The future of coral does not appear to be bright. Reef-building creatures, which provide the living architecture for some of the world’s most varied ecosystems, are susceptible to climate change and have already suffered significant losses. Their future is becoming increasingly bleak unless they can achieve significant carbon reductions in the following decade.
That isn’t to say that the end is unavoidable. Humans can still help reefs survive in a warming environment, as two new studies show.
“We truly believe our tale is good news,” says Mary Donovan, an Arizona State University professor of conservation science and a principal author on one of the research. “The consequences of climate change on coral reefs are enormous. So being able to identify additional effects [on reefs] puts it in everyone’s hands—we’re saying, human influences are making things worse, but we can also improve things.”
Bleaching is the most climatic severe danger to coral. Coral polyps, the invertebrates that construct the reef structure, have a symbiotic relationship with colored algae, which supply food for them. During maritime heatwaves, however, the polyps expel their algae mates, leaving the reef bare. A healthy reef can recover after a blow, but if conditions deteriorate to the point where it dies completely, the entire structure will disintegrate.
According to Donovan’s research, which was published this week in Science, those bleaching events might be worsened by two prominent local trends: pollution and overfishing. As a result, local restoration initiatives may assist reefs in surviving the majority of heatwaves.
The reefs that suffered the most damage during bleaching episodes had an overgrowth of seaweed, which thrives in nutrient-rich runoff and is generally kept in control by herbivorous reef fish. This seaweed produces toxins that cause the reef to suffer.
“Coral is extremely worried when it gets extremely hot,” Donovan explains, “and if you have anything else stressing them out, that’s pretty bad.”
Bleaching was also linked to an oversupply of sea urchins, which was surprising. Those urchins would typically eat the seaweed under normal circumstances. The urchin population can potentially expand if its predators are overfished.
“There’s a Goldilocks zone where they’re doing more damage than benefit at extremely excessive abundances,” Donovan adds. “There’s nothing left to eat since there are so many of them, and they’ve gorged themselves at the bottom, so they keep descending. They have these strong teeth, for want of a better term, and they begin destroying the reef itself.”
Humans can also assist in the repair of harm.
For one thing, local fisheries managers may conserve seaweed-eating species like parrotfish, surgeonfish, and unicornfish. Donavan claims that this is already happening in Hawai’i’s Kahekili Marine Reserve. “In many of the regions where coral reefs occur, people living near the reefs and the reefs themselves are strongly connected,” she said. Often, society places a great deal of reliance on the reef for subsistence. It’s critical not to portray fishing as a harmful activity. For many individuals all across the world, it’s a question of life and death.”
Reduced nutrient pollution (typically accomplished by cleaning up sewage) might benefit both reefs and human health.
Another study, published in the National Academy of Sciences Proceedings, reveals that people may influence coral evolution towards bleach resistance.
Conservationists have already begun planting tens of thousands of corals from nurseries in the Caribbean and the Great Barrier Reef. In an ideal world, they’d choose corals that looked to withstand heat waves without bleaching. However, bleach resistance must be a feature that persists when a coral is transferred. It’s possible that certain corals were hardy due to their environment or that the shift would stress them out to the point where they’d become vulnerable.
A team in Hawai’i plucked fragments from corals that have survived back-to-back heatwaves and transferred them over the reef to see how to bleach resistance spreads.
It took them many months to adjust to their new surroundings, but they both appeared to retain their heat-resistant qualities in lab testing once they did. Because a coral contains hundreds of genetically similar creatures, the transplant is expected to make the entire piece of reef more heat tolerant as it matures.
The two studies, when taken together, provide a diverse strategy for protecting coral.
“Many of the individuals who are most affected by coral reduction are not in charge of climate policies,” Donovan adds. Massive bleaching occurrences, such as the one that occurred on the Great Barrier Reef in 2015, “leave a lot of people dismayed.” As a result, our findings convey a strong warning that action at all levels is required to ensure the survival of corals.”