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The Outer Reefs of Moorea Island: Back From the Dead!

Paper Reviewed
Adjeroud, M., Kayal, M., Iborra-Cantonnet, C., Vercelloni, J., Bosserelle, P., Liao, V., Chancerelle, Y., Claudet, J. and Penin, L. 2018. Recovery of coral assemblages despite acute and recurrent disturbances on a South Central Pacific reef. Scientific Reports 8: 9680, DOI:10.1038/s41598-018-27891-3.

A topic of frequent discussion in the global change debate is that of the vulnerability of coral reefs, as it has been claimed by some that many of these key biological species will become extinct within mere decades in consequence of the so-called anthropogenic-induced disasters of coral bleaching, ocean acidification and more frequent and more intense hurricane activity. Findings reported in a new study by Adjeroud et al. (2018) suggest otherwise.

According to this team of nine researchers, there exists "a critical knowledge gap in the ecological drivers of coral community trajectory, particularly during recovery processes that follow disturbances." Consequently, predictions of coral demise from disastrous events -- be they of natural or human origin -- are often based on limited data that fail to properly incorporate information on coral resilience and/or recovery processes, which, if appropriately accounted for, would likely temper estimations of future reef peril.

And so it was, that Adjeroud et al. set about to analyze a unique long-term dataset from the outer reef slope of Moorea Island in French Polynesia to determine the interannual variability in cover and abundance of three benthic stages of corals (recruits, juveniles and adults) since 1991. Moorea Island provided the perfect location to study "the temporal dynamics and recovery trajectories of coral populations affected by acute and recurrent disturbances" because "since the 1980s, this reef has been affected by a remarkably high frequency and intensity of the three most destructive large-scale disturbances that affect Indo-Pacific reefs: coral bleaching events, cyclones and outbreaks of the coral-killing crown-of-thorns seastar."

The results of their analysis revealed that the destructive episodes indeed took their toll on the outer reef slope of Moorea, e.g., combined impacts from a hurricane and a bleaching event reduced live coral cover from ~51% in 1991 to ~22% in 1993, and a crown-of-thorns seastar outbreak followed by another major hurricane event reduced live coral cover from ~49% in 2005 to less than 1% in 2010. However, and despite the magnitude of these disturbances, Adjeroud et al. report that coral cover has gradually increased "and has since recovered to pre-disturbance levels (~50%) at several reef slope locations in 2016."

In commenting on the magnitude of this finding, the authors write that "despite the frequency and severity of the disturbances that have affected Moorea in the last three decades, and the two major declines in coral cover and abundance that rival some of the biggest losses reported elsewhere, this reef has shown, on two occasions, a remarkable capacity to quickly recover." And what makes this recovery even more remarkable is that fact that it was achieved "despite constantly low recruitment rates," whereas Adjeroud et al. note that the typical recovery "is generally associated with significant inputs of coral recruits. Another uncharacteristic observation with the Moorea reef recovery was that a "proliferation of fleshy macroalgae was not observed at Moorea" following the major disturbance events, which absence, once again, defies conventional wisdom in these matters.

The take-home message of this study is that there still is a lot that remains unknown with respect to the resilience and recovery of coral reefs. As such, it is premature (and likely the height of folly) to be forecasting their doom, especially just a few short decades from now, as climate alarmists frequently do.

Posted 27 July 2018