Article by Shreya Dasgupta.
Amid all the gloomy news about coral reefs from around the world, a glimmer of hope has emerged: scientists have identified coral reef “oases” that are flourishing despite threats.
From mass bleaching events caused by a climate change-induced rise in ocean temperatures, to threats from overfishing, pollution and invasions from coral-devouring sea stars, coral reefs are under attack. But some coral reefs seem to be doing better than their neighbors, according to a new study published in the Journal of Applied Ecology.
“Over the last 12 to 24 months, bad news about coral reefs has appeared at an alarming rate, notably because of the severe coral bleaching that occurred on the Great Barrier Reef due to the 2016 El Niño,” study co-author Peter Edmunds, a marine biologist at California State University, Northridge, U.S., said in a statement. “The overall message is ‘Oh my goodness, coral reefs all over the world are dying, and many have already gone.’ We felt there was a fragment missing from that story.”
To fill this gap, Edmunds and his colleagues analyzed data from long-term reef-monitoring programs in the Pacific and western Atlantic, and identified 38 coral reefs that have either “escaped,” “resisted” or “rebounded” from declines in coral cover while neighboring coral reefs have not. These reef oases offer hope in times of rapid coral cover loss, the researchers say.
“I think the concept of oasis reefs really resonates with any coral reef ecologist who has dived extensively on a wide range of reefs,” James Guest, a marine biologist at Newcastle University, U.K., told Mongabay. “Personally, my experiences working in Singapore — one of the most heavily impacted coral reef environment in the world — made me aware of how some reefs can be remarkably resistant despite chronic and acute disturbances.”
Crown-of-thorns sea stars have led to the decline of many coral reefs. Image by Rhett A. Butler/Mongabay.
The team defines “escape oases” as coral reefs, such as those in Hawaii’s Molokini crater, that have managed to avoid major disturbances largely because of the physical and environmental characteristics of the site that the reefs are located in. “Resist oases” are coral communities that appear to tolerate environmental disturbances because of specific traits of the corals at the site. For example, some reefs in the Florida Keys occur in areas with highly variable turbidity and temperature conditions, which the authors say “might have favored higher tolerance to acute thermal anomalies” — that is, they’re more resistant to higher-than-normal sea temperatures.
“Rebound oases” are corals that have suffered damage like many other reefs, but have recovered quickly to a better state. Some of the most striking examples are the coral reefs in Moorea, French Polynesia, in the Pacific. The reefs there went from having 47 percent coral cover in 2005 to less than 1 percent in 2010, mainly because of crown-of-thorns starfish (Acanthaster planci) chomping through the corals, and the impact of Cyclone Oli in 2010. But by 2015, the coral cover had rebounded to about 54 percent.
“We started working there in 2005, and almost immediately encountered hordes of coral-eating sea stars that quickly consumed the tissue of the corals,” Edmunds said. “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 percent of the sea floor is now covered by live coral. It is a remarkable example of an oasis.”
At other Indo-Pacific reefs that have recovered well, Guest said that two of the most important things that have helped their recovery are “the amount of herbivorous fish, as they help keep reef surfaces clear of seaweeds that can compete with baby corals, and the amount of new baby corals ‘recruiting’ to the reef.”
“If you have a high supply of new corals each year and if the reef is kept reasonably clean by herbivores, then reefs stand a good chance of recovering,” he said.
Reefs without these characteristics may also rebound.
“The reefs I studied in Singapore are interesting in that they have pretty low rates of recruitment of new corals and low rates of herbivory, but they recovered well from a big coral bleaching event in 1998,” Guest said. “I think what happens here is that most corals in Singapore are quite stress tolerant and that when they get hit by bleaching, only part of the colony dies. The remaining fragments that are left over can grow back very quickly so coral cover returns to normal in a few years.”
While these success stories do not discount reports that many coral reefs are under grave threat, they do offer examples of places where corals are doing better, or not as bad, as coral communities elsewhere. “These places provide a focus of attention that might be used to enhance coral conservation efforts,” Edmunds said.
The study’s results also show that coral reefs vary a lot in their response to disturbances, even at very small scales of tens to thousands of meters, Guest said. “The reasons why one reef survives may be very different from another reef and these differences may be important in informing management decisions,” he said. “We really need to have more long-term data on coral reefs be made publicly available.”
The team is hopeful that the framework it has developed to identify the coral reef oases will be helpful in pinpointing oases across other ecosystems as well.
“The motivation for me to develop this method was to help managers and scientists figure out which reefs are doing well and which reefs are doing poorly, and then to figure out what drives these differences and decide what actions to take in terms of protection and conservation,” Guest said. “I really hope the paper is useful to [conservation] managers and that it inspires others to examine the ideas in other ecosystems.”
Corals off Komodo islands, Indonesia. Image by Rhett A. Butler/Mongabay.
James R. Guest, Peter J. Edmunds, Ruth D. Gates, Ilsa B. Kuffner, Andreas J. Andersson, Brian B. Barnes, Iliana Chollett, Travis A. Courtney, Robin Elahi, Kevin Gross, Elizabeth A. Lenz, Satoshi Mitarai, Peter J. Mumby, Hannah R. Nelson, Britt A. Parker, Hollie M. Putnam, Caroline S. Rogers, Lauren T. Toth. A framework for identifying and characterising coral reef “oases” against a backdrop of degradation. Journal of Applied Ecology, 2018; DOI: 10.1111/1365-2664.13179