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Alternative States of Coral Reefs

In existence for nearly 500 million years, coral reefs survived the extinction of the dinosaurs and the ice ages. Today, this previously resilient ecosystem faces a multitude of natural and man-made threats, including coral bleaching, disease, overfishing and global climate change. Many of these threats have caused reef systems to shift phases, from coral-dominated communities to communities dominated by other marine organisms. The results -- "alternative stable states" -- appears to be irreversible, raising the question posed by Bernhad Riegl of the National Coral Reef Institute: Has mankind "created the final crisis that will push these long-lived systems into extinction?"

Macroalgal Dominance
- Nearly all references to "phase shifts" in coral reefs relate to a change from a hard coral-dominated community to a macroaglal-dominated community. In a landmark 1994 study, Terrence Hughes concluded that a combination of natural and man-made pressures -- notably hurricanes and overfishing -- resulted in a marked decline in coral cover in Jamaica. When the population of Diadema, an herbivorous sea urchin, crashed, the system was left with too few grazers, causing the system to shift to one dominated by large, fleshy (corticated) algae. Even when the pressures were removed, the new, algae-dominated community persisted as an alternative stable state. Examples of similar phase shifts to a macroalgal-dominated ecosystem have been observed across the Caribbean, the eastern Pacific, eastern Africa and Australia. While very few examples demonstrate a reversal of the alternative state, conservation managers argue that reducing overfishing can limit phase shifts.
Corallimorpharian Dominance
- Corallimorpharians, like sea anemones, are in the class Anthozoa.
  
  Corallimorpharians are soft-bodied marine organisms, closely related to sea anemones. In a multi-year study, Yossi Loya and his team have documented a phase shift from hard corals to corallimorpharians in the reef flats of the Eliat Coral Nature Reserve in the northern Red Sea. An extreme low-tide event in 1970 reduced hard coral cover from 36 percent to 5 percent. Chronic oil spills between 1970 and 1980 prevented recovery of this coral community, leading to a dramatic increase in the abundance of the corallimorpharian Rhodactis rhodostoma. Long-term monitoring of the site reveals Rhodactis has thrived in the shallow reef flats and continues to be the dominant organism in the ecosystem. Shifts to an alternative, corallimorpharian-dominated state have been documented elsewhere in the western Indian Ocean and Indo-Pacific.
Soft Coral Dominance
- Soft coral species may outcompete new recruits, or young, of hard coral species, thus delaying recovery.
  
  Common organisms in many of the world's reef ecosystems, soft corals have increased in abundance as a result of coral mortality due to outbreaks of crown-of-thorns sea stars as well as extensive use of blast fishing. In 1997 and 1998, the most severe incident of coral bleaching on record led to a loss of 16 percent of the world's coral populations. The event was particularly severe in the Indian Ocean, including the shallow waters of the Aldabra Atoll in the southern Seychelles, where 66 percent of the hard corals were killed. By 2004, the soft coral Rhytisma had increased from 0 to 26 percent cover, resulting in a phase shift to a soft coral-dominated, alternative state.
Sponge Dominance
- Sponge
  
  Species of the sponges Cliona and Chrondilla have been documented to monopolize reef substrate following declines in populations of hard corals. Following the 1998 coral bleaching incident, hard coral cover dropped precipitously on the Channel Cay portion of the Belize barrier reef. A species of Chondrilla, the chicken liver sponge, colonized unoccupied areas. A decade later, a study by Richard Aronson revealed hard coral populations had not recovered, and the sponge-dominated alternative state persisted. The presence of a herbivorous sea urchin seems to have prevented a shift to a macro-algal dominated state, as occurred on other parts of the Belize barrier reef, devoid of this species of urchin.
Urchin Barren States
- Species of Diadema, an herbivorous sea urchin, eroded much of the reef base in Uva Island, Panama.
  
  Herbivorous urchins play a significant role in the health of reef ecosystems as their grazing controls algal cover. However when their populations increase significantly, urchins can have negative impacts on the reef, both preying on juvenile corals and removing calcium carbonate from the coral framework. On the Uva Island reef off the west coast of Panama, an El Nino event in 1982 and 1983 caused extensive loss of coral. In a multi-year study, C.M. Eakin documented a subsequent, dramatic increase in the population of Diadema mexicanus, an herbivorous urchin. While the population remained high, a significant amount of reef bioerosion destroyed much of the reef framework. As Eakin described, the urchins literally "ate themselves out of house and home" as the lack of suitable habitat caused Diadema populations to crash by 2000.

Nature