Crown-of-thorns: A nasty star?

The predatory crown of thorns starfish is threatening Indonesia's portion of the "coral triangle," the richest area of coral reef biodiversity on the planet, scientists warned.

Scientists said they feared the growth in numbers of the starfish was caused by poor water quality and could be an early warning of widespread reef decline.

"We witnessed a number of active outbreaks of this coral predator. There was little to suggest that the reefs have been much affected by climate change as yet. The threats appear far more localised," said Andrew Baird from the Centre of Excellence in the statement.

full article on wildsingapore news

More background

What is considered a Crown-of-thorns outbreak?
When a population of these sea stars on a reef reaches the level where coral is being consumed faster than it can grow. (from the Australian Institute of Marine Science (AIMS) website, which also includes an FAQ on Crown-of-thorns)

Can reefs recover from an outbreak?
Within 10 to 20 years, reefs can have good coral cover again, although the corals are predominantly the fast growing varieties. Massive coral (i.e., boulder-shaped ones) takes longer to regrow and must compete with the faster growing corals for light and space. Some reefs which were affected by outbreaks two decades ago have now recovered so completely that they are major tourist attractions. (from the CRC Reef Research Centre website)

AIMS surveys since 1984 have found that the average coral cover on reefs with an outbreak declines at a rate of about 6% per year to reach a low of about 9%. For comparison, coral cover on reefs with no history of outbreaks ranged from 16 to 40%. Coral cover has not increased on 25% of reefs that have suffered outbreaks. Increases in hard coral after outbreaks on the remaining 75% of reefs were significantly greater than on reefs with no outbreak history. Estimates of median recovery times for these reefs (representing an increase of 30% coral cover above post-outbreak levels) ranged from 10 to 25 years. (from AIMS)

What causes a Crown-of-thorns outbreak? Are the outbreaks natural?
No one knows the answer to this question. Nor is it known whether outbreaks are caused by a single factor or a group of factors. Currently two hypotheses concerning the cause(s) of outbreaks have received most attention from scientists. One invokes natural causes; runoff from high land masses after periods of dry weather creates phytoplankton (algal) blooms which provide food for the larvae and enhance their survivorship. The other suggests that outbreaks are a result of man's interference since he has (inadvertently?) removed the main predators of the starfish. (from the Australian Institute of Marine Science (AIMS) website, which also includes an FAQ on Crown-of-thorns)

Research conducted in the 1980's has shown that the starfish has an in-built natural ability to undergo sudden population explosions. The ability to produce great numbers of offspring in a single spawning season suggests that outbreaks may occur naturally following particularly good spawning years. Whilst outbreaks are likely to be a natural phenomenon there remains some uncertainty over the possible role of certain human activities that may affect both the frequency and intensity of outbreaks. (from the CRC Reef Research Centre website)

How much coral can the Crown-of-thorns starfish eat?
Crown-of-thorns starfish have been estimated to consume about 5-6 sq m of coral tissue per year. (from the Australian Institute of Marine Science (AIMS) website, which also includes an FAQ on Crown-of-thorns).

More links
Animation of Crown-of-Thorns outbreak in the Great Barrier Reef 1985-2007 on the Australian Institute of Marine Science website: quite a scary animation.

Should the Crown of Thorns Starfish be exterminated? a discussion

No comments: