Nutrient Pollution, Overfishing and a loss of Herbivorous species: Phase Shifts
Within the past few decades, the combined effects of overfishing, increased nutrient pollution and a reduction in the long spined sea urchin species, Diadema antillarum, have caused coral-algal phase shifts on many coral reefs in the Caribbean region (McManus and Polsenberg 2004). Coral-algal phase shifts occur when coral cover is replaced by fleshy macro algae on a coral reef, causing a reduction in coral reef structural complexity (McManus and Polsenberg 2004).
Coral-algal phase shifts are instigated by natural disturbances that cause physical damage to corals such as hurricanes, disease outbreaks and bleaching events (McManus and Polsenberg 2004). After such disturbances, damaged coral colonies and coral fragments are rapidly colonized by fleshy macro algae (McManus and Polsenberg 2004). Historically, high levels of grazing by herbivorous fish, and the long-spined sea urchin, would decrease macro algae abundance, allowing for coral cover to return to a high level (McManus and Polsenberg 2004).
However, within the past few decades, a combination of human induced stressors such as overfishing and irresponsible land clearing practices have stimulated macro algae growth and abundance on coral reefs. (McManus and Polsenberg 2004). In the Caribbean region, the herbivorous long-spined sea urchin, Diadema antillarum, plays a fundamental role in the control of macro algae growth and abundance (McManus and Polsenberg 2004). Between 1983 and 1984, an unknown disease caused Diadema antillarum populations to decline by approximately 93% throughout the Caribbean region (McManus and Polsenberg 2004). It is believed that this catastrophic mortality event combined with the effects of overfishing, have lowered the resilience of coral reefs throughout the region (McManus and Polsenberg 2004).
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reversible declines in coral cover? Lack of recovery of two coral reefs in St. John, US Virgin Islands. Mar Ecol Prog Ser, 306:103-114.
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Coastal development, dredging, land reclamation, deforestation and poor agricultural practices have caused increased levels of sedimentation on coral reefs. Suspended sediments block sunlight from reaching corals, reducing coral growth and survival (Spalding et al. 2001; Cesar et al.2003; Fabricius 2005; Rogers 1990). Although corals can remove sediment from their surface through mucus production, ciliary action or tentacle waving, these processes require a lot of energy (Rogers 1990). As a result, prolonged exposure to sediments may cause reductions in coral growth, reproductive potential (their ability to reproduce) and their ability to compete with other sessile organisms for space (Spalding et al.2001; Cesar et al. 2003; Fabricius 2005; Rogers 1990). Studies also suggest that increased levels of sedimentation can weaken a corals immune system, making them susceptible to bleaching and disease (Cesar et al. 2003; Fabricius 2005; Raymundo et al. 2008; Jackson et al.2014 ).
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