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These underwater worlds are even more biodiverse than tropical rainforests – and just like rainforests, coral reefs face significant threats to their existence. So what’s causing so many of these incredible ecosystems to die out – and what’s being done to preserve them?
Imagine a supercity so big that it can be viewed from space. A complex environment of massive structures and high-rises that’s home to wildly diverse cultures and communities. It’s a busy, noisy place that’s filled with life and traffic.
This isn’t London, New York City or even Shanghai. We’re talking about the Great Barrier Reef, the world’s largest coral reef system.
Although they cover less than 0.1% of the earth’s surface, coral reefs are the most biodiverse marine ecosystems in the world. The Great Barrier Reef, located off the coast of Queensland, stretches over 1,400 miles of the eastern side of Australia. It’s composed of more than 2,900 individual reefs and is home to a dynamic array of sea life – from fish, turtles and sharks to manatees and dugong. A quarter of all marine species live on coral reefs, and there are more types of fish living within a two-acre area of healthy coral reef than there are species of birds in North America.1
And it’s been around for a lot longer than any man-made city: estimates suggest the Great Barrier Reef first started forming 240 million years ago.2
Occupying less than 1% of the ocean floor, coral reefs are home to more than 25% of marine life.3 These magical underwater worlds are quite unlike our own. But just what makes coral so special, and why are they increasingly threatened by natural disasters?
Contrary to what many people assume, coral aren’t plants. In fact, they’re animals, most closely related to jellyfish and anemones.4 A reef is composed of thousands of soft-bodied animals called coral polyps – a mouth surrounded by tentacles. Plants inside these photosynthesise during the day, while at night the polyps come alive and feed. These tiny predators have tentacles with stinging cells called nematocysts. They extend these tentacles out and catch whatever tiny organisms (such as zooplankton and small fish) happen to be floating by. The captured prey is then moved into the polyps' mouths and digested in their stomachs. The polyps then secrete layers of calcium carbonate beneath their bodies, which build up over time. This is how coral reefs grow, at an average rate of just two centimetres a year.5
Coral reefs can be divided into three types: fringing reefs, barrier reefs and atolls.6 Fringing reefs are found close to shore, whereas a barrier reef lies further out to sea, protecting shallow waters from the open sea (hence the name). Atolls, meanwhile, are often mistaken for islands because they are so large and generally appear on the rim of a lagoon. They come in a wide array of shapes, colours and sizes. Some more unusual specimens resemble deer antlers, trees, giant fans, brains and honeycomb.
Coral needs sunlight to grow, which is why they thrive in shallow water. That’s why you’re unlikely to find a coral reef deeper than 45 feet. Their biomes must maintain a temperature of 70-85ºF (21-29ºC).7 They tend to prefer tropical seas, as the sea water is warmer and clearer here. Hence the world’s other biggest reef systems are located in the Caribbean Sea (the Mesoamerican Barrier Reef System) off the coast of the Florida (the Florida Keys Reef) and the Philippines (Apo Reef). They also need saltwater to survive, which is why they don’t live in areas where rivers drain fresh water into the ocean.
Coral reefs also improve the structural integrity of the seabed, helping seaweed, seagrass and other marine plants to survive.8 These plants lessen the impact of storms and help prevent the ocean bed from being washed out. They also provide food and protection for a broad variety of marine animals: fish, manatees and countless other species feed and raise their offspring in the reefs. They’re also one of nature’s great recyclers – taking nutrients filtered from the water and producing waste products that feed lots of other reef species. This helps to improve the quality of the surrounding water.9
But it’s not just sea life that benefit from the coral: scientists have discovered that many parts of a coral reef can be harvested to make medications. The National Oceanic & Atmospheric Administration commented: “Coral reef plants and animals are important sources of new medicines being developed to treat cancer, arthritis, human bacterial infections, Alzheimer’s disease, heart disease, viruses and other diseases.”10 In addition, coral’s unique skeletal structure has been used to make our most advanced forms of bone-grafting materials. They truly are the medicine cabinets of the 21st century.
As a giant, living, breathing organism, corals are essential to regulating carbon dioxide levels in the ocean. These ecosystems therefore play an important role in managing the effects of global warming. But they’re under increasing threat – from rising sea levels, ocean acidification and rising global sea temperatures. Small but prolonged temperature increases force coral colonies to expel their symbiotic, food-producing algae, a process known as ‘bleaching’.11 This stress response to warmer temperatures sees the dying reefs turn ghostly white.
Although they can, in theory, recover from such events, many do not. There have been five mass bleaching events so far in 1998, 2002, 2016, 2017 and 2020 – all caused by rising ocean temperatures. Over half of the world’s reefs have been lost in the past 30 years.12 In the 2016 occurrence alone, a staggering 29% of coral on the Great Barrier Reef died.13
Bleaching is not the only thing threatening our reefs. Burning fossil fuels adds carbon dioxide (CO2), the same gas used to give fizzy drinks their fizz, into the atmosphere. And just as CO2 is absorbed into the drink, the ocean absorbs it from the air. When it enters the ocean, it makes it more acidic. This interferes with coral’s ability to calcify their skeletons. They means that they can no longer grow, and therefore die.
As much as a third of all reef-building corals are at risk of extinction. Scientists predict that all corals will be threatened by 2050, with 75% facing high to critical threat levels. However, this is not only a problem for sea species that depend on coral reef for survival, but also for human life. Coral reefs provide food for a variety of fish which, in turn, provide food for us. It’s estimated that around 500 million people in the world consume the fish found on coral reefs. Coral reefs also play an essential part in tourism: the total economic value of coral stands around $30 billion.
But there is reason for optimism. At a local level, when we reduce direct threats, such as pollution, overfishing and unsustainable tourism, reefs are healthier and more than capable of withstanding the effects of climate change. At a regional and global level, the likes of the Coral Reef Alliance are establishing ‘adaptive reefscapes’.14 These networks try to create an environment in which coral can adapt to climate change. Methods include reducing localised threats, diversifying the reefs and species within the network, and varying the environmental conditions.15 Meanwhile, marine scientists from 50 Reefs, The Nature Conservancy and other NGOs are coming up with innovative solutions, trying to breed and train new corals to better withstand rising ocean temperatures.16
As much as cities do not exist without buildings, coral reefs do not exist without thriving coral within them. Let’s hope we can enjoy the wonder and amazement they inspire for many generations to come.
Featured image © Razvan Ciucai I Getty
1. Fish species living in coral reefs, 2. Great Barrier Reef formation, 3. Coral reef statistics, 4. Coral as animals, 5. Growth rate of coral, 6. Types of coral reefs, 7. Coral biomes, 8. Coral helps other marine plants to survive, 9. Coral reefs help to improve the quality of the water, 10. The role of coral reefs in medicine, 11. Bleaching, 12. Decline of coral reefs in the past 30 years, 13. 2016 bleaching event, 14. Adaptive reefscapes, 15. How adaptive reefscapes work, 16. NGO solutions