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Coral reef diversity

Coral reefs are important habitats with high ecological, social and economic significance. Watch Assoc Prof Andrew Olds explain more.
In this section you will learn about the biodiversity, socio ecological significance and changing condition of coral reef ecosystems. Hi, I’m Associate Professor Andrew Olds. I’m a specialist in marine spatial ecology at the University of the Sunshine Coast in Australia. Have you ever been to a coral reef? They are breathtaking oases of life in tropical and subtropical seas. But what are reefs? And how do they form, reefs are created by corals and other calcifying organisms who build towering multifaceted and brightly coloured structures from the sea floor. These can be as large as city skyscrapers and conform interconnected seascapes the size of some countries. Today I’m going to tell you about the ecological, social and economic importance of coral reefs.
We’ll discuss how these values are changing in response to a diversity of stressors and look at bright spots for reef recovery. Coral reefs harbour remarkable biodiversity that cover less than 1% of the sea floor, but also support at least 25% of all marine species. So what is biodiversity? And how high is it on coral reefs really, biodiversity is the variety of all forms of life from genes to ecosystems. Estimates of global reef diversity converge at close to 1 million species, but also range from point five to 9 million species. When we think of reefs, we see exquisite corals and brightly coloured fish, but these are far from the most diverse groups.
There are close to 4000 fish species, and 800 species of hard corals. Most reef species, however, are small, rare and like to hide, such as worms, crustaceans, and mollusks. These groups have not been adequately surveyed and as a result, it is estimated that up to three quarters of all reef species still remain to be discovered. So why are there so many species on coral reefs? Reefs harbour more species than other marine ecosystems because of their high complexity and the variety of micro habitats and feeding opportunities these provide.
There is also intense competition between species to utilise these resources, which are often in short supply, the upshot of which is high specialisation and speciation, with each species adopting a slightly different approach to both defend their turf and claim resources in their neighbourhoods. So reef species make terrible neighbours, always fighting and borrowing things from each other that they will never return. Why does this diversity matter? reefs are beautiful to look at, and are also immensely important to humanity. Their complex structures and the diversity of species they support underpin the safety, wellbeing, food and economic security of hundreds of millions of people globally.
In fact, the value of ecosystem services reefs deliver is estimated to be worth 2.7 trillion US dollars each year. This includes an estimated 36 billion from tourism, 7 billion from fisheries and 6 billion from the protection of built shorelines. Reef fisheries yield 1.4 million tonnes of fish each year and support up to 70% of the dietary protein needs for people in tropical island countries. So there are many reasons we need coral reefs. But as you might have heard, they are also in quite a bit of trouble. So how are reefs doing?
Reefs are actually in pretty bad shape because of global impacts from climate change and ocean acidification and local impacts from eutrophication - that’s lots of nutrients, pollution, starfish predators like crown of thorns starfish, overfishing, and destructive fishing practices. Coral cover is declining on most reefs, and this is primarily because of recurring large scale coral bleaching events that have been caused by elevated sea surface temperatures. There have been three global coral bleaching events in 1998 2010, and 2017. All led to widespread coral mortality. Roughly 8% of the world’s corals died after the first event, which equates to more coral than there is in the entire Caribbean Sea.
And other 14% of the world’s coral has died since the second event, which is more coral than there is on all of Australia’s coral reefs. This has had a big impact on reef biodiversity, as you might expect, and also on reef fisheries, both of which are estimated to have declined by over 60%. So is it really all bad news for coral reefs? No, there are bright spots of coral recovery. Coral cover increased globally between the first and second bleaching events. It has also been increasing again since the last bleaching event. This recent recovery is particularly encouraging on the Great Barrier Reef were in a period without severe heat stress, coral cover has increased by roughly 10%.
Recovery was due to rapid expansion of tabular and branching hard corals, which grow very quickly, but are also the first to die following disturbance. So the Great Barrier Reef is resilient, but it is also likely sensitive to future heat waves and tropical cyclones, which are projected to occur more frequently in the future. The most diverse reefs in the world in the Coral Triangle, Indonesia and Southeast Asia are also proving to be quite resilient. Coral was he had been impacted by severe bleaching events. But coral cover has also recovered quickly. It is now actually higher than it was before the first bleaching event in the 90s.
This is encouraging because it suggests that higher coral cover and diversity might have conferred these iconic reefs with the level of natural resistance to marine heatwaves. So reefs are in trouble, but they appear to have a few tricks left in their coral bags. We’re learning from these and working to integrate them into restoration initiatives to promote reef recovery.

Coral reefs are among the most diverse ecosystems on our planet. In this section you will learn about the biodiversity and the socio ecological significance of these habitats. You will also learn about the various stresses they face and how surprisingly resilient they have been so far.

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