Longevity & Causes of Death

Coral & Coral Reefs

Longevity & Causes of Death

Longevity

  1. Little is known about the lifespan of corals. Generally, coral colonies may live for several decades to centuries.

Predators

  1. Coral polyps face many predators including parrotfishes, butterflyfishes, and sea stars.
    • Since the 1960's, unnaturally high rates of crown-of-thorns sea star predation in certain regions of the Great Barrier Reef resulted in as much as 95% loss of living coral in these areas.
    • An increase in crown-of-thorns predation events may be at least partly caused by human activities. Increased runoff from forest clearing and agriculture may lower the salinity and increase nutrients, which may cause crown-of-thorns populations to flourish. In addition, targeted fishing may reduce populations of crown-of-thorns predators such as pufferfish, triggerfish, emperor fish, and tritons.
  2. During the larval stage, corals are particularly subject to predation. They may also drift into areas where the substrate isn't suitable for coral growth.

Human & Climate-Based Threats

  1. Ocean pollution poisons coral polyps. Pollution takes on many forms including oil slicks, pesticides and other chemicals, heavy metals, and garbage.
  2. Fertilizer runoff and untreated sewage introduce added nutrients to coastal ecosystems. These elevated nutrient levels promote algae growth. Unfortunately, high concentrations of algae or solid sewage can overwhelm and smother coral polyps. Under normal conditions, herbivorous fish and some invertebrates keep the algae population balanced.
  3. Deforestation degrades more than just land habitats. When tropical forests are cut down to clear land for agriculture, pasture, or homes, topsoil washes down rivers into coastal ecosystems. Soil that settles on reefs smothers coral polyps and blocks out the sunlight needed for corals to live.
  4. Coastal development and dredging ravages reefs. This development includes building seaside homes, hotels, and harbors.
  5. Fishing with dynamite, cyanide, or bleach has killed coral reefs in the Indo-Pacific region. Between 1986 and 1991, half of the coral reefs in the Philippines have been demolished by these and other destructive fishing methods.
  6. Besides fishes, fishermen harvest a variety of exotic seafood from the reef including conchs and lobsters. Overharvesting could lead to these species' demise. Careless handling of nets, lines, and lobster traps has led to some reef damage.
  7. International seashell and aquarium trades have put a strain on coral reefs and reef inhabitants. Excessive collecting decimates reef species and upsets the balance of the reef ecosystems. Careful monitoring of these trades will help make sure that the demand for reef species doesn't exceed the sustainable supply.
    • The souvenir trade has created an international market for coral skeletons, shells, sponges, and other reef animals. Coral skeletons are used as aquarium decorations or fashioned into jewelry and sculptures.
    • Coral skeletons are also sold as "live rock". Live rock is popular in home saltwater aquariums because it is permeated with living bacteria and algae and acts a natural biological filter.
    • The tropical fish trade has created a demand for reef fishes. These attractive fishes are popular in saltwater aquariums.
  8. Careless water recreation damages reefs. Divers and snorklers that sit or stand on or handle corals can injure the delicate polyps. Dropped boat anchors can gouge the reef and crush corals.
  9. Coral bleaching results when the coral polyps expel their zooxanthellae symbionts from their tissues revealing their underlying, white skeleton. Without zooxanthellae, the coral polyps lose nutrition and have less energy available for growth or reproduction.
    • Coral bleaching is linked to higher than normal temperatures, pollution, and exposure to air.
    • Within a region, massive coral bleaching events can occur with as little of a temperature increase as 1 to 2°C above normal for a few weeks.
    • Since 1979, these mass bleaching events seem correlated with severe El Niño Southern Oscillation events.
    • Full recovery of coral reefs from these events often takes decades.
  10. In the oceans, global climate change - caused by excess emission of carbon dioxide and other greenhouse gases - is linked to a rise in global ocean temperatures, which may lead to a variety of impacts for ocean animals, plants, and even entire ecosystems such as coral reefs.
    • As the ocean warms, the melting of the polar ice caps is projected to raise sea level. A rise in sea level would decrease the amount of available sunlight necessary for the zooxanthellae symbionts to use in photosynthesis and may ultimately inhibit coral growth.
    • As ocean temperatures warm due to climate change, incidences of coral bleaching may increase.
    • Major tropical storms can strip corals from miles of reef habitat. Extreme weather events, such as hurricanes, may become more frequent as a result of climate change and provide a further risk to coral reefs.
  11. In addition to climate change effects, increases in atmospheric carbon dioxide increases ocean acidification. Traditionally, corals have removed excess carbon dioxide from the atmosphere. However, the amounts of carbon dioxide in the atmosphere have gone beyond what corals can uptake, and the oceans are becoming more acidic as higher amounts of carbon dioxide dissolve into the water. Increasing acidity reduces corals' ability to construct their calcium carbonate skeletons.
  12. The coral reef is an intricate ecosystem and contains a diverse collection of organisms. If coral reefs decline, populations of fishes and other animals that rely on coral reefs for food and shelter may decrease as a result.