Testing the ‘macroalgal dominated coral reefs’ paradigm

The paradigm of ‘coral vs algae’ has become entrenched in coral reef science over the last few decades. The classic example of this paradigm in the Caribbean was from a paper published byTerry Hughes in a 1994 article in the journal Nature, entitled “Catastrophes, Phase Shifts and Large-Scale Degradation of a Caribbean Coral Reef”. The paper documented a series of disturbances in the late 1970’s and early 1980’s, including two major hurricanes, a disease outbreak and the loss of a seaweed-grazing urchin, after which coral cover declined dramatically from ~70 percent cover to less than 10%, and macroalgal cover to rose to almost complete dominance >90% .


Temporal trends of coral and algae on Jamaican Reefs (top left, 1975 - 1995, Discovery Bay showing dramatic declines and corals and corresponding dominance of macroalgae, top right 1995 - 2004 from Dairy Bull Reef showing a rapid recovery and reversal of the macroalgal phase shifts)

Since then, reefs throughout the Caribbean have undergone dramatic declines in coral cover, leading to the regionwide collapse of the two dominant reef building corals, Acropora cervicornis and Acropora palmata. One ‘good news’ story did come out from a neighbouring reef in Jamaica called ‘Dairy Bull’ reef, where Joshua Idjadi and a team reported a doubling of live coral cover over the last decade, resetting the balance from a macroalgal dominated reef to a coral reef.


Photograph of Dairy Bull Reef in 2003 showing the recovery of the branching coral Acropora cervicornis. No macroalgae to be seen!

Since this iconic case study, a considerable quantity of scientific literature has been devoted to management principles, herbivorous grazing pressure and the reversal of macro-algal dominated reefs. However, documented examples of regional ‘phase shifts’ between coral and algae in the literature are surprisingly few and far between (asides from a few notable exceptions). John Bruno & Elizabeth Selig, two coral reef researchers who have developed a considerable dataset on coral reefs throughout the world, decided to test this assumption by randomly samply for regional trends and patterns in algal cover – much the same as an epidimiologist would determine the generality of case reports in the medical literature.

John and his team trawled through an immense number of reef surveys (3,500 to be exact) from over 1,800 reefs across the globe between 1996 and 2006, and developed a ‘phase shift index’ based upon corals and macroalgae. They then tested this index in four geographic regions (Greater Caribbean, Florida Keys, Indo-Pacific and the Great Barrier Reef) to see if the severity of phase shifts altered over the decade between 1996 – 2006. Their findings were  surprising, and might prove to be somewhat controversial…


A coral reef in the Caribbean dominated by macroalgal cover

Whilst phase shifts were indeed more common in the Caribbean than elsewhere, very few of the worlds reefs fell into either a stable ‘coral reef dominated’ or a ‘macroalgal dominated’ category. Furthermore, the ‘severity’ of phase shifts at a regional level was much less severe than the classic examples of macroalgal dominance, such as the Jamaican coral decline story. The data also suggested that there was no trend (>1995) towards macroalgal dominance in the Florida Keys or Indo-Pacific. Coral cover during this period (1996 – 2006) did decline (primarily due to crown of thorns starfish plagues), but their was no corresponding increase in macroalgal cover at all during this time.

Bruno et al argue that the apparent mismatch between the local scale descriptions of macroalgal dominance and regional scale patterns was caused by a gross generalisation of a relatively small number of ‘atypical’ case studies. This in itself is no small finding, and may go along way to altering the way we manage coral reefs. These findings may be somewhat controversial, it’s hard to disagree with the data. In what’s bound to throw the proverbial cat amongst the pigeons, the authors conclude:

“Since the Jamaica story was an anomaly, it makes a poor foundation for general models of reef ecology (e.g., Knowlton 1992, Bellwood et al. 2004 ). The current paradigm of reef management and ‘‘resilience’’ is based in large part on the perception that most of the world’s reefs are being overrun by seaweed (Szmant 2001, Precht and Aronson 2006, Knowlton 2008). This belief led to the argument that reef managers should focus primarily  on conserving herbivores or water quality (Szmant 2002, Pandolfi et al. 2003, Bellwood et al. 2004 ). While these are clearly important objectives of management, our analysis suggests that the macroalgae problem has been exaggerated.

Overfishing and poor land use practices may trigger widespread coral to macroalgal phase shifts in the future, but to date, the principal form of coral reef degradation has been the loss of reef-building corals, with only limited and localized increases in macroalgae. Therefore, the primary goal for reef managers and policy makers should be the conservation of coral populations, without which the entire system would collapse.”

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