Coral Reef Ecology Lecture 13 - Beyond Coral Loss: Further Indicators of Reef Degradation

Coral Reef Ecology Lecture 13 - Beyond Coral Loss: Further Indicators of Reef Degradation

Further Reading

  • Prescribed Reading

    • Rogers A, Blanchard JL, Mumby PJ (2017). "Fisheries productivity under progressive coral reef degradation." Journal of Applied Ecology, 55: 1041-1049.

    • Cornwall CE et al. (2021). "Global declines in coral reef calcium carbonate production under ocean acidification and warming." PNAS, 118: e2015265118.

    • Pisapia C, Edmunds PJ, Moeller HV, Riegl BM, McWilliam M, Wells CD, Pratchett MS (2020). "Projected shifts in coral size structure in the Anthropocene." Advances in Marine Biology, 87: 31-60.

    • Tebbett SB, Connolly SR, Bellwood DR (2023). "Benthic composition changes on coral reefs at global scales." Nature Ecology & Evolution, 7: 71-81.

Main Topics Discussed

Coral Community and Population Changes

  • Diversity: Changes in coral community structure, species diversity, and colony size.

  • Loss of Structural Complexity: Impact on habitat and the ecosystem functions of coral reefs.

  • Declining Coral Biodiversity: Examination of the factors affecting coral biodiversity.

  • Declining Calcium Carbonate Production: Effects of ocean acidification and its long-term implications.

  • What Replaces Corals?: Exploration of macroalgae, algal turfs, and rubble and their influence on reef ecosystems.

Data Representation

  • Coral Cover (%) Over Time: A study by Pratchett (unpublished data) tracks coral cover from the 1960s to 2010s.

  • Sustained Coral Loss Impact: Consideration of sustained coral loss on a reef system.

Declining Measurements from Coral Loss

  • Declines in Coral Cover

    • Coral diversity (not richness)

    • Coral size

    • Structural complexity

    • Overall biodiversity

    • Calcium carbonate production (not accretion)

Coral Replacement Considerations

  • Potential replacements for corals include:

    • Macroalgae

    • Algal Turfs

    • Rubble

Species Richness vs. Species Diversity

  • Species Richness: The number of species within a given assemblage.

  • Species Diversity: Considers both richness and evenness; assemblages dominated by few species are less diverse, measured by Simpson’s diversity index.

  • Marked reductions in the abundance of many species, especially in the Caribbean.

Coral Size and Population Structure

  • Size Declines: Increased mortality or reduced growth and survival contribute to decreased coral size.

  • Consequences for extinction risk, reef function, and resiliency are discussed.

  • Notable declines in coral size structure coinciding with changes in coral species.

  • Most research primarily focuses on cover rather than the population structure of corals.

Structural Complexity Metrics

  • Definition: Structural complexity refers to the heterogeneity in the three-dimensional (3D) physical structure of coral reef habitats.

  • Rugosity: Defined as small-scale variation in topography; can be quantified as a ratio of horizontal distance to surface distance (Risk 1972).

  • Advanced measurement using 3D landscape photogrammetry models to derive additional metrics such as factual dimension and vector dispersion.

Recovery and Disturbances

  • Types of Disturbances:

    • Biological Disturbances: E.g., Crown-of-Thorns Starfish (COTS) and bleaching which cause coral mortality but not directly affect structural integrity.

    • Physical Disturbances: E.g., cyclones that directly damage corals, leading to immediate declines in structural complexity.

    • Natural decomposition occurs post-disturbance, often leading to collapse within 3-5 years.

  • Illustrative time series of habitat structure changes following mass-bleaching events (Pratchett et al. 2008).

Coral-Algal Phase Shifts

  • Definition of Phase Shifts: Marked changes in ecosystem state caused by persistent environmental changes. Often observed after acute disturbances.

  • Coral-Algal Phase Shift Dynamics: Examining shifts to macroalgal dominance as an alternate stable state, raising questions about recovery potential once disturbances cease.

  • Historical accounts of shifts on Caribbean reefs and debates surrounding community re-assembly, particularly in relation to species like Diadema.

Projected Future Implications

  • Predictions for coral reef accretion rates, focusing on:

    • Expected accretion rates declining to ≤0 in most reefs with current warming rates (Cornwall et al. 2013).

    • Pacific reefs may maintain carbonate production by 2050 with moderate carbon emission reductions.

  • Impacts of degradation on reef productivity and ecology, particularly with coral-algal shifts in mind.

Overall Biodiversity Considerations

  • Quantifying Biodiversity: Challenges in assessing true biodiversity metrics beyond fish and readily distinguishable corals.

  • Questions regarding loss of species and biodiversity among less described organisms.

  • Current research calls for standardized sampling and quantifying distinct operational taxonomic units (OTUs).

  • Discussions surrounding evidence of diversity loss or increases amongst certain assemblages, reflecting ongoing debates in reef ecology.