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To the brink of extinction... and back

22 Mar 2023 | Science Notes

From the CO2Science Archive: Gilmour et al. write that “coral reef recovery from major disturbance is hypothesized to depend on the arrival of propagules from nearby undisturbed reefs,” and, therefore, they say that “reefs isolated by distance or current patterns are thought to be highly vulnerable to catastrophic disturbance,” which could well spell their local extinction. The authors “document the recovery of coral assemblages at Australia’s largest oceanic reef system, where changes in assemblage structure and key demographic parameters were quantified for 16 years, through a regime of disturbances beginning with a catastrophic mass bleaching event in 1998.” This they did at the Scott system of reefs, which is “surrounded by oceanic waters on the edge of Western Australia’s continental shelf,” and which is “more than 250 km from the mainland and other reefs in the region, and more than 1000 km from a major center of urbanization.”

Paper reviewed: Gilmour, J.P., Smith, L.D., Heyward, A.J., Baird, A.H. and Pratchett, M.S. 2013. Recovery of an isolated coral reef system following severe disturbance. Science 340: 69-71.

What was learned
The five Australian researchers report that “for 6 years, recruitment rates were <6% of those prior to the disturbance,” and “on the basis of these rates of change,” they say that “recovery was projected to take decades.” However, within a mere 12 years, they indicate that “coral cover, recruitment, generic diversity, and community structure were again similar to the pre-bleaching years.” And they say that the coral recovery “may have been even faster if not for a series of more moderate disturbances, including two cyclones, an outbreak of disease, and a second bleaching.”

What it means
Gilmour et al. say their results demonstrate that “even corals with a negligible supply of larvae from outside can recover relatively quickly from disturbances in the absence of chronic human pressures [italics added].” And they thus suggest that “addressing local pressures, such as pollution and overfishing, is as important to the recovery of coral reefs as the establishment of networks of marine protected areas.” Indeed, they conclude in the final sentence of their paper that “managing local pressures to promote resilience will be critical to preventing the global degradation of coral reefs,” which is the same thesis we have long promoted in describing several offending phenomena, many of which have particular application to Australia’s Great Barrier Reef, including (1) rising nutrient levels caused by runoff from agricultural activity on land, (2) outbreaks of the coral-devouring crown-of-thorns starfish, (3) the barbed hooks and scything nets used in fishing, (4) tourists and the developers who build resorts and marinas for them, (5) increased sediment levels, (6) the nets of prawn trawlers stirring up the growing load of sediments, (7) the 6-10 tons of “bycatch” for each ton of prawns netted that are caught and die, which dramatically changes the composition of reef life, (8) sea life depleted to the point of exhaustion by over-fishing, (9) huge catamarans and dive boats that take thousands of visitors to the Barrier Reef each day and dump their sewage in the sea on the way home, (10) the live reef-fish trade, (11) fishermen using dynamite and cyanide, (12) coral diseases, and (13) pollution.

Clearly, if all countries bordering on the world’s oceans adequately dealt with these problems within their own jurisdictions, earth’s corals have the capacity to do whatever else is needed to insure their continued existence.

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