The Great Barrier Reef
- Nikhil Patel
- 12h
- 4 min read
The Great Barrier Reef is the largest and most complex ecosystem ever developed in the history of the Earth. Spanning over 2,300 km (1,400 mi) and covering 344,400 km², it is composed of nearly 3,000 individual reefs and 900 islands. As the largest living structure on Earth, it is visible from space and supports immense biodiversity, including 600 types of coral and 1,600 fish species .
Around 15,000–20,000 years ago, during the last Ice Age, global sea levels were much lower than they are today. Large volumes of water were locked up in ice sheets, and the continental shelf off present-day Queensland was exposed land. (See first image below) Rivers flowed across plains that are now submerged, and coral reefs could not exist because the water was too shallow or absent altogether .
As the climate warmed and glaciers melted, sea levels rose steadily. Between about 10,000 and 6,000 years ago, the ocean flooded this shelf. (See image 2 on the right) Clear, sunlit waters—essential for coral growth—created the right conditions for reef-building corals to colonize the area.
But, what are these Coral Reefs, and how are they formed? In the simplest terms, Coral Reefs are nothing but billions and billions of skeletons of tiny animals called ‘Polyps’.
Polyps’ skeletons are made up of Calcium Carbonate. Over thousands of years, billions and billions of polyps died in this region, and their skeletons piled up over one another to make the complex structure of the Coral Reef that we see today.
This is an extraordinarily slow process. Roughly two million coral polyps are required to produce about one tonne of calcium carbonate, forming a layer only around one millimetre thick. Multiply this process over thousands of years, and you begin to understand the timescale required to build a reef that is up to 250 kilometres wide in places and contains nearly 3,000 individual reef systems
Importantly, coral growth must keep pace with rising sea levels. If the water becomes too deep, sunlight cannot reach the symbiotic algae living inside coral tissues, and reef growth stops. The Great Barrier Reef is therefore the result of a precise balance between geology, sea-level change, sunlight, and biological activity.
Coral reefs are often called the “rainforests of the sea.” Although they cover less than 1% of the ocean floor, they support about 25% of all marine species. The Great Barrier Reef provides habitat for thousands of species of fish, molluscs, crustaceans, sea turtles, sharks, rays, and marine mammals.
At the base of this ecosystem are microscopic algae known as zooxanthellae, which live inside coral tissues. (See diagram A above) These algae use sunlight to photosynthesize, producing energy that feeds the coral. In return, the coral provides shelter and nutrients. This partnership is the foundation of the reef ’s productivity and the broader marine food web. (See image B)
Small fish feed on plankton and algae, larger fish feed on smaller fish, and top predators maintain balance within the ecosystem. This intricate network supports not only biodiversity but also human livelihoods, including fisheries and tourism.
Why is The Great Barrier Reef Dying?
One of the most destructive natural threats to the Great Barrier Reef is the Crown-of-Thorns starfish (Acanthaster species). This large, spiny marine invertebrate feeds by everting its stomach over living coral and digesting the tissue externally. A single starfish can consume several square metres of coral each year. Under natural conditions, Crown-of-Thorns starfish exist in low numbers and are part of the reef ecosystem. However, periodic population explosions—known as outbreaks—can devastate large areas of coral in a short time. Scientists estimate that Crown-of-Thorns outbreaks have been responsible for a significant proportion of coral loss on the Great Barrier Reef over recent decades.
But, why do their population suddenly go up? This is where humans became responsible.
There are two major reasons.
One is the reduction of natural predators, such as large triton or helmet snails, which prey on juvenile starfish. These snails have been heavily collected and sold as ornamental shells, reducing their numbers in the wild. In the image above, you can see a Helmet Snail eating the Crown of Thorns.
Another major factor is nutrient runoff from agriculture along the Queensland coast. Fertilizers washed into rivers and then into the ocean increase plankton growth. Crown-of Thorns larvae feed on plankton, and higher food availability increases their survival rates dramatically. The result is a sudden surge in starfish numbers capable of overwhelming coral reefs.
To combat outbreaks, Australian reef management agencies have deployed trained divers to manually remove or kill Crown-of-Thorns starfish. Earlier efforts included injecting the starfish with chemicals such as copper sulphate, but this method raised concerns because copper can harm other marine organisms.
Modern control programs now use more targeted injections, such as bile salts or vinegar, which kill the starfish without spreading toxins through the reef. Even so, manual control is labour-intensive and expensive. During major outbreaks, starfish populations can reach into the millions, making complete eradication impossible.
Because large-scale manual control is not sustainable on its own, scientists are increasingly focused on restoring natural ecological balance. Research has examined the role of reef predators and healthy fish populations in suppressing starfish numbers before outbreaks occur.
Some studies have explored the potential role of crabs and other reef-dwelling species in preying on juvenile starfish. While no single species is a “silver bullet,” maintaining a diverse, intact ecosystem appears to be the most effective long-term defence. Healthy reefs with balanced food webs are far more resilient to Crown-of-Thorns outbreaks than degraded ones.
Climate Change: The Bigger Threat
While Crown-of-Thorns starfish are a serious problem, climate change poses an even greater threat to the Great Barrier Reef. Rising ocean temperatures cause coral bleaching, a stress response in which corals expel their symbiotic algae. Without these algae, corals lose their colour and, more importantly, their primary energy source.
If high temperatures persist, bleached corals can die. Mass bleaching events have become more frequent and severe in recent decades, affecting large sections of the reef. Ocean acidification, caused by increased absorption of carbon dioxide, further reduces corals’ ability to build calcium carbonate skeletons.
Protecting the reef requires more than controlling starfish populations. It demands reducing land-based pollution, safeguarding predator species, managing tourism responsibly, and—most critically—addressing global climate change. The future of the world’s largest coral reef depends on whether human actions can shift from short-term exploitation to long-term stewardship .
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