“What is it to blame?” is frequently asked when discussing what damages the Great Barrier Reef. Hurricanes, water quality, climate change, bleaching, acidification, crown-of-thorns starfish (COTS). Arguments arise to isolate one of them, but when having a look at the crown-of-thorns it becomes clear how he combination of all turns into a deadly cocktail.

It is no secret that the Great Barrier Reef lost half its coral cover between 1985 and 2012, hence before hurricane Debby passed by (2017) and before the main bleaching events of 2016, 2017 and 2020, yet after the ones of 1998 and 2002. According to the Australian Institution of Marine Science (AIMS), the three main causes of that loss were storm damage (48%), crown-of-thorns (42%), and bleaching (10%).

Obviously, we cannot stop storms but we can definitely make the reef more resilient by reducing other stressors such as crown-of-thorns, run-off, and climate change.

Feasting on the Coral

Let’s zoom in on one major stressor: crown-of-thorns. Research by the AIMS for that period showed that the absence of the crown-of-thorns would have given the Reef the chance to recover and even increase at 0.89% per year, despite the losses caused by cyclones and bleaching. It would be slowly, but surely recovering.  

This makes sense because an adult COTS feasts on about 10 square meters of coral a year, while a female crown-of-thorns produces up to 50 million eggs a year. It takes the larvae only 6 months to become and adult and consume that proportion of coral, and only 2 years before it becomes sexually mature, producing the same amount of eggs.

“So, let’s kill them all”, one might think, and there are a lot of Crown-of-Thorns control programs out there in which divers give them a deadly injection. However, considering the 344,400 square kilometres of the Great Barrier Reef, which is for the Australians bigger than Victoria and Tasmania combined, or for the Europeans bigger than the UK, Switzerland and Holland combined, or for the Americans about half the size of Texas, and for the football lovers: about 70 million football fields. Hence, one can guess: killing them all might not be the most efficient solution.

Sucking it up

Moreover, crown-of-thorns might be considered as a pest, but they are a natural inhabitant of the Reef. Outbreaks of COTS are recorded every 15 year since the 1960s. Usually, they start in the north of the Reef, and migrate southward over a period of about 15 years. High-likely there have been more outbreaks before the 1960s, however they are off the record and still little understood.

Yet, the Reef wasn’t entirely lost. Surveys of the AIMS show that healthy Reef recovered in the 10 to 20 years between the outbreaks if it had enough time without additional stressors.

Deadly Cocktail

And here is where the question of the famous discussion comes back: “What is it to blame?”

The Reef could recover from the crown-of-thorns outbreaks. However, stressors such as storms, intensified and multiplied by climate change; bleaching and acidification; water contamination and run-off; all weaken the resilience of the Reef, and take away the time it needs to recover before the next outbreak starts.

Moreover, these factors might be in favour of the crown-of-thorns. The run-off of sediments and nutrients might feed plankton, on which the larvae of the COTS feed themselves, hence increasing their survival chances. Further on, the few natural predators of the COTS and of its larvae might be threatened by the changing ocean conditions, and hence no longer threaten the COTS, not to speak about overfishing, since it is known that there are less COTS outbreaks in marine protected areas.

So, let’s come back to the essence of the discussion: “What is it to blame?” Water quality issues, run-off, climate change, crown-of-thorns, bleaching and acidification are all part of the deadly cocktail of the Reef. Therefore, the good news is: we can break that cycle by we can break that cycle by both strengthening the Reef as weakening the stressors such as the crown-of thorns. It’s not an or-or question, but an and-and answer.