Global threats to coral reefs: coral bleaching, global climate change, disease, predator plagues and invasive species

Abstract

A series of new and emerging threats to coral reefs has become a focus of attention in recent decades with clear evidence of widespread and even global damage. This chapter focuses on these threats:
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\n* coral bleaching and global climate change;
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\n* diseases of corals and other reef organisms;
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\n* plagues of predators like the crown-of-thorns starfish (COTS – Acanthaster planci) and other damaging organisms such as the sea urchin Echinometra mathei; and
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\n* invasive species which have been introduced onto new coral reefs.
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\nThese threats are in addition to natural stresses that have always existed on coral reefs such as storms, freshwater inundation and seismic and volcanic events. Direct human pressures on reefs have, until recently, been the dominant factors damaging coral reefs through a range of stresses, many of the which can co-occur:
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\n* the delivery of 'pollution' from unsustainable land-based human activities such as deforestation, poorly regulated agriculture, and urban and industrial development resulting in the release of excess amounts of sediments and nutrients. This is exacerbated by the release of nutrients and other pollutants from untreated or poorly treated sewage and industrial and agricultural wastes;
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\n* over-fishing and over-exploitation of coral reef fisheries and coral rock and sand resources. Within the last 2 decades there has been an alarming increase in damaging fishing activities involving the use of home made bombs, cyanide and damaging practices such as muro ami that involves dropping weighted rocks onto corals to drive fish into set nets; and
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\n* modification and engineering practices such as building of ports, airports and groynes on coral reefs, including the practice of 'reclamation', which pours sediments onto shallow areas, displacing sea area in exchange for increased terrestrial amenity.
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\nCoral reefs managers and scientists now suspect that these apparently newer global threats (bleaching, disease and predators) are increasing rapidly in frequency and severity, coincidentally with direct human disturbances. Predator plagues like COTS are increasingly reported around areas of human activities with 2 strong hypotheses advanced: the plagues may be initiated and certainly exacerbated by either over-fishing of key starfish predators; and/or increases in nutrient runoff from the land favours the planktonic stages of the starfish. Coral disease has caused major disruptions to coral reefs in the Caribbean with a range of human disturbances potentially implicated, and there are now increasing reports of similar disturbances from the Indo-Pacific region. Evidence linking severe coral bleaching and mortality to increasing rates of global climate change attributed to rising levels of anthropogenic greenhouse emissions is growing stronger.
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\nCoral reef managers and policy makers urgently need guidance from the research community on appropriate responses to these mounting levels of global stresses. Management has largely been based on controlling the direct pressures of pollution and over-exploitation. There are now urgent questions for researchers: are there linkages between human activities and the increasing reports of global stresses on reefs and if so, how are they manifested and how can they be controlled or at least minimised?