturbances.

      • If demographic rates (recruitment, growth, mortality) remain unchanged, populations are presumed to return to equilibrium abundance.

    • For corals affected by major disturbances, resilience closely aligns with recovery in rate or capacity

  • Time Dynamics Representation:

    • Variables: Time, Population Size, Disturbance, Resistance

    • Carrying Capacity, K

Community Resilience

  • Definition of Ecological Resilience

    • Similar to that of population resilience concerning the ability to withstand disturbances without altering fundamental structure or function.

      • Can manifest as stability (resistance) or re-assembly (recovery) in community structure.

  • Community Dynamics

    • Emergent Trends: After disturbances, coral assemblages may initially be dominated by opportunistic "weedy" coral species (e.g., Pocillopora).

    • Need to assess if this represents a successional phase in recovery or if it's sustained by ongoing disturbances.

    • Recovery may be guided by a select few taxa, potentially resulting in recovery without overall community re-assembly.

Recovery Capacity and Factors

  • Factors Supporting Coral Recovery

    • Instances exist where coral reefs demonstrate significant recovery and community reassembly after major disturbances (Gilmour et al. 2013).

    • Promoting Factors Noted:

      • Limited exposure to chronic anthropogenic pressures (e.g., coastal runoff) promotes resilience at remote reef locations.

      • Conversely, such remote locations may also face acute disturbances due to reduced connectivity with other reefs.

Coral Recovery Observations

  • Data Sources on Recovery

    • Data from Australian Institute of Marine Science (AIMS) indicates notable coral recovery from 2016-2024, particularly in the northern Great Barrier Reef, mainly driven by rapid proliferation of Acropora spp.

    • The recovery process highlights a significant vulnerability to future disturbances caused by shifts in coral assemblages and lack of community re-assembly.

  • Monitoring Methods:

    • AIMS Long-Term Monitoring Program (LTMP) employs MANTA-tow surveys recording multiple metrics

      • i) Coral Cover (11 categories, nearest 5-10%)

      • ii) Number of Crown-of-Thorns Starfish (CoTS)

      • iii) CoTS feeding scars

      • iv) Number of coral trout

      • v) Coral bleaching (6 categories, nearest 5-15%)

    • Note limitations exist in testing community reassembly through these methods.

  • Coral Cover Trends:

    • Data indicates a decline of 40% in coral cover on northern GBR during 2024, particularly where Acropora spp. showed high initial coverage (Emslie et al. 2025).

Coral Community Shifts

  • Community Changes: Analysis of coral cover proportions across different species based on unpublished data from Pratchett shows significant fluctuation in community composition.

  • Taxa Represented: Pocillopora, Acropora, Montipora, Porites, and others quantified across varying times, emphasizing shifts in hard coral cover.

Coral Recovery Rates

  • Variables Influencing Recovery

    • Coral recovery rates (Ortiz et al. 2018) depend on both intrinsic and extrinsic factors:

      • Intrinsic Factors: Demographic rates of corals.

      • Extrinsic Factors: Availability and suitability of substrate.

  • Annual Rate of Change Observations:

    • Following extensive coral loss, intrinsic constraints indicate slow annual rates of coral cover recovery ~$1 ext{%} ext{ p.a.}$ while higher coral cover situations yield equally slow recovery attributed to extrinsic limits, including competition and space constraints.

  • Average Coral Recovery Rate:

    • Observed across 22 studies from 48 different locations, average recovery rate is $3.56 ext{%}$ per annum (95% CI = 2.89 – 4.43) following acute disturbances >10% decline in coral cover (Graham et al. 2011).

    • Key variations correlate to geographic locations and management approaches.

    • Recovery rates significantly ameliorate when coral cover is between 6-10%, as opposed to above 20%, indicating a non-linear relationship based on coral density and stress.

Recovery Processes and Influences

  • Coral Growth Dynamics:

    • Recovery rates are significantly dictated by the growth of surviving corals in cases of moderate loss (Pratchett et al. 2015).

    • Rapid growth expected from branching and tabular coral species, surpassing massive coral growth rates by tenfold.

    • Re-sheeting is a rare phenomenon allowing rapid recovery under specific conditions but is primarily confined to Acropora spp.

  • Process After Extensive Loss:

    • Recovery rate subsequently relies on larval recruitment and growth/survival of new recruits.

    • Severe disturbances may limit further recruitment and recovery due to declines in reproductive outcomes of surviving corals.

  • Factors Affecting Recruitment Rates:

    • Sustained declines in coral replenishment rates identified via meta-analysis (Edmunds 2023) are notably observed across Pacific and Caribbean regions, despite absolute higher rates in the Pacific.

  • Mechanisms of Recruitment Limitation:

    • Breakdown into various factors impacting recruitment capacity, including:

      • Supply-Side Limitation: Limited larval supply and reproductive viability.

      • Settlement Constraints: Insufficient suitable habitat for settlement and cues.

      • Post-Settlement Mortality: Challenges faced by newly settled larvae reflecting environmental quality and habitat suitability.

Historical Context and Implications

  • Major Historical Establishments:

    • References to critical die-offs (1983 Diadema die-off, 1980 Hurricane Allen) and subsequent studies on coral-algal shifts in places like Discovery Bay, Jamaica highlight pertinent phase shifts in coral dynamics.

  • Understanding Phase Shifts:

    • Distinction noted that while phase shifts mark significant ecological changes, they may not inherently represent alterations in self-organizing processes.

Experimental Observations in GBR

  • Study Design:

    • Inshore GBR herbivore exclusion experiment, using control and cage plots to ascertain herbivorous fish regulation of macroalgal cover.

  • Results of Herbivore Exclusion:

    • Enhanced macroalgal growth observed in caged areas relative to control plots affirms fish role in controlling algal biomass.

Macroalgae Impact Dynamics

  • Coral and Macroalgal Interaction:

    • Elevated macroalgal biomass exhibited a negative correlation with coral resilience by impeding reproductive and recruitment capabilities in corals.

Summary Key Points

  • Resilience exists in many coral reef systems, with average recovery rates around $3.56 ext{% p.a.}$ (Graham et al. 2011).

  • Regional differences noted, with higher recovery rates generally present in western Pacific (including GBR) versus Caribbean, albeit with notable outliers.

  • Recovery dynamics can shift rapidly, particularly in species like Acropora, and are influenced by structural population and community attributes.

  • Recruitment limitations represent increasing constraints on coral recovery, echoing the prevalent need for habitat restoration and environmental management strategies.

*Individual studies suggest that recovery may be quicker (5-6 years; Emslie et al. 2024) driven by fast-growing species as primary recovery agents. Coral recovery limitations relate to ecological viability within local contexts.