Resilience and Resistance of Coral Reefs to Disturbances

Global climate change impacts, such as severe bleaching events, can be more destructive if there are local anthropogenic influences undermining the resilience of the coral reef ecosystem. Following a bleaching event, it could take corals about three years to recover to pre-bleaching extension rates. However, the recovery period for corals will be extended or prevented altogether by local impacts compounding a severe destructive disturbance. For example, coral colonies experiencing higher local stress prior to the 1998 bleaching event were more severely affected and recovered slower. While repetitive bleaching may increase coral's ability to withstand future heat stress due to climate change, local stressors such as increasing sedimentation may dampen a coral's energy reserves making it less likely to survive or recover from a bleaching event. Local conservation efforts that reduce stress caused by anthropogenic influences could have significant impacts on the ability of corals to withstand the effects of climate change. These local influences can include overfishing of important herbivores, increased levels of disease, and excess nutrients. These regional or local-scale disturbances can act together to significantly degrade reef ecosystems to the point where they are unable to recover from even minor disturbances. Chronic anthropogenic impacts cause persistently elevated rates of coral mortality due to degraded resilience. Coral reefs with compromised resilience after acute-disturbances (bleaching or hurricanes) can lead to an algal-dominated phase shift.

Phase shifts are extensive decreases in coral cover coinciding with substantial increases in some alternative benthic organism, such as macroalgae, soft corals, or sponges. Most of the phase shifts documented are from hard coral-dominated to macroalgae-dominated communities. However, soft coral- or sponge-dominated reef systems are other alternative stable states which have received less attention. Phase shifts can be initiated by disturbances which cause high coral morality, and may become stable through positive feedback mechanisms. These disturbances can include chronic or acute climate change and local anthropogenic impacts, including but not limited to overfishing of important herbivores, increased nutrients from coastal development, hard coral diseases, sedimentation, recurring storm events, and severe bleaching. The death of hard corals provides solid substrate for other benthic organisms to settle and flourish. The increased colonization of macroalgae, soft corals, and sponges on the hard substrates inhibits hard corals recovery potentially leading to a positive feedback mechanism favoring other benthic organisms. The limited research evidence indicates that phase-shifts to soft coral-or sponge-dominated reefs is linked to declining water quality. It is possible that these other benthic organisms, macroalgae, soft corals, and sponges, exhibit traits that allow them to tolerate the changing environmental conditions.

Caribbean reefs are among the most heavily affected by phase shifts, specifically shifts to macroalgae-dominated systems. While there are multiple drivers of phase shifts, a main foundation of the phase shifts throughout the Caribbean was due to the mass mortality of Diadema antillarum. Urchins, such as D. antillarum, promote reef resilience by controlling algal cover. The mass mortality of D. antillarum left the main herbivore role to parrotfishes (Scarids). Unfortunately, these large herbivores had already been exploited for decades and even healthy, unexploited populations of parrotfishes are unable to match the grazing intensity of urchins. The mass die-off of D. antillarum, overfishing and increased nutrient input from increasing coastal development explains the wide-spread phase shifts from hard coral-dominated to macroalgae-dominated reef ecosystems in the Caribbean, including the Bahamas.