Ecological Assessment of Coral

Palmyra Atoll, NOAA photo by Erin Looney.
Palmyra Atoll, NOAA photo by Erin Looney.

Corals are the defining species in one of the most diverse marine ecosystems: coral reefs. The Coral Reef Ecosystem Division (CRED) studies of corals focus on basic questions concerning the distribution, abundance, and condition of corals and coral reefs in selected areas across the tropical and subtropical Pacific under U.S. jurisdiction.

CRED studies address the following questions:

  • What are the diversity and relative abundance of corals?
  • How can the habitats in which corals occur be qualitatively and quantitatively analyzed?
  • What is the spatial distribution of habitats in which corals occur?
  • What is the capacity of corals to replenish populations over space and time?
  • What are appropriate indicators of reef health?
  • Are reefs healthy?
  • How are reefs changing over time?

Several methods are used to address these questions. Any one method can address only a certain subset of questions; no one method can address them all. Methods used by CRED and its partners include benthic towed-diver surveys, Rapid Ecological Assessments (REAs), benthic line-point-intercept (LPI) surveys, and permanent transects. The metrics that CRED uses to answer these questions include percent cover, diversity, species richness, coral condition, and coral demographics. Why are all of these metrics important?

Coral percent cover data are important not only because percent cover is the most widely used metric of coral reef condition but also because changes in cover likely reflect the set of environmental and disturbance regimes that characterize each reef system (Rogers et al. 1994; Jokiel et al. 2005).

Rose Atoll. NOAA photo by Erin Looney.
Rose Atoll. NOAA photo by Erin Looney.

Marine biodiversity is predicted to be indirectly affected by climate change and ocean acidification (Worm et al. 2006; Riebesell 2008) because of the implicit alterations in community structure, synergism among organisms, and functionality and the anticipated increases in species extinctions and invasion (Ives and Carpenter 2007; Cheung et al. 2009). Coral reefs, change greatly in diversity and community composition across spatial and environmental gradients. Although the details of how this variation influences reef resilience and response to climate change and ocean acidification are poorly understood, ecological resilience is generally considered to be enhanced by increased activity.

Over the last three decades, outbreaks of coral bleaching and disease have resulted in global reductions in coral reef diversity and resilience. As such, assessments of coral condition are a metric of coral health and have the potential to identify possible causes of changes in benthic community structure. For a detailed description of the various types of coral lesions and diseases commonly recognized on reefs in the U.S. Pacific and information about their distribution and abundance, please click here.

Kingman Reef. Photo by Erin Looney.
Kingman Reef. Photo by Erin Looney.

Size-class distribution, or coral demographics, is important because it can serve as a telling indicator of disturbance on a reef. For example, a distribution in which an abundance of corals occur within large size classes (>50 cm) suggests a region in which disturbance events have not interfered with the growth of corals over long periods of time. Conversely, a distribution in which the majority of corals occur within small size classes (<10 cm) suggests either a recent, severe disturbance or frequent recurrences of moderate disturbance, after which recruitment and growth have been relatively recent processes. Other factors to consider when interpreting the implications of size-class distribution include species-specific life history strategies and environmental regimes to which corals are exposed (e.g., frequent high-energy waves). Recruitment of juvenile corals also is an important measure obtained from size-class distribution data.

For more information, please contact Bernardo Vargas-Ángel.

References

Cheung WWL, Lam VWY, Sarmiento JL, Kearney R, Watson R, Pauly D
2009. Projecting global marine biodiversity impacts under climate change scenarios. Fish and Fisheries 10(3): 235-251.
Ives AR, Carpenter SR
2007. Stability and diversity of ecosystems. Science 317: 58-62.
Jokiel PL, Rodgers KS, Brown EK, Kenyon JC, Aeby G, Smith WR, Farrell F
2005. Comparison of coral cover measures: comparison of methods used to estimate coral cover in the Hawaiian Islands. Report to NOAA/NOS NWHI Coral Reef Ecosystem Reserve, Honolulu, HI. Available at http://cramp.wcc.hawaii.edu/Downloads/Publications/TR_Methods_Comparison.pdf
Riebesell U
2008. Acid test for marine biodiversity. Nature 454: 46-47.
Rogers C, Garrison G, Grober R, Hillis Z-M, Franke MA
1994. Coral reef monitoring manual for the Caribbean and western Atlantic. National Park Service, Virgin Islands National Park, St. John, Virgin Islands.
Worm B, Barbier EB, Beaumont N, Duffy JE, Folke C, Halperm BS, Jackson JBC, Lotze HK, Micheli F, Palumbi SR, Sala E, Selkoe KA, Stachowicz JJ, Watson R
2006. Impacts of biodiversity loss on ocean ecosystem services. Science 314: 787-790.