For each human, there are more than 70 coral colonies in the Pacific Ocean and the most common coral species outnumber humans globally. Our findings provide new insights about the risk of global extinction in corals. The urge to count is innately human. We count everything: our savings, our likes on social media, even the number of people at our presidential inauguration. For corals, comparable estimates did not exist prior to our study, despite the global ecological and economic importance of reef corals.
Similar to trees, corals create the complex three-dimensional habitat that an estimated one million species call home. Coral reefs take thousands of years to form, and they protect our shorelines, attract tourists and feed millions of people around the world.
The abundance of corals has been in decline for decades and, as the planet continues to warm, heat stress is killing more and more corals, and their increasing frequency is preventing coral populations from fully recovering. The alarming rate of decline has stirred concerns about the risk of widespread global extinction in corals and sparked a debate among scientists and managers about the arsenal of policies and technologies required to halt the decline.
Inspired by similar research in trees, which revealed new insights into the extinction risk of Amazonian tree species and led to more ambitious global reforestation targets , we decided to estimate how many corals there were in the Pacific Ocean. The project was daunting at first, to say the least. Primarily because ecological data are notoriously scarce in corals at large spatial scales. For instance, the availability of remote sensing data, which has been instrumental to comparable estimates in trees and other organisms, is still limited in coral reef ecosystems, though rapidly expanding.
Furthermore, coral abundances are typically measured as percentage cover of benthic habitat, rather than counts of individual coral colonies, and are rarely recorded at the species level. We faced the following four challenges.
First, to map the extent of coral habitat across the Pacific Ocean. Second, to split coral habitat into the three most common reef habitat types: the reef crest, slope and flat, across which the abundances of corals and individual species are known to vary widely.
Third, to estimate how the density of coral colonies varies between these habitats and across the Pacific Ocean. And four, to estimate the relative abundances of more than coral species in the three habitats and in different parts of the Pacific. To overcome these challenges, we combined spatial data on the global distribution of coral reefs, detailed maps of coral reef habitat, species abundance data collected across the Pacific, and a new approach to modelling species abundances.
The nature of the available data made three things abundantly clear. First, a project like this would not be possible without the generosity of fellow scientists sharing their data.
Second, although estimates at such scales are inevitably uncertain, they can reveal valuable and robust insights into the functioning and conservation of our biosphere. And third, science is a work in progress: as more detailed data become available in the future, our estimates will improve.
So how many corals are there in the Pacific Ocean? Most reefs in the world have experienced bleaching events in the last few decades. All have been affected, but some more than others. In this article, I look at how coral bleaching varies from ocean to ocean, and whether this has changed with warming waters in recent decades. Before we look at the bleaching trends for specific oceans, there is a larger meta-point about the disproportionate impact on some reefs versus others.
But there appears to be a marked distinction in the vulnerability of corals across these zones. Reefs that lie closest to the equator have often fared better; those across mid-latitudes experienced worse bleaching.
This is despite the fact that corals in both zones have been exposed to similar levels of thermal stress. Purple markers highlight reefs where very little only one to a few percent of the corals were bleached.
Yellow markers show reefs where most of the corals were affected. Reefs closer to the equator might be less vulnerable to bleaching because they have adapted to larger swings in temperature over time. Equatorial reefs often experience much more variability in temperature over short timeframes: they see large swings of heating and cooling on a daily basis.
This exposure to constant fluctuations might have made them more resilient and adaptable over time. Reefs at higher latitudes will have less experience of this. When larger changes in sea surface temperature occur, they are less prepared. In a study published in Science , Terry Hughes and colleagues tracked the frequency of coral bleaching events across pantropical locations from to In the charts here we see the number of moderate and severe bleaching events that occurred in each ocean from onwards.
There are large geographical differences in the timing, intensity, and frequency of these events. The Western Atlantic warmed earlier than elsewhere, and therefore also saw an earlier uptick in the frequency of bleaching. Its reefs were experiencing regular bleaching much sooner — as early as the s. By its was seeing an average of four bleaching events per location, compared to 0. Over the course of the entire period — from to — the Western Atlantic has seen the highest number of bleaching events, averaging ten events per location.
This is two to three times higher than other regions. It has also seen the greatest number of severe mass bleaching events. The Western Atlantic might have been the most exposed to bleaching in the s, but this has also changed a lot over time. In the s bleaching risk was highest in the Western Atlantic, followed by the Pacific, with relatively low risk in the Indian Ocean and Australasia.
But this has since flipped. We see in the charts that the Indian Ocean and Australasia have seen the strongest rise in bleaching over time. In the s, bleaching events were rare. This is even true of severe mass bleaching events. The return times of severe bleaching events — how long following an event that we would expect another — have declined in all regions apart from the Western Atlantic.
In contrast to the earlier period, the Western Atlantic managed to escape a major bleaching event between and For most, this is 10 to 15 years. Getting hit by one bleaching episode after another means corals will struggle to recover. But they differ in how much pressure and rate that this has changed over time.
What might explain these differences? The corals that had seen the largest rise in bleaching were not necessarily the ones that had encountered the biggest increase in average water temperature. This might seem counter-intuitive since bleaching is driven by warming waters. This point matters for our understanding of how reefs will be affected by bleaching in the future. This is relatively simple to do. But the relationship to bleaching is a bit more complex.
Instead, we need to understand how warming oceans will affect the frequency and intensity of short-term episodes of extreme warming.
An important question is whether reefs will be able to adapt to these extreme temperatures. And if some, but not all of them can, whether it will mean very different coral ecosystems from what we have today. In our follow-up article we look at how adaptable our corals might be to a warming ocean. This is when they expel their algal symbionts — which are their primary source — due to environmental stress. Some corals die immediately when exposed to extreme temperatures.
Others become bleached then either recover or slowly die over the coming months. As the time between successive bleaching events get shorter and shorter, corals do not have the time they need to recover. Some have suggested that there might be a silver lining. Perhaps coral reefs are more resilient than we give them credit for, and can adapt to a warming ocean. The broader question of whether corals can adapt touches on two related but distinct questions. First, the question of whether an individual coral becomes more resilient to bleaching over time.
If a coral experiences coral bleaching, are they more resilient to future events? Effectively, is there a protective effect of past bleaching? Second, the question of whether coral reef systems can adapt. If a coral experiences and recovers from an intense bleaching episode, will they be protected from another event years later? There are a couple of ways in which organisms can acclimatize or adapt to different conditions.
Organisms can often adjust to new environments — such as a change in temperature, pH, moisture level or altitude — to make sure they can survive across a range of conditions.
We see many examples of this in nature. Sheep, for example, grow thicker coats in colder climates then shed them in the Spring or Summer.
We even see examples in humans. When we climb a mountain our bodies produce more haemoglobin — the protein in red blood cells — which transport oxygen around the body. This helps us to adapt to higher altitudes. When we move to a hot climate our bodies adjust by sweating at a lower core body temperature while also reducing the amount of salt in our sweat.
In corals, there are a couple of ways that this might work. First, the coral hosts can release higher levels of specific genes involved in stress-resistant traits. By releasing heat shock proteins and antioxidants, they can respond efficiently to pulses in warming and cooling. One study, published in Science, took the species Acropora hyacinthus — an important coral in the Pacific that tends to bleach easily — and transplanted it to an environment with very frequent swings in temperature.
This resistance that comes with being in a rapidly shifting environment mirrors the heat tolerance we see in corals elsewhere: corals close to the equator tend to experience much less bleaching than those at mid-latitudes; this is because they experience much larger swings in temperature on a day-to-day basis.
Second, acclimatization traits could be passed onto offspring. When corals reproduce, most of the inheritance of key traits is based on genetic factors. But experiments on species such as Acropora , Goniastrea , Platygyra and Porites , suggest that some acclimatization from generation-to-generation could influence the tolerance of offspring. The third involves the selection of particularly heat-resistant symbionts.
There are large differences in how well different types of Symbiodinium can repair photosynthetic damage — the cause of heat stress and bleaching. These are often much more resistant to bleaching.
There is mixed evidence on how well these three processes work to protect reefs in practice. Some results suggest that the threshold for coral bleaching has increased over the course of decades, or even years. In contrast, more recent studies on the Great Barrier Reef have found no evidence of a protective effect of past bleaching. They bleached just as much.
This would suggest that previous exposure did not improve their resilience to subsequent bleaching events. It was the largest mass bleaching event on record in Australasia. This might point towards an explanation for why we have conflicting results — that some studies show that previous bleaching provides some adaptive protection while others do not.
Perhaps reefs can and do adapt to previous heat stress, but only up to a certain point. There may be a limit to what they can adapt to. Their adaptive success might show in more moderate bleaching episodes but when extreme events — like the Great Barrier Reef bleaching in — arrive, they are pushed beyond their limits.
How well corals can acclimatize or adapt to climate change will therefore depend on the frequency and intensity of these very extreme bleaching episodes. Coral B is still going strong and soon takes the place of the disappearing Coral A. In the end our reef system looks very different to how it started. How does this play out in the real world? Do we really see such large differences in the tolerance of different coral species? We already know that different species can tolerate very different environmental conditions.
Rather than comparing corals in different environments we should compare the bleaching or mortality thresholds of species within their current environments. Even there we see large differences in response from species to species.
In a study in Moorea in the French Polynesia, researchers looked at how different species of coral were affected in a mass bleaching event in The results are shown in the chart. Montipora experienced moderate bleaching. The susceptibility of Pocillopora was very different: small colonies of these species were very tolerant to heat stress, but the largest ones experienced a lot of bleaching.
Finally, at the bottom we have Porites, a stony coral that tends to form small finger-like structures. It was very resistant to heat stress — almost none of its corals experienced bleaching. This variability in response to heat stress is not unique to these particular coral taxa. A number of studies have shown the same across a range of coral taxa and reefs across the world.
This is true of all coral taxa. But there are large differences in the sensitivity of each. As we saw from our study in the French Polynesia, Acropora is highly susceptible to bleaching and comes out on top. Other taxa — such as Siderastrea siderea and Stephanocoenia intersepta — experience small amounts of bleaching even at very low levels of heat stress. The difference is that these corals are much less responsive to even more heat stress. This means that not only do different corals respond differently to warmer temperatures, the magnitude of these differences really depends on how extreme the warming event is and the amount of stress they are put under.
Changes in reefs will also reflect differences in how quickly corals recover and grow. If some corals bleach — and possibly die off — much more easily than other species, it seems likely that reefs will begin to be dominated by the most resilient ones. This seems likely. But sensitivity to bleaching is just one part of the story.
We also need to consider how quickly corals can recover and grow. If corals that bleach easily then bounce back quickly, they might be able to maintain their spot on the reef. Conversely, if more resilient corals that experience only moderate bleaching take 10 to 20 years to recover and grow back slowly, they could lose theirs. Researchers have looked at this dynamic between some specific corals in detail. The two corals which dominate many reefs in the Indo-Pacific region — which includes corals off the coasts of Southeast Asia and the Great Barrier Reef — are Acropora palmata and Porites.
These corals could not be more different. Porites is much more resilient. But their growth rates and recovery times are also very different: Acropora grows quickly; Porites grows slowly.
This means infrequent but severe disturbances tend to favor Acropora because it can grow back quickly. But moderate, frequent events tend to favor Porites which is much less-affected by moderate warming. Models that look at the dynamics of these two corals on a reef system suggest that Acropora continues to dominate as long as the interval between bleaching events is more than two years. If the interval between events is less than two years then Porites starts to dominate because even the fast-growing Acropora cannot recover quickly enough.
Researchers therefore expect susceptible corals like Acropora to decline in abundance as a result of increased warming. But depending on the frequency of bleaching events they may not decline by as much as their response to heat stress would suggest. The corals themselves can adapt and acclimatize to changing temperatures through genetic changes in the host, or changes in the selection of heat-resistant symbionts. But reefs as a whole can also change and adapt, with more resilient species becoming more dominant while others die away.
Reef assemblages will be different. Just how different, and how much coral cover we will lose completely will depend on the intensity and frequency of extreme bleaching events. The research suggests that corals themselves can adapt to changes — but only up to a certain point.
More and more extreme events will push beyond these limits. Human emissions are driving climate change.
The intensity of coral bleaching depends on how much greenhouse gases we emit. When we think of the impacts of carbon dioxide CO 2 emissions we tend to focus on its impact on climate change. But for marine organisms, these emissions pose a double threat.
CO 2 emissions could risk the future of marine life, including coral reefs through ocean acidification. If we emit more than can be absorbed by forests, other vegetation and the ocean, then the atmospheric concentration increases. The Global Carbon Project shows this nicely in the graphic.
What this means is that the ocean absorbs a lot of CO 2. As we can see from the chart — the amount of absorbed CO 2 has been increasing over time as a result of increased emissions from fossil fuels.
Now, on to chemistry. When CO 2 is absorbed in water, it reacts to form other substances. When we combine CO 2 and water H 2 O we get carbonic acid. This reaction occurs:. So we can see that through these reactions, our seawater becomes increasingly acidic. This chain of reactions was started by adding CO 2 to the water. The pH of water does decrease being more acidic when we add CO 2 , but not quite as much as we might expect.
It creates reactions to work against this process. Overall, there are two outcomes of adding CO 2 to water. First, the pH decreases a bit and our water becomes more acidic. Second, some carbonate CO 3 ions are consumed to buffer against this process. Why does this matter? Why are carbonate ions so important? Corals and other marine life build their shells and exoskeletons using the mineral calcium carbonate CaCO 3. To build them, they need carbonate ions in the water.
Clearly, as carbonate reacts with CO 2 , we end up with less and less in the water. Less carbonate for marine life to form their shells. Corals then risk the dissolution of their current skeletons and struggle to form new ones. When the erosion rate of organisms is higher than the rate that they can build new skeletons, they slowly disappear.
This leads to the question of how much ocean acidification we might expect to see in the coming century. Or will reefs struggle to survive and grow? There are two elements of this question. First, how do we expect our emissions of CO 2 to affect the chemistry of seawater? Second, how will corals respond to these changing conditions?
Scientists can try to test future acidification in controlled experiments. They can expose corals to waters with different concentrations of dissolved CO 2 — mimicking the process of human emissions — and monitor how they respond. Many experiments have tried to look at the response of corals to this acidification. This is because most of the experiments have exposed corals to CO 2 concentrations that are much higher than we expect to see in the real world. As we cover in a related article , Hughes et al.
Another third ranged from to ppm, and the remaining third were over 1, ppm. Atmospheric concentrations of CO 2 would have to almost double from around to ppm. Even the worst and unrealistically higher scenarios of CO 2 emissions barely reach ppm by the end of the century. They continue to grow as normal.
But above ppm, their rate of calcification begins to decline. Most of their nutrients come from the zooxanthellae. Like plants, zooxanthellae use the sun to make food for themselves and the coral.
This is why it is important for corals to live in clear, shallow waters where they can get lots of sunlight. Corals also eat plankton — these are tiny animals or plants which drift around in the water. Some corals also consume very small fish. To catch these animals, the corals use their tentacles to paralyse their prey with specialised stinging cells called nematocysts. They can also feed on tiny plants or from the zooxanthellae that live within their cells. Home The Reef Corals.
There are two main types of corals — hard and soft. Hard corals Hard corals act as building blocks for the Reef. Common types of hard coral on the Reef include brain coral and staghorn coral. Soft corals Soft corals are flexible because they lack a solid skeleton which means they are often mistaken for plants. How fast do corals grow?
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