NEWS

Many factors, such as different environmental conditions and the presence of other marine life, can influence coral resilience and recovery after bleaching events. 

Research efforts looking into coral resilience and climate adaptation have assessed many components that can influence the survival rate of coral reefs. One PI-CASC project observing coral conditions in the Palmyra Atoll is finding that a combination of nutrients from seaweed and bird excrement contribute to the resilience and recovery rate of corals. In a research article published last February from the National Academy of Sciences, a heat-tolerant symbiont algae found in some Pacific Islands was reported to have helped corals become more resilient to heatwave events.  

Some coral species stave off bleaching events better than others, though the reason for this changes from environment to environment. And with more than 6,000 known species around the world, the characteristics for resilience become more complex. 

“It is always hard to predict with a high level of accuracy which species will be more susceptible,” said John Burns, an associate professor of marine science at the University of Hawaiʻi at Hilo and a former PI-CASC researcher. 

Since Hawaiʻi’s last major bleaching event in 2014 and 2015, Burns said that scientists and natural resource managers have been working hard to better understand these variations. Using advanced technologies for monitoring, in addition to standardized survey techniques, is crucial in determining the specific species and habitats that are more vulnerable to heat stress, he added.  

“Past events have also provided insight into certain species, such as Pocillopora meandrina and Montipora dilatata, that were impacted heavily in the 2014-2015 bleaching,” Burns said. 

Tolerant vs. Susceptible 

For scientists, what defines a coral species as stress-tolerant or stress-vulnerable is its mortality rate and recovery rate after a bleaching event. Coral species that are more resilient to heat stress may have some individuals die, but most will survive and recover from any bleaching. Stress-vulnerable coral species have a much more challenging time recovering from bleaching, resulting in more deaths, a decline of coral cover, and loss of habitats. 

The Guam Long-Term Monitoring Program’s 10-year report on the island’s coral reefs, which harbor more than 5,000 coral species, provided examples of stress-tolerant and stress-susceptible species: 

Examples of stress-tolerant corals: 

• Mounding Porites spp. 
• Goniastrea retiformis 
• Porites rus 
• Pocillopora setchelli 

Examples of stress-susceptible corals: 

• Staghorn Acropora spp. 
• Bushy Acropora spp. 
• Stylophora mordax 
• Montipora spp. 

David Burdick, a coral ecologist and research associate at the University of Guam Marine Laboratory who authored the report, said that while there are typical characteristics that distinguish vulnerable and resilient corals, other factors are also at play. 

“Generally, but with some exceptions, faster growing species, like many branching and encrusting corals, tend to be more susceptible to stress. The slower growing species, like the mounding Porites and other mounding corals, tend to be more stress-tolerant,” Burdick said. “It is likely a lot more complicated than that, though. Other aspects of the holobiont, such as the zooxanthellae and the microbial community, also play a role in susceptibility to heat stress and other stressors.” 

A holobiont refers to the symbiotic relationship between a host species and other species that live around or inside of the host, functioning as one unit. In this case, microbes living inside coral may have a beneficial role in the coral’s resilience to thermal stress. Burdick provided an example in which vulnerable taxa, such as Acropora spp. and Montipora spp., were still found to be dominant in certain shallow reef areas around Guam that had been hit with bleaching. 

“In 2021, biologists encountered some reef areas, such as a site near Pati Point and another south of Talo’fo’fo Bay, that still hosted cover of these stress-susceptible taxa, suggesting that these sites are more resilient to thermal stress,” according to the report. Scientists and natural resource managers are looking further into why these sites were spared coral bleaching. 

Adaptation strategies 

While studies on the many factors of coral bleaching resilience are still underway, some research projects are intent on providing natural resource managers with up-to-date monitoring tools to help them manage coral reefs more efficiently and more effectively.  

A PI-CASC funded project led by Monica Moritsch, for example, produced a summary of climate trends and conditions for coral reefs around Guam and American Samoa to assist managers with developing resilience-based management strategies for vulnerable coral cover areas. The project categorized coral sites into four types of management strategies: conservation and restoration of robust corals; restoration of declining corals; conservation of genetic material of robust corals and stressor mitigation; and no clear strategy for declining corals. A science article on the project, published last December, profiled these sites and compiled trend maps for both Guam and American Samoa. This data can also be viewed on an online map tool produced from the project. 

Producing tools and knowledge to keep natural resource managers well-equipped in monitoring coral bleaching is a critical effort in climate adaptation. Browse our “Climate Resources” and “Publications” sections of our website to learn more about PI-CASC products and publications.