The Great Barrier Reef located off the coast of Queensland Australia is the one on almost everybody’s bucket list; but one that will soon become not possible if conditions continue.
Aerial view of the Great Barrier Reef. Source: Hamiltonisland.com
Figure 1. Aerial view of the Great Barrier Reef. Source: Hamiltonisland.com

One of the natural wonders of the world

The Great Barrier Reef is undoubtable a place of wonder (as seen in figure 1) and a completely natural environment that is slowly dying faster and faster. The Great Barrier Reef is the world’s largest coral system consisting of over 2900 individual reefs and 900 islands stretching for over 2300 kms (1400 miles) over an area of approximately 344,400 square kilometres (133000 square miles).

Threats

The Great Barrier Reef has lost 50% of its coral cover in the last 27 years. This decrease was largely due to storm damage (48%), crown of thorns starfish (42%) and bleaching (10%).  The biggest destruction is from the storm damage which unfortunately cannot be stopped but closely behind is the impact of crown of thorns starfish which is a more manageable problem however still a massive issue. Bleaching of the corals is probably the most well known and covered cause of coral degradation. Smaller factors such as pollution, loss of coastal wetland, eutrophication, sediment runoff and pesticides.

Healthy reef shown on the left and then a degraded dying reef on the right: Source
Figure 2. Healthy reef shown on the left and then a degraded dying reef on the right: Source

Figure 2 illustrates the difference in structure and diversity of a healthy reef compared to that of a dying and degraded reef. The left image of a healthy reef with low threats is not only more colorful with the corals alive and present but a lot of other reef dwelling species are present showing a healthy ecosystem. The right image shows a degraded reef with dead corals and a destroyed ecosystem.

Crown of Thorn Starfish

Coral eating crown of thorn starfish, Acanthaster planci (COTS) are marine invertebrates that feed on the coral and are a major problem for the coral reefs of the Indo-Pacific and have cause major widespread damage over the last five decades. Whilst in normal reef conditions they are a natural part of the reef ecosystem, in the population explosions they cause coral reef degradation. It has become the leading cause of damage to the Great Barrier Reef ahead of other major causes such as cyclones, bleaching and coral diseases.

crown-of-thorns-starfish
Figure 3. Large Crown of Thorn Starfish in the great barrier reef captured from underwater done footage: Source

There have been three previous major outbreaks of COTS on the Great Barrier Reef: 1962-1976, 1978-1991 and 1993-2005. Current estimates are that we are now at the beginning of the next explosion which has been recorded as to have started in 2009 just off Cairns. Larval dispersal transports these organisms up and down the Great Barrier spreading their plague. They even mange to be dispersed throughout the center of the reef if current waves are flowing as conditions need. These outbreaks are somewhat similar to the locust plagues and definitely cause as much damage to their environment. Figure 3 shows an adult COTS and the size that they can reach to in the great barrier reef and it is easy to see how these individuals can take over an environment when there are high density explosions (as can be seen in figure 4).

Cause

The cause of these outbreaks is debated heavily and there are a combination of factors that contribute but it is confirmed that elevated nutrient levels cause the crown of thorn starfish explosions. Although it is not going to be the sole cause for these outbreaks. Some researchers suggest that these population outbreaks are a natural phenomenon and will occur naturally because of the unstable  population sizes of the COTS. In explosions population numbers can get to their millions and extremely high densities can be reached as can be seen from figure 4 which is a picture from the waters of the Great Barrier Reef.

Figure 4. Population explosion of the COTS can be seen in this image, high densities are reached in outbreaks. Source: Courier mail
Figure 4. Population explosion of the COTS can be seen in this image, high densities are reached in outbreaks. Source: Courier mail

The cause most accepted for the outbreaks seen in the Great Barrier Reef is the nutrient enrichment from agricultural land run-off which is high in nitrogen and phosphorus, as this causes phytoplankton blooms which consequently feed the COTS larvae. The Great Barrier Reef has seen doubled concentrations of  large phytoplankton correlating with the increased outbreaks of the COTS. Many organisms benefit from phytoplankton blooms, but it is a major driver for the larvae survival of COTS which then makes the way for future outbreak.

A secondary cause for these outbreaks is potential over fishing of the key predators. This means that the COTS have fewer predators so more are surviving, as described in the video below. This disruption in the natural chain of the ecosystem is seriously damaging the Great Barrier as it is all happening at an alarmingly fast rate.

The video above describes and shows the problems caused by the Crown Of Thorns Starfish. Explaining how they destroy the coral and the features of these organisms that make it an impossible project to control.

The King destroyer

In their masses the Crown Of Thorns starfish are in their millions. With a single adult COTS eating up to 10 square meters of living coral a year, extremely large damage can be done by these king destroyers causing significant coral cover loss. They have a number of adaptations and traits that lead to booms and busts in their population. Rapid growth and early age of sexual maturity are a deadly combination for the reefs but if this wasn’t enough; they have some of the highest fertility rates of any known invertebrate. All this means that they are able to grow and spread quickly once conditions have been breached and they can then spread their plague all across the Great Barrier Reef.

Figure 5. Crown of Thorns Starfish feeding on Branching Acropora coral. Source
Figure 5. Crown of Thorns Starfish feeding on Branching Acropora coral. Source

They are also specialist coral eaters that usually prey on reef coral polyps. It mounts a section of the living coral using its tube feet to effectively climb onto the coral and positions itself. The stomach of the starfish excretes its digestive enzymes that allow the COTS to absorb the nutrients from the coral. Figure 5 shows the COTS feeding on the coral Acropora and how they climb on top of the coral so they can position themselves. The interconnected nature of the Great Barrier Reef may make it more vulnerable to the COTS than other reefs as it means they can spread easily. Research into the Great Barrier Reef consistently shows the crown of thorns starfish as a key driver and degradation. Declines in coral cover could be reduced if outbreaks were managed.

In conclusion it is clear that COTS are the greatest threat to the corals of the Great Barrier Reef and the coral reef ecosystem. Whereas it is impossible to remove each and every individual starfish when these outbreaks occur, the removal of each adult means one less mouth to feed and so reduces the rate of coral cover loss.

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