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Coral reef ‘oases’ vital insights for saving world’s most threatened ecosystem

International study developed a framework that can identify small communities of corals that are flourishing

As the world is working for saving the coral reefs as one of the world’s most threatened ecosystems, the identification of small oases in the world’s oceans, where corals appear to be thriving, could offer vital insights in the race to save them, according to an international study.

The study, which was conducted by an international team of academics, including the lead author James Guest, from Newcastle University, UK, has developed a framework that can identify small communities of corals that are flourishing against the odds, while many around the world are dying.

The researchers of the study focussed on four key locations in the Pacific and Caribbean region, they also used data from sites that have been surveyed for at least a decade. The research team has identified small pockets of life, where the coral appeared to be either escaping, resisting, or rebounding from changes to their environment.

Findings of the study appeared on Monday in the Journal of Applied Ecology. The results of the study sparks hope that it will encourage further study into why these small communities of corals are surviving, while so many more are not, and inspire efforts to identify similar “oases” in other ecosystems.

“Coral reefs are in rapid, global decline, but the severity of degradation is not uniform across the board and what we have identified are coral reefs that are doing better than their neighbours against the worst effects of climate change and local impacts. This glimmer of hope does not mean we can be complacent about the severity of the crisis facing most of the world’s coral reefs. But it does give us a starting point from which to understand why some ecosystems might be more resistant than others and to identify areas that warrant stronger protection or specific management strategies, such as restoration or mitigation,” said Guest, the lead author on the paper and currently a European Research Council Fellow at Newcastle University.

The team worked together until they were able to identify 38 oases, that they tentatively categorised as either “escape,” “resist,” or “rebound” oases. They described escape oases as coral communities that have been able to avoid disasters, such as bleaching, invasions from coral-eating sea stars, or the wrath of hurricanes.

According to the study, resist oases are coral communities that appear hardy and resistant to environmental challenges. Rebound oases are coral communities that have suffered damage like many other reefs, but have “rebounded” to a coral dominated state.

“There are a number of reasons why one coral reef might survive while its neighbour dies,” said Guest and he further explained that “it could be that the location is simply better for survival – deeper water that is outside the storm tracks, for example. The coral communities could possess biological or ecological characteristics that make them more resilient and able to resist damage. Or there may be ecological processes at play, which means that the reef community is able to rebound more quickly after a disturbance. Identifying cases in which individuals or communities perform better than their neighbours, despite being at equal risk, is common in public health and medical fields and using a similar approach in ecology can help us to identify areas that can be prioritised for conservation.”

Peter Edmunds, from California State University Northridge, who studies coral reefs in St. John, US Virgin Islands in the Caribbean, and in Moorea, French Polynesia, in the Pacific, said he had been blown away by the capacity of the reefs in Moorea to rebound following devastation.

Edmunds who was involved in the study, added “we started working there in 2005, and almost immediately encountered hordes of coral-eating sea stars that quickly consumed the tissue of the corals. By 2010, there was as close to zero coral on the outer reefs as I have seen in my entire career. And yet, within eight years, that coral has regrown. In places, about 80% of the seafloor is now covered by live coral. It is a remarkable example of an oasis. This does not contradict reports of coral reefs suffering huge losses across the world and that the overall situation is very bad. However, there are kernels of hope in places where corals are doing better, or where they are doing less badly than elsewhere, and these places provide us with a focus of attention that might be used to enhance coral conservation efforts.”

In April, Nature, the international journal of science, published a study saying that corals on the world’s largest reef system Great Barrier Reef (GBR) suffered five death events, largely driven by changes in sea level and associated environmental change during the past 30,000 years. The Great Barrier Reef (GBR) lies on the Australian eastern coast.  

The GBR experienced a catastrophic die-off following the extended marine heatwave of 2016, transforming the ecological functioning of almost one-third of the 3,863 reefs that comprise the world’s largest reef system.

Findings of the study reinforce the need for risk assessment for reef ecosystem collapse, especially if global action on climate change fails to limit warming to 1.5-2 °C above pre-industrial levels.

The researchers of the study found that although many corals died immediately from the heat stress, others died more slowly following the depletion of their zooxanthellae, the yellowish brown symbiotic algae that live within most reef-building corals.

Zooxanthellae are single-celled creatures that can live in symbiosis with marine invertebrates, such as corals, jellyfish, and sea anemones.

Coral death was highly correlated with the amount of bleaching and level of heat exposure, with the northern third of the GBR most affected. The coral die-off also led to radical changes in the composition and functional traits of coral assemblages on hundreds of individual reefs, with mature and diverse assemblages transformed into more degraded systems.

The authors of the study noted that a full recovery to the pre-bleaching assemblages is unlikely to occur, because many surviving coral colonies continue to die slowly, and the replacement of dead corals will take at least a decade even for fast-growing species.

Moreover, the GBR experienced severe bleaching again in 2017, causing further extensive damage. As such, coral reefs throughout the tropics are likely to continue to degrade until climate change stabilizes, allowing remnant populations to reorganize into heat-tolerant reef assemblages, according to the paper.

Speaking to Daily News Egypt, Terry Hughes, from James Cook University in Australia and a co-author of the study, said: “Our study found that one in every three corals on the Great Barrier Reef, the world’s largest reef system, died following bleaching in 2016. In the following summer in 2017, another 20% died, bringing the total loss to close to half of the corals.”

Hughes added that some types of corals died much more than others, changing the mix of species, adding “branching corals died off the most, reducing coral habitat for fish and other species that depend on corals for food and shelter.”

He explained further that it takes 10 years at least for the fastest corals to recover, and much longer to replace 50 or 100-year-old corals that have died. He expected the “mix of species to continue to change as global warming intensifies.”

The GBR supports an important tourism industry that employs 65,000 people. Hughes believes that the only way to protect the GBR from global warming is to immediately reduce greenhouse gas emissions, and to reach the Paris Agreement target of no more than 1.5-2 °C of global average warming. “If we can achieve that target, we’ll still have a GBR, but with lower diversity and a very different mix of mainly heat-tolerant species,” he added.

Achieving the ambitious Paris Agreement climate goal to limit global warming to 1.5 °C above pre-industrial levels will require substantial investment in mitigation efforts.

To limit the warming to 1.5°C “we need to fully eliminate our emissions of the main cause of warming: carbon-dioxide,” according to Joeri Rogelj, from the International Institute for Applied Systems Analysis explaining to Daily News Egypt that “this means that global carbon-dioxide emissions have to be brought to zero levels. This does not mean that no carbon-dioxide is produced at all. However, it means that we need to actively remove as much carbon-dioxide from the atmosphere as we are putting into it.”

Rogelj added that there are various ways and strategies to achieve this goal. “Two aspects that jump out is the need for reaching zero global carbon-dioxide emissions by roughly mid-century and a need to strongly limit energy demand by adopting efficient and sustainable lifestyles,” he said.

Moreover, a recent large-scale analysis suggest that local management actions may not protect coral reefs from climate change, yet most local threat-reduction strategies have not been tested experimentally. The study, which was published on Monday in ecology and evolution journal part of Nature journal, shows that removing coral predators is a common local action used by managers across the world, and that removing the corallivorous snail Coralliophila abbreviata from Caribbean brain corals before a major warming event, increased coral resilience by reducing bleaching severity resistance and post-bleaching tissue mortality (recovery).

Findings of the study highlight the need for increased evaluation and identification of local interventions that improve coral reef resilience. Despite the importance of coral reefs for biodiversity and services to humans, local, and global threats are degrading reefs across the globe. According to the study, in response to local stressors, such as overfishing and pollution, coral reef managers commonly conduct threat-reduction strategies—an approach grounded in ecological theory suggesting that multiple stressors act additively or synergistically to reduce a system’s resilience to physical stress.

Although several studies support this conservation strategy for coral reefs, recent global studies argue that local actions have not improved coral reef outcomes in response to global warming and mass bleaching events, such as those in 2015 and 2016. However, these large-scale studies may fail to detect the positive effects of specific actions made at smaller scales, such as individual coral colonies. Furthermore, few experimental studies have tested whether localised management interventions can increase coral resilience to climate stress, including both resistance and recovery. After several consecutive years of mass coral mortality following climate-related warming, there is an urgent need to understand whether local actions can help to protect corals from climate change, according to the paper.

Topics: coral reef

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