Outbreaks

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Outbreaks

Besides coral disease, natural disasters, and other anthropogenic sources of coral degradation, many of the threats to coral reefs are outbreaks of another kind: changes in the number of biotic threats such as predators and competitors. One of the largest direct threats to coral was the exponential population growth of its major predator, Acanthaster planci, or the Crown-of thorns Starfish. An equally harmful, but indirect threat to coral, was the die-off of one of coral's major allies, Diadema antillarum, or Black Sea Urchins, which keep coral competitors such as algae in check.


Decline of the Black Sea Urchin

Diadema antillarum Image courtesy of

D. antillarum tend to remain in more sheltered aquatic habitats, such as depressions in coral, during the day and migrates out to more open seagrass areas to feed during the night. The sea urchin has established itself as a major herbivore in reef and seagrass habitats. Its preferred food appears to be benthic algal turf and macroalgae, but it's diet is wide and unrestricted.[1]

Starting in 1983, D. antillarum experienced a two-year period of mass-mortality with some areas losing up to 97% of their black sea urchin populations. Presence of a disease among the sea urchins was first noticed along the Caribbean coast of Panama. [2]. Affected organisms initially developed an accumulation of sediment, lost most of their dark pigment, and often had their spines break off. As the mysterious disease progressed, the urchins did not seek shelter during the day, were unable to remain attached to their substrates, and eventually became easy targets for predacious fish looking for an easy meal.[1] The source of the disease has been linked to a waterborne, host-specific pathogen that impacted Caribbean sea urchins as it traveled with ocean currents and in the ballast water of ships crossing the Panama Canal. In one 1985 study, healthy urchins kept in an aquarium were exposed to seawater, developed diseased symptoms, and eventually died. Two types of Gram-positive Clostridium bacteria were cultured from these urchins, suggesting the bacterial basis for the disease was from the highly aggressive and resistant Clostridiaceae family. [3]

The survival of these urchins is critical to the survival of coral, as the sea urchins provide the herbivorous control of algae that compete for the coral for habitat space, sun, and nutrients. The period of D. antillarumwas followed by a dramatic increase of fleshy benthic algae. In Jamaican reefs, algae was increased 50% in just 2-weeks following the sea-urchin die-off, responsible for a 40% reduction in coral cover over the next 10 years. [2] While it is clear that this reduction in 'top-down" control of algae can have detrimental effects on coral reefs, the "bottom-up" impacts on algae can not be forgotten. Increased run-off from farming results in excessive nutrients entering ocean habitats, supplying algae with all the supplements they need to grow and thrive.[1] While the disease outbreak on D. antillarum had a more dramatic and sudden impact on algae, and therefore coral, it was further compounded by human pollution.


Growth of the Crown-of-Thorns Starfish

Acanthaster Planci

This predatory seastar feeds on corals, destroying them with digestive enzymes and their unrestricted, carnivorous diet. While these outbreaks have been characterized as “slow and methodical,” the starfish posed a major threat to an area known as the Coral Triangle in 2012, one of the richest regions in terms of biodiversity.

    A. Causes of Outbreak
                1. Increased pollution and rise in nitrogen levels
                2. Surge in phytoplankton population due to agricultural runoff
                3. Shifts in weather patterns and climate such as El Niño, noted in the extreme El Nino of 1997-1998 
    B. Possible Solutions
                1. Reduce run-off pollution promoting phytoplankton as a source of food for COTS
                2. Survey and control population of COTS
                     a. Ethical Concerns: Is this safe for the surrounding environment?
                3. Regulate and restrict industrial and agricultural development along the reef
                     a. Invest in programs like Project Catalyst to promote responsible and sustainable agricultural effort


Notes

  1. 1.0 1.1 1.2 Loya, Yossi. Coral Health and Disease. Springer-Verlag; New York NY, 2004
  2. 2.0 2.1 Birkeland, Charles. Life and Death of Coral Reefs. Chapman & Hall; London UK, 1997
  3. Engman, James. Mass Mortality in Diadema antillarum: A Large-Scale Natura. Henderson State University; Arkadelphia AK, 2002


           http://www.pnas.org/content/109/44/17995.full.pdf+html (Science Journal) 
           http://www.sciencedaily.com/releases/2008/01/080114112308.htm (Science Journal) 
           http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0047363 (Science Journal) 
           Birkeland, C. (1997). Life and death of coral reefs. New York: Chapman & Hall.
           Letcher, T. (2009). Climate change: Observed impacts on planet Earth. Amsterdam: Elsevier.
           Loya, Yossi. (2004). Coral health and disease. Berlin: Springer-Verlag.
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