Alternative states on coral reefs: beyond coral–macroalgal phase shifts

Abstract

Degradation of coral reefs is often associated with changes in community structure where macroalgae become the dominant benthic life form. These phase shifts can be difficult to reverse. The debate on coral reef phase shifts has not focused on reports of coral reefs becoming dominated by other life forms following disturbance. A review of the primary and grey literature indicates that reefs dominated by corallimorpharia, soft corals, sponges and sea urchins can enter an alternative state as a result of a phase shift. Shifts can be triggered by pulse disturbances that cause largescale coral mortality, and may become stable as a result of positive feedback mechanisms. However, they may differ from the archetypical coral-macroalgae shift, depending on the factors driving the shift; whereas coral-macroalgae and coral-urchin shifts seem to be driven by loss of top-down control through overfishing, shifts to corallimorpharian, soft coral and sponge dominance seem more associated with changes in bottom-up dynamics. Understanding the differences and similarities in mechanisms that cause and maintain this variety of alternative states will aid management aimed at preventing and reversing phase shifts of coral reefs.

Conditions of coral communities in the Gulf of Thailand: a decade after the 1998 severe bleaching event

“…A long term monitoring program should be effectively implemented at unhealthy communities in the Gulf of Thailand in order to examine degradation trends or natu ral recovery processes (Cooper et al 2009;Sutthacheep et al 2009). Degradation of coral reefs is frequently associated with changes in community structure where macroalgae become the dominant benthic component (Done 1992;McManus and Polsenberg 2004;Norstrom (Sudara et al 1994). The impact of climate change, especially thermal stress related coral bleaching on coral reefs in the Pacific and other parts of the world has been well documented (Obura 2005;Baker et al 2008;Smith et al 2008;Wilkinson 2008).…”

Cryptic regime shift in benthic community structure on shallow reefs in St. John, US Virgin Islands

“…However, changes in abundance of scleractinians and macroalgae alone do not fully capture the range of community dynamics occurring on coral reefs (Norström et al 2009).…”

“…Although most studies of community change on coral reefs have focused on scleractinians and macroalgae (Mumby et al 2013 and references therein), other transitions in community structure have occurred (Norström et al 2009), and they have received little attention. These alternative transitions include changeovers from scleractinians to corallimorpharians or soft corals in the Indo-Pacific (Work et al 2008), and to sponges (Loh & Pawlik 2014) or gorgonian soft corals (Ruzicka et al 2013, Lenz et al 2015 in the Caribbean.…”

Corals shed bacteria as a potential mechanism of resilience to organic matter enrichment

“…There have been substantial discussions in the literature about how to define resilience (Nystrom et al, 2008), how to manage reefs in ways that encourage it (Hughes et al, 2010), and how to synthesize case studies to elucidate the large-scale mechanisms that determine whether a stony-coral-dominated ecosystem can resist a phase-shift or rebound from one (McClanahan et al, 2002;Norstrom et al, 2009). However, very little attention has been given to the small-scale mechanisms relevant to the microbial processes that determine the resilience of individual corals to a given stressor.…”