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.
Whilst 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).
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.
A classic example of this is the dominance of a macro-algae on Belizean patch reefs. Following wide spread coral mortality over two decades ago, the subsequent increases in dead coral substrates lead to a dramatic increase in free space for macro-algae (Lobophora, Sargassum, Turbinaria). As these fleshy brown algae colonised and proliferated, the number of coral-algal interactions increased, affecting coral growth rates, recruitment of new coral juveniles and recovery of coral tissues, resulting in a community dominated by fleshy brown macro-algae. When combined with increases in nutrient loading on reefs (further fuelling macro algal growth), this scenario creates a system of ‘positive feedback’ processes, sustaining algal blooms and making a reversal back to coral-dominated states less straight forward.
In the case of the Belizian reefs, manually ‘re-setting’ a phase shift by removing the algae failed to return the system to a coral-dominated state, even in marine park regions where herbivorous fish were abundant. Norström and co-authors argue that similar feedback mechanisms are likely to exist for ASS (sponges, soft corals, urchins) and urge further research on large scale experiments to increase our understanding of why these stable states persist in regions with no obvious external environmental change (such as the Belizian reefs).
Norström et al conclude that the projected increases in human-induced disturbance (coral bleaching, disease, cyclones) are likely to result in significant increases in open substrate for colonisation by such alternative organisms, resulting in an increased occurence and persistence of these alternative stable states on reefs throughout the globe. A better understanding of these states will help managers to ‘shake’ or reverse ASS, with an ultimate goal to preventing phase shifts on coral reefs.
Norström, A., Nyström, M., Lokrantz, J., Folke, C. (2009) Alternative states of coral reefs: beyond coral-macroalgal phase shifts. Marine Ecology Progress Series 376: 295-306