A scuba diver records data next to a large pink coral.
Sara Cannon conducted field work on Kiribati’s coral reefs to measure how the ecosystem responded to pressures from the local population. Credit: Simon Donner
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Recent events have taken a toll on Kiribati’s atolls.

Reefs in the western portion of the island nation experienced mass bleaching—when heat stress causes corals to expel the tiny algae that live in their tissues—in 2004–2005 and 2009–2010, followed by a 2014 crown of thorns starfish outbreak. This massive, spiny sea star is a notorious coral predator that can devastate coral communities.

Now coral reef scientists are taking a deep dive into data collected from these stressed-out systems to look for answers about resilience to, and recovery from, climate change, taking into account local activities that influence reef composition and health.

The location of the Gilbert Islands—a group of the 16 westernmost of Kiribati’s 33 islands—makes its reefs susceptible to variations in sea surface temperature from central Pacific El Niño events. “This part of Kiribati is experiencing an increased frequency of heat stress events that hasn’t happened in other places yet,” said Sara Cannon, a Ph.D. candidate at the University of British Columbia. “It gives us a chance to look at what could happen elsewhere as climate change continues to intensify.” So far, reefs in the Gilbert Islands have endured years with prolonged heat stress that is greater than that experienced by 99% of the world’s coral reefs.

Cannon and her adviser, Simon Donner, a climate scientist at the University of British Columbia, teamed up with researchers from Kiribati’s Ministry of Fisheries and Marine Resources Development to survey and compare coral populations at two atolls in the Gilbert Islands that have experienced different degrees of human influence. In a recent paper in PLoS ONE, they describe how the reefs have responded to more than a decade’s worth of disturbances—and how those responses might inform management strategies near and far.

A Tale of Two Atolls

The Abaiang and Tarawa atolls sit just 11 kilometers apart, but they occupy opposite ends of the spectrum of human influence.

The Abaiang and Tarawa atolls sit just 11 kilometers apart, but they occupy opposite ends of the spectrum of human influence.

More than 70,000 people—or nearly 60% of Kiribati’s total population—live in the capital, Tarawa, according to a 2020 census. Of that number, some 90% are concentrated in communities on the atoll’s southern rim. This high population density corresponds to intense pressures on the local reef in the forms of fishing, nutrient loading from sewage, and sedimentation from construction activities.

Abaiang, on the other hand, has a population of around 5,800 people. And although its reefs aren’t completely untouched, Cannon said, “they do have a lot less pressure on them.”

This stark gradient of stressors can be traced to Kiribati’s history of colonialism, according to the study. The British controlled the Gilbert Islands from 1892 to 1979, during which time they built causeways that altered patterns of water flow and sedimentation and actively incentivized mass relocation to Tarawa.

Diving Coral Diversity

To survey the shifting composition of the coral communities in Abaiang and Tarawa, the researchers collected data from 19 study sites at 2-year intervals between 2012 and 2018. (A trip planned for 2020 was postponed indefinitely because of the COVID-19 pandemic.)

Diving 10 meters beneath the water’s surface at each site, scientists laid out a 50-meter tape and identified and measured all coral colonies within 25 centimeters on either side of the tape. Every 50 centimeters along the tape, they photographed an area covering one third of a square meter. Later, they used the photos to calculate the percentage of the reef covered by each species and to count feeding scars from crown of thorns attacks.

Cannon and colleagues found that the reefs in Tarawa, which experience significant pressures from nearby coastal communities, have shifted away from hosting diverse groups of coral and are now dominated by a single, heat-tolerant species called Porites rus. This species, which Cannon described as “weedy…like a dandelion,” emerged from the bleaching events relatively unscathed and accounted for 81% of all coral cover in the Tarawa study sites by 2018.

Close-up of Porites rus, a heat-tolerant species of coral.
Porites rus is a weedy, heat-tolerant species of coral that dominates the reefs of Tarawa, the most populated island in Kiribati. Credit: Simon Donner

In Abaiang, which sees only light human activity, the team found a wider array of species but very low overall coral cover: just 18%. Instead, the sites were overgrown with turf algae. Researchers observed that the diverse, sensitive corals that had previously thrived in Abaiang were almost completely wiped out by the bleaching events. Massive colonies of mounding coral did manage to survive the bleaching in Abaiang; however, they were later decimated by the crown of thorns outbreak.

Tarawa’s ecosystem, the authors write, has likely “settled in” to its current composition, making it all but impossible to recover its previous diversity without human intervention like coral transplanting.

But it’s still unclear what trajectory Abaiang’s continued recovery will take. The authors suggest three potential scenarios: Abaiang’s reefs continue to be dominated by turf algae, a large green algae called Halimeda takes over, or heat-tolerant coral species eventually recover. Only time will tell the outcome.

More Stress, Fewer Problems?

The new findings add evidence to the counterintuitive possibility that more degraded coral reefs may be more resistant to climate impacts.

The findings add evidence to the counterintuitive possibility, raised by other scientists, that more degraded coral reefs—like Tarawa’s—may be more resistant to climate impacts. In other words, local stressors might actually help reefs survive by causing hardier species to thrive. Conversely, policies of strict protection might inadvertently make corals vulnerable to climate disasters by favoring sensitive species.

Whether or not these degraded-yet-resistant reefs are functioning in the way that local communities need them to, however, remains an open question. It’s unclear, for example, whether reefs dominated by Porites rus offer erosion control or support healthy fish populations. And if a disturbance comes along that affects Porites rus, the majority of the reef could be in serious trouble.

“This study brings up the importance of making sure there are clearly defined, and accurate, goals for coral reef management,” said Jessica Carilli, a coral reef scientist with the Naval Information Warfare Center Pacific. Carilli has worked in Kiribati but was not involved with the recent study. “If the goal is simply to maintain a certain amount of live coral cover, then South Tarawa may be succeeding—but the true goal is more likely maintenance of sufficient ecosystem services to support the local human population.”

Going forward, Cannon hopes to continue to study the trade-offs between reef resistance and utility.

—Clara Chaisson (@clarachaisson), Science Writer

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Citation: Chaisson, C. (2021), Degraded coral reefs may be more resistant to climate change, Eos, 102, https://doi.org/10.1029/2021EO163528. Published on 29 September 2021.
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