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Acanthaster planci has been identified as a coral predator. Studies have shown that it has been undergoing a population explosion in many parts of the Pacific Ocean. Acanthaster is a large, sixteen armed, spiny sea star also known as the crown of thorns starfish. It is this star fish that is said to have been destroying large areas of living coral along many Pacific coastal lands. It has been said that other animals feed on coral but none has been as efficient as Acanthaster planci. Since 1967, this starfish has consumed almost 90% of coral cover in many coastal regions including the Great Barrier Region. It is said that each Acanthaster planci can consume up to 40 cm2 of live coral on a single day. So when many of these starfishes feed on a single reef, then devastating effects on the coastal habitats can occur. This article is going to describe how this starfish has destroyed coral reef along the Great Barrier Reef in Australia (Lourey et al 2000).

The Great Barrier Reef have faced predation by the crown of thorns starfish causing major damages to the coral communities in the region since the 1950s. Large populations of this starfish kill entire colonies of many coral species despite that there exists individual preferences for different types of corals in some occasions. The first ever record of A. planci outbreak on the Great Barrier Reef was done in the 1960s and covered reefs in the central third of the ecosystem. When most of corals on the reef had been destroyed, the starfish disappeared. This followed a rapid growth and establishment of new coral cover on many parts of the reefs. These coral formations have since been studied on many reefs of the Great Barrier Reef in particular, the Green Island Reef and the John Brewer Reef.  There followed a second outbreak in the 1980s that was experienced on many reefs of the GBR. Studies have shown that the time required for hard coral cover to return to pre-outbreak levels is estimated to be between 10 years t0 50 years on the Great Barrier Reef. An outbreak of A. planci can only occur if the reef population reaches levels where corals are being consumed at a faster rate than they can grow leading to depletion (Lourey et al 2000).

To clearly understand the impact of A. planci on the Great Barrier Reef, Manta tows were used to study different categories of coral. It was observed that the difference in hard coral cover between outer shelf reefs and inner shelf reefs was largest in the northern region with no observable difference in other regions. The percentage of reefs having a history of outbreaks of A. planci ranged 6% of outer shelf reefs in region to the south to 50% of inner shelf reefs in the north. It was also observed that inner shelf reefs had a higher likelihood of having a history of outbreaks of A. planci than the outer shelf reefs. Those in the southern region on the other hand had low probability of having a history of A. planci outbreaks than reefs in the other regions. Hard coral cover in the Great Barrier Reef was reduced 6% averagely every year on those reefs with sustained Acanthaster planci outbreaks. This rate of decrease of coral cover was almost the same in all the regions of the Great Barrier Reef. After a year of sustained outbreak, the reef’s hard coral cover ranged from 2 to 28% which averaged to 9%. Contrastingly the hard coral cover on reefs with no outbreak history ranged from 16 to 40%. There was an average increase in hard coral cover on those reefs recovering from sustained A. planci outbreaks than on those with no outbreak history. There was a large difference in the rate of change of hard coral cover between the southern region and other regions. It was estimated that hard coral cover for recovering reefs was increasing at a rate 0.8 per year in the far northern, northern, and central regions of the Great Barrier Reef. This was very different to regions having no outbreak history of A. planci that had an increase of 0.2% per year. Southern reefs had an increasing rate in hard coral cover averaging 4% on recovering reefs and 2% on those without an outbreak history (Lourey et al 2000).

Recovery time after an outbreak was also considered whereby the percentage of recovering reefs where there was an increasing coral cover, ranged from 88% in the far north to 70% in the north. All recovering reefs had an increasing coral cover of 78%. Now, if a linear growth is assumed, then this meant that for hard coral cover to increase by 30% after a sustained outbreak of A. planci, it will require time ranging from 5 to 1000 years in the three northern regions of the Great Barrier Reef, and time ranging from 2 to 300 years in the southern region. What this means is that, regions having coral cover could be destroyed for many years because outbreaks occur more frequently, even if hard coral cover has not fully recovered from the previous outbreak. There was a lower hard coral cover on reefs without an outbreak history of Acanthaster planci on the inner shelf reefs in central and northern regions. This was because the regions are adjacent to water sheds that have been modified by human activity over a long period of time. These include factors like the nutrient enriched run-off that hinders the growth and subsequent recruitment of corals (Lourey et al 2000).

Reasons for the sudden increase of Acanthaster planci population has not been that clear. Questions as to how the infestations come about have seen many theories put forward. There are those who claim that population increases are brought about by the anthropic factors that cause disturbances in the environment, while others claim that it is all about natural fluctuations in population sizes. It is widely claimed that the depletion of tritons by shell collectors is the cause of increase in A. planci population. Tritons, Charonia tritonis, are gastropods that feed on Acanthaster. Their depletion may have favored the proliferation of the A. planci species.  Another cited cause has been marine pollution that contributes enormously to the weakening and subsequent destruction of coral.  Some studies suggest that population explosion in Acanthaster planci are just but natural events. Their evidence is based on the fact that such trends have occurred in the past. Historical evidence such as remnants of Acanthaster skeletons found in both recent and fossilized sediments on the Great Barrier Reef proves their argument (Chesher 1969).

There are also theories put forward by researchers in this area. One is that adult Acanthaster populations of normal densities regroup in the wake of cyclones that destroy coral reefs. Another theory suggests that an increase in the survival rates of larva may bring about sharp increases in the rates of recruitment. Basing on the first hypothesis, increase in population density happens suddenly especially after a reproductive season that is accompanied by heavy rainfall. When based on the second theory, heavy rainfall in the highlands bring about enrichment of lagoon waters with nutrients, this lead to growth of other plants that Acanthaster larvae thrive on. All these are possible causes although more research is needed to ascertain the real cause (Chesher 1969).

To manage the situation, suggestions have been made that adult Acanthasters should be prevented from infesting new coral areas. If infestations are identified earlier then their control can be made simple because the population of seeds is localized and the fish is easy to find. Topographical zones through which the starfishes can not move should be established.  Then, adult starfishes can be contained in these zones where starvation will bring about a natural, rapid decline in their population. After a certain period most of the fishes could be eliminated.  If zones can not be established, the preservation of some sections of the reef can be done by extermination.  Isolation of major reef sections that are connected to other coral zones can be done to prevent migration. This will cause starvation and therefore elimination. It is argued that coral destruction in an effort to eliminate A. planci can be an economic disaster especially for small Isles because their inhabitants derive their livelihood from them. This can also lead to severe land erosion that is caused by storm waves. Other measures aimed at controlling population increase of the Acanthaster planci can include the presence of an efficient predator to the starfish such as the Charonia tritonis. Such predators if introduced in a population of Acanthaster planci can bring about a natural extinction to this fish species (Chesher 1969).

Conclusion

Acanthaster planci has caused a lot of destruction to many coral reefs. In Great Barrier Reef the effect has been their since the 1960s and has caused major concerns to the people living in those areas. Several causes in their increase have been cited including man’s activity. Many measures to prevent the increasing population can be taken. These include, introducing predators to the starfish, starvation to death and extermination among others.

Code: Sample20

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