Why the existence of ‘heat tolerant’ corals does not mean that coral reefs will be able to resist climate change.

A recent study published by Tom Oliver and Stephen Palumbi from Stanford University in the journal ‘Marine Ecology Progress Series‘ seems to suggest yet another miraculous and novel mechanism by which corals will ‘escape’ the pressures of global warming. In a nutshell, the researchers found that corals from ‘warm pools’ at Ofu Island (American Samoa) … Continue reading Why the existence of ‘heat tolerant’ corals does not mean that coral reefs will be able to resist climate change.

Caribbean reef fish decline: where have all the big fish gone?

Humans have been overfishing Caribbean reefs for decades or even centuries.  And you don’t have to be a scientists to have noticed.  Countless reefs once dominated by vertebrate predators are now nearly devoid of large fish. Sharks and large grouper and snapper are a rarity on most Caribbean reefs.   The reason isn’t a mystery; … Continue reading Caribbean reef fish decline: where have all the big fish gone?

Doom and Boom on a Resilient Reef: Climate Change, Algal Overgrowth and Coral Recovery

Our lab has just published a new paper in PLoS ONE, detailing the interactions of coral and algae on the Great Barrier Reef, and uncovered just how resilient some reefs can be following coral bleaching events. The southern end of the Great Barrier Reef was exposed to extended periods of high sea surface temperatures in … Continue reading Doom and Boom on a Resilient Reef: Climate Change, Algal Overgrowth and Coral Recovery

“Macro-algal dominated coral reefs: shake that ASS”

In recent years, coral reefs have been hit hard by an array of anthropogenic impacts – coral bleaching, coral disease, overfishing and eutrophication to mention but a few – resulting in significant declines in coral cover and species diversity. One of the classic examples of coral reef decline was discussed by Terry Hughes in a 1994 article in the journal Nature, entitled “Catastrophes, Phase Shifts and Large-Scale Degradation of a Caribbean Coral Reef”. Hughes concluded that the synergistic impacts of overfishing, hurricane damage and disease resulted in a ‘phase shift’ from a coral dominated ecosystem (52% coral cover, 4% algal cover) to a macro-algal dominated ecosystem (2% coral cover, 92% algal cover). Similar examples of phase-shifts from coral to macroalgal dominated ecosystems have been observed across the Caribbean region, throughout the Eastern-Pacific, Indian Ocean and on the Great Barrier Reef.

asdasdWhilst macro-algal dominated reefs and phase shifts have recieved considerable attention in the scientific literature, a recent paper questions the role and driving factors of such ‘alternative stable states’ (ASS), and implicates the dominance of several other organisms that take rise following the loss of coral cover.

First establishing that a ‘phase shift’ must result from a decline of coral and subsequent increases in an other ‘alternative’ organism that must last for a significant period of time (in this case >5yrs), Norström et al conducted a survey of the literature to determine exactly what alternative organisms were dominant on reefs following a phase shift.

The authors argue a timely point that phase shifts associated with coral reefs are not exclusively coral – macroalgal shifts, and often result in shifts to ‘other’ states, including ‘soft coral’  dominance (corallimorphs and octocorals), sponges and urchin dominated states.

One of the key findings of the research suggests that whilst these different alternative states are common, the factors driving the shift may be considerably different. Whilst macro-algal states are driven by ‘top down’ factors (a loss of herbivorous fish or urchins through overfishing or disease), soft coral and sponge states are more closely associated with ‘bottom up’ factors (declining water quality).

Site specific examples of phase shifts in coral reefs: a) Israel, b) Seychelles, c) Belize
Site specific examples of phase shifts and the persistence of alternative stable states in coral reefs: a) Israel, b) Seychelles, c) Belize

So what does it take to ‘shake that ASS’? (Alternative Stable State, of course). Once a coral reef has shifted to an alternative stable state, simply removing the stressor that triggered the shift might not be sufficient to produce recovery back to a coral dominated state – partly due to feedback mechanisms, or a longer-term decline in environmental conditions.

Continue reading ““Macro-algal dominated coral reefs: shake that ASS””