This study looks at the potential for isolated coral reef ecosystems to recover after severe disturbance, in this case ocean warming events. The study system that is examined by Gilmour et al. is Scott Reef, an oceanic reef at the edge of Western Australia’s continental shelf. Because Scott reef is so far removed, it means that there is a lack of connectivity that makes it far more susceptible to disturbances because recruits cannot easily come from elsewhere. Additionally, this isolation means that there is negligible human activity that could stress the system and inhibit its recovery. In 1998 there was a catastrophic bleaching event caused by a sudden extreme warming in sea surface temperatures, which led to a mass coral die-off in regions across the Pacific. It was not expected that the Scott Reef system would be able to return to pre-disturbance conditions, and if it did recover the projections were that it would be decades before it reached anything comparable because all recruits would have to be locally produced—none could come from outside. However, since the area is not fished there are large populations of herbivorous fish to keep down the algae that would otherwise take over a disturbed reef and inhibit recruitment. Within twelve years of the bleaching event, Scott Reef was able to recover to a point where reproductive output and recruitment were comparable to pre-disturbance levels. —Dawn Barlow
Gilmour, J.P., Smith, L.D., Heyward, A.J., Baird, A.H., Pratchett, M.S., 2013. Recovery of an isolated coral reef system following severe disturbance. Science 340, 69-71.
In this study, Gilmour et al. did not manipulate pieces of the ecosystem. Instead, they used an existing system for which they had data for coral abundance and recruitment rates as well as ocean temperatures, and then monitored these same things during and after the mass bleaching event. They surveyed permanent transect lines on the reef, which they categorized as either reef crest (~3m), reef slope (~9m), and upper reef slope (~6m), depending on the depth of the coral. Recruitment was monitored using settlement tiles, which were deployed at regular intervals along the transect line. Thus they were able to observe the correlation of these factors and influences as the reef recovered after severe disturbance.
In February of 1998, seawater temperatures at Scott Reef rose to an unheard of 13.3°C and remained at extreme high temperatures for the next two months. In the next six months, 80-90% of live coral cover was lost from the reef crest and reef slope, and nearly 70% was lost from the upper reef slope. Recruit numbers decreased to zero from the previous years’ 2600—5600, and in the following six years recruitment rates were less than 6% of years prior to the disturbance event.
In other systems that have been studied following severe disturbance, what usually takes place is a phase shift to macroalgae which outcompetes the corals so that there is no longer a substrate on which recruits can settle. But what happened at Scott Reef was that the substrata that were made available by the dead corals were colonized by fine turfing and crustose coralline algae. There was also an observed increase in the densities of herbivorous fishes following the event, and the inability of macroalgae to colonize the dead coral was very likely due to the dramatic increase in grazing capacity by large herbivorous fishes. This surplus in grazing capacity in the Scott Reef system was able to assist subsequent coral recruitment, allowing for more coral to be locally produced as well as giving them a place to settle and therefore increasing overall survival. On reefs experiencing chronic pressures from human impact, the mean survival of recruits tends to average around 50%. On Scott Reef during this recovery period however, mean survival of recruits ranged from 83—93%. Within a decade of the bleaching, reproductive output and recruitment were similar to the levels prior to the bleaching, and after 12 years the overall health of the reef was determined to be comparable to the time before the disturbance.
This study shows that anthropogenic pressures, particularly fishing, are dramatically inhibiting coral reef recovery after disturbance if a system as isolated as Scott Reef was able to recover from 90% coral loss without recruits from other populations within a decade, simply because of the absence of human activity. This observational study demonstrated the importance of having an intact ecosystem—the fact that the herbivorous fish had not all been caught—and the importance of limiting anthropogenic stressors if we aim to protect the reefs from further degradation. The authors of this study suggest that creating marine protected areas could be a key to promoting coral reef resilience through protecting healthy, intact ecosystems.