Isolated coral reefs are often thought to be exceedingly vulnerable to catastrophic disturbances such as cyclones and bleaching events because they do not have an external supply of larvae recruits. Gilmour et al.(2013) studied Scott Reef, an isolated reef 250 km off the coast of Western Australia that suffered mass mortality following a bleaching event in 1998. Twelve years after the bleaching event, live coral cover in Scott Reef had increased by 35 percent despite extremely low (six percent) recruitment rates for the first six years of recovery. Their results show that isolated reefs are able to recover relatively quickly from severe disturbances even though they have limited connectivity—suggesting that benefits of being away from constant human pressures such as pollution and overfishing may outweigh benefits of access to large quantities of larval recruits.—Kelsey Waite
James P. Gilmour, Luke D. Smith, Andrew J. Heyward, Andrew H. Baird, and Morgan S. Pratchett. Recovery of an Isolated Coral Reef System Following Severe Disturbance. Science 5 April 2013: 340 (6128), 69-71.
Gilmour and colleagues studied Scott Reef, a large (~650 km2) and isolated system of reefs that is more than 250 km from the mainland of Australia. There is negligible commercial and recreational fishing at Scott Reef, and the reefs isolation means it is free of many other anthropogenic pressures as well. Permanent transects were established in regions of Scott Reef in 1994, and data has been collected according to the Australian Institute of Marine Sciences Long Term Monitoring protocols every year since.
Water temperatures at Scott Reef rose quickly in February of 1998 and remained above average for another two months—the estimate of the severity of the anomaly was 13.3°C, which is the most severe anomaly ever recorded at Scott Reef. Over the next six months, a massive bleaching event occurred in which 80–90 percent of live coral on reef crests (~3m) and reef slopes (~9m) died, while live coral cover on upper reef slopes (~6m) decreased 70 percent. This extreme decrease in coral cover was followed by recruitment failure—over the next six years recruitment rates were less than six percent of what they had been prior to the bleaching event, meaning almost all initial coral growth was from local colonies that had survived the bleaching. Mean survival of recruits was between 83 and 93 percent each year during the recovery period, which is much higher than the mean survival of recruits (less than 50 percent) on reefs that are constantly experiencing anthropogenic pressures.
Gilmour and colleagues also suggest that the corals in Scott Reef would have likely recovered even faster if they hadn’t experienced a series of more moderate disturbances: two cyclones, an outbreak of disease, and a second less severe bleaching event. Their results show that coral reefs with negligible supply of larvae from outside sources (isolated reefs) can recover quickly from disturbances in when they are not under chronic anthropogenic pressures—suggesting that addressing these local pressures from human activities such as overfishing and pollution will help promote resilience against global degradation of coral reefs.