Review: Connell JH, Hughes TP, Wallace CC (1997). A 30-year study of coral abundance, recruitment, and disturbance at several scales in space and time. Ecological Monographs, 67(4):461-488.

Feature Paper: Connell JH, Hughes TP, Wallace CC (1997). A 30-year study of coral abundance, recruitment, and disturbance at several scales in space and time. Ecological Monographs, 67(4):461-488.

Author Abstract: Observations over a 30-yr period revealed a considerable degree of natural variation in the abundance of corals on Heron Island, Great Barrier Reef, Queensland, Australia. Cover ranged from 0.1% to 0.80%, with a similar large range in colony density, at several temporal and spatial scales. Much of this variation was due to the type, intensity, and spatial scale of disturbances that occurred. Coral assemblages usually recovered from acute disturbances, both on Heron Island and on other Indo-Pacific reefs. In contrast, corals did not recover from chronic disturbances of either natural or human origins, or from gradual declines. Recovery was slower after acute disturbances that altered the physical environment than after disturbances that simply killed or damaged corals. The space and time scales of declines and recoveries in abundance were much smaller on the wave-exposed side of the reef than on the side protected from storms. Recruitment rates were reduced by preemption of space by corals or macroalgae, and by storms that altered the substratum. Thus, the dynamics of abundance in this coral community can be largely understood through the variation in types and scales of disturbances that occurred, and the processes that took place where disturbances were rare.

Note to Readers: Follow links above for author email, full article text, or the publishing scientific journal. Author notes in my review are in quotes.

Review: Considering that today's Saturday Science Corner Post falls on Christmas Day (in the Western Hemisphere), many people are preparing for a New Year and considering the passage of time. With another year, many feel the need to make various resolutions. Therefore, I thought it appropriate that today we'd discuss a paper covering 30 years of data… plenty to reflect upon!

This paper is even more relevant today than before because in 1998 a global El Niño-Southern Oscillation event killed roughly 18% of corals worldwide (through temperature-induced coral bleaching, which we've discussed in several past Science Corner posts). Because this paper was published in 1997 (based on coral reef surveys between 1962 and 1992), it means that the study provides a baseline for coral health pre-1998 El Niño-induced bleaching. Granted, the study described by the authors was specific to Heron Island on the central Great Barrier Reef, meaning that results may not apply to other reef types in other areas of the world.

However, during the 30-year study period, 17 cyclones (hurricanes / typhoons) struck the reefs meaning that responses of corals and benthos to disturbances were recorded (which may have ramifications for understanding coral recovery following El Niños, which are becoming much more frequent). 

The authors' study methods including replicating quadrats and transects over several reefs at Heron Island, including photographs of quadrats (photoquadrats) for individual census areas for 16 censuses.

All coral colonies and other benthos (zoanthids, algae, etc.) were recorded and individual colonies were tracked as they recruited or died within quadrats. Growth and percentage cover of benthos was recorded, as were percentages of non-living substrates (rock, sand, etc.). Belt transects and line-intercept transects, as well as larger quadrats, supplemented permanent quadrats, some of which were resampled periodically throughout the study period.

The authors also tried to "investigate the reasons that some cyclones were more destructive than others to corals" by using "changes in abundance of corals during those 11 intervals between censuses when cyclones passed within 200 km of Heron Island." 

Recruitment rates of corals was calculated using "sequential sets of photographs of the permanent quadrats, or from maps made at successive censuses of the permanently marked belt transects" at one site.

Individual questions the authors addressed in their paper were:

1. "Are abundance estimates from the permanent quadrats representative of the local habitat?"

2. "Spatial and temporal autocorrelation in abundance among samples."

3. "Long-term patterns of change in abundance of corals."

4. "Acute disturbances that affected only the corals."

5. "Recovery after acute or chronic disturbances that affected only the corals."

6. "Acute disturbances that directly affected both the physical environment and the corals."

7. "Recovery after acute disturbances that affected both the physical environment and the corals."

8. "Effects of gradual changes in the physical environment and the corals, with little or no disturbance."

9. "Differences in effects among the different cyclones."

10. "Patterns of recruitment of corals."

11. "Scales of variation in abundance and recruitment of corals."

12. "Scales of variation in recruitment."

Several important conclusions of the authors were:

1. "Studies at all scales of time and space are necessary to understand the mechanisms that determine the dynamics of coral reefs."

2. "Studies over short time spans complement longer term ones. The questions posed should determine the scales of observation needed to answer them."

3. "The great variations in abundance and recruitment of corals at different scales at Heron reef, and at other reefs as well, underline the need for observational and experimental studies at as many scales as possible, if we are to understand the mechanisms underlying these variations."