Coral Transitions

Feedback mechanisms

The feedbacks maintaining each regime can be defined by the competitive relationships among corals, macroalgae, coralimorpharians, soft corals, sponges and urchin barrens (local, proposed). Each regime reinforce itself by occupying space and consuming nutrients that then become inaccessible to other species groups.

Coral dominated reefs

  • Symbiosis with zooxanthellae feedback (local, well-established): The coral regime is reinforced by its symbiosis with zooxanthellae. Microscopic algae called zooxanthellae live inside the polyps. Corals receive food from the algae, which have the ability to photosynthesize. The algae also facilitate skeletal growth and provide corals with their color. In return, corals offer the algae nutrients (nitrogen, phosphorous and carbon dioxide) and protection from predators. When coral abundance is reduced by external disturbances, competitors may establish and restrict the ability of corals to regrowth.

Algae dominated reefs

  • Unpalatability feedback (local, well-established): If macroalgae reach certain size, it becomes unpalatable for herbivores (Scheffer et al. 2008, Hughes et al. 2010), creating a reinforcing feedback that favors further macroalgae recruitment.

Coralimorpharians, soft corals and sponge

  • Competition feedback (local, well-established): Coralimorpharians, soft corals and sponges are set of functional groups of species that usually compete with corals for space and resources. After strong disturbance events such as hurricanes, bleaching events, El Niño or La Niña events, low tides, disease outbreaks, oil pollution or euthropication that reduce coral populations, coral reefs are prone to be overgrown by its competitors.

Urchin barrens

  • Bioerosion feedback (local, well-established):The formation of urchin barrens usually happens in environments with low biodiversity and absence of predators. Urchin are herbivores in coral reefs, but when they form barrens, they may predate corals as well. The urchin barren regime is established when the bioerosion rate is higher than the reef accretion rate (Norström et al. 2009). 


Shift from Coral dominated reefs to alternative regimes

Important shocks (eg droughts, floods) that contribute to the regime shift include:

  • Thermal anomalies (regional, well-established): Thermal anomalies produce coral bleaching, a phenomena seen when corals release its zooxanthellae due to thermal stress, leading to the loss of coral color. If temperatures exceed summer maxima by 1º to 2ºC for over 3 weeks, then bleaching will occur, with more severe bleaching as thermal anomalies intensify and lengthen (Hoegh-Guldberg et al. 2007). Berkelmans et al. (2004) found that in the Great Barrier Reef, Australia, the maximum sea surface temperature (SST) over any 3 day period during the bleaching season (summer) predicted the presence/absence of bleaching with 73.2% accuracy. Massive events of coral bleaching have been recorded more recently (Berkelmans et al. 2004). According to model predictions, coral bleaching events are expected to become increasingly frequent and severe in the coming decades due to global warming (Wooldridge et al. 2005, Hoegh-Guldberg et al. 2007). Corals may survive and recover from bleaching after mild thermal stress, but typically show reduced growth, calcification, and fecundity and may experience greater incidences of coral disease (Bruno et al. 2007, Hoegh-Guldberg et al. 2007). 
  • Hurricanes (regional, contested): Hurricanes are shock events that can destroy coral structure, reducing coral populations.  These shocks can provide opportunities for algal populations to dominate over coral reefs, especially when these shocks occur in synergy with other drivers. However, hurricanes might also have a beneficial role when passing several hundred kilometers from coral reefs as they potentially cool down water from thermal anomalies (Eakin et al. 2010). 
  • Coral disease outbreaks (local to regional, contested): Coral disease outbreaks may also act in synergy with other drivers by weakening coral's resilience to other shocks. The same works the other way around. For example, warmer water temperature and nutrients inputs have been correlated with increase of disease outbreaks (e.g. white band syndrome)(Bruno et al. 2007, Houk et al. 2010a). However, in cases where disease probability is density dependent, outbreaks are less likely to happen. It depends on the disease dynamics, transmission patterns and contextual factors. This is why it is considered a contested driver.
  • Low tides (local, well-established): Low tides have been reported to induce mass coral mortality (Norström et al. 2009). They expose the coral to higher solar radiation, luminosity and temperature. 
  • Oil spills (local, controversial): Oil spills reduce luminosity inhibiting photosynthesis, change the chemical composition of water, and may induce coral mortality directly by the effect of toxins or by affecting larvae survival and dispersal.

The main external direct drivers that contribute to the shift include:

  • Fishing (local, well-established): Overfishing reduces herbivory, leading to increasing macroalgal abundance due to reduced grazing (Mumby et al. 2007). Fishing can also reduce the diversity of herbivores.  When coral cover is reduced there is increased colonization by algae, which in turn inhibits coral recruitment – i.e., a positive feedback exists (Mumby et al. 2007, Norström et al. 2009). When the abundance of fish is reduced, the ability to react to algal growth peaks is reduced as well, and mats of algae may establish and cover portions of a coral reef.
  • Atmospheric CO2 (Global, well-established): ocean acidification reduces the concentration of carbonate-ions in the oceans, which is a fundamental compound required for corals to grow. With less carbonate accretion corals cannot fix their skeletons, hence the coral reef structure will be reduced.  Many coral are expected to be unable to cope with CO2 concentrations above  480ppm and temperature increase above +2ºC (Hoegh-Guldberg et al. 2007).
  • Global warming (Global, well-established): Global warming is expected to increase sea surface temperature, furthermore by increasing temperature it also increases CO2 concentration in water and therefore ocean acidification.

The main external indirect drivers that contribute to the shift?

  • Deforestation (regional, proposed): Deforestation increases the leakage of nutrients and runoff of sediments from soils to water bodies. Sedimentation and nutrients inputs are in turn a source of stress for coral reefs and by fertilizing algal populations stimulate algal growth while decreasing coral growth rates (Hoegh-Guldberg et al. 2007).
  • Urbanization (Local, proposed): Similar to deforestation, coastal development increase the amount of sewage on waterbodies, leading to more sedimentation and nutrient inputs, which stimulates algae at the expense of corals (Hoegh-Guldberg et al. 2007).
  • Food demand (Global, well-established): Food demand is a driver that stimulates fishing that in turn is a direct driver for coral degradation (SALVAT 1992). Market connectivity, trade facilities, government policies and development of technologies allow local fishermen to increase fishing effort (Deutsch et al. 2011).

Slow internal system changes that contribute to the regime shift include:

  • Herbivory (Local, well-established): Herbivores (mainly fish) control the growth of algae through grazing, scraping, and bioeroding (Nyström and Folke 2001). However, once algae grow beyond a certain size they become unpalatable to herbivores (Scheffer et al. 2008). If the abundance of fish is reduced, the ability to react to algal growth is reduced as well.  Consequently, reduction in herbivore diversity and populations decreases the ability of herbivory to regulate temporary increases algal growth due to disturbance.  Diseases key herbivore species (Nyström and Folke 2001) by reducing herbivory can initiate a positive feedback that can lock the system into an alternative regime dominated by macroalgae.
  • Connectivity loss (Regional, well-established): For all types of regime shifts described, there is a common pattern in regard to spatial resilience.  Spatial connectivity is needed to allow larvae interchange, which increase genetic variability.  Well-connected reefs are more resilient to disturbances. However, when connectivity is broken, corals rely on self-seeding and are more vulnerable to depletion of local stocks, bleaching events and other disturbances (Elmhirst et al. 2009, Hughes et al. 2010). 

Summary of Drivers

# Driver (Name) Type (Direct, Indirect, Internal, Shock) Scale (local, regional, global) Uncertainty (speculative, proposed, well-established)

Key thresholds

If temperatures exceed summer maxima by 1º to 2ºC for over 3 weeks, then bleaching will occur, with more sever bleaching as thermal anomalies intensify and lengthen (Hoegh-Guldberg et al. 2007)

Coral reef is expected to be lost if carbon dioxide exceeds 480ppm and temperature increase +2ºC (Hoegh-Guldberg et al. 2007) 

Summary of Ecosystem Service impacts on different User Groups

References (if available)
Provisioning Services
Food Crops
Feed, Fuel and Fibre Crops
Wild Food & Products
Regulating Services
Air Quality Regulation
Climate Regulation
Water Purification
Soil Erosion Regulation
Pest & Disease Regulation
Protection against Natural Hazards
Cultural Services
Aesthetic Values
Cognitive & Educational
Spiritual & Inspirational