Coral skeletons improve reef recovery after bleaching

UC Santa Barbara scientists have conducted research into how coral reefs are affected by different kinds of natural disasters. The study, led by doctoral student Kai Kopecky and published in the journal Ecology, found that coral is less likely to recover from bleaching than from storms, even when the mortality rate is similar between the two events.

“Natural disasters can devastate a region, abruptly killing the species that form an ecosystem’s structure,” wrote the researchers. “While fires scorch the landscape to the ground, a heatwave leaves an army of wooden staves in its wake. Storm surges and coral bleaching do something similar underwater.”

Most shallow-water corals host symbiotic algae that provide them with food in exchange for a safe home and nutrients. However, extreme conditions can cause the coral to expel the algae, known as bleaching, which can be fatal.

The researchers studied coral and their reef ecosystems around the island of Moorea, French Polynesia, where they have been conducting research since the late 1980s. During Kopecky’s second visit to the island, a major bleaching event occurred, and he noticed that seaweed, a major competitor with coral for space on the reef, began colonizing the bleached skeletons left behind.

Kopecky wondered whether the presence of the skeletons was setting the reef on a pathway toward a more algae-dominated state. Previous work at Moorea had shown that tropical reefs can host either coral- or seaweed-dominated communities. 

These distinct states are resilient to small disturbances, but a large shock can flip the ecosystem from one to the other in a process called hysteresis, after which the reef won’t revert to its previous state even if conditions do. The system finds a new equilibrium.

To compare reef dynamics after a bleaching event, which leaves skeletons in place, and after a storm, which scours the reef bare, Kopecky developed a mathematical model using a system of five differential equations to capture the transition between empty space, live and dead branching coral, and seaweed cover on the reef.

“The results were telling,” said Kopecky. “Just the fact that those skeletons are left on the reef results in these fundamentally different patterns of recovery.”

Coral skeletons play a protective role for young algae by shielding them from herbivores that would otherwise consume them. The crevices in the coral structure provide a safe haven for the algae to grow and spread, which is not possible in the open reef. 

However, this protection does not appear to provide the same benefits to young coral. The authors of the study suggest that coral does not face as much pressure from predators as algae does. Furthermore, algae can outgrow coral when given the opportunity.

Holly Moeller, an assistant professor of ecology, evolution, and marine biology, explained: “Coral is literally laying down rock, while the algae are mostly just fast-growing, soft, leafy material.”

Reef buildup is a slow process, and new growth incorporates dead skeletons into the larger reef structure. However, bleaching kills a lot of coral at once, particularly the oldest and youngest, and the skeletons eventually become brittle through erosion. This is not a sturdy foundation for young coral to build upon.

The idea of removing dead coral skeletons to promote coral recovery is gaining popularity in other ecosystems. 

“Think of prescribed fires or the thinning of dead trees in forests so that the system is more resilient to future disturbances,” explained Kai Kopecky, a postdoctoral researcher at UC Santa Barbara.

However, coral skeletons provide numerous benefits, including providing habitats for various types of animals. Additionally, some evidence suggests that the structural complexity of a reef correlates with faster coral recovery. “The effect really depends on what the nature of that structure is,” Kopecky said. “Those aspects need to be taken into consideration before you go out and just start jackhammering the reef.”

The researchers have ongoing experiments in Moorea, including one that examines how the reef recovers when dead coral skeletons are removed. Others are testing the assumptions Kopecky used to create his model. For example, how much does dead coral actually reduce herbivory? And how do the skeletons affect the growth of living corals?

Moeller believes that the study is a classic example of the value of mathematical models in ecology. Coral can live for hundreds of years, and reef recovery can take decades. “That’s just not an experiment that you can realistically do,” she said. “But if you have a model, and you trust the way you set that model up because you’ve done other experiments, then you can make these projections decades into the future.”

Coral reefs are incredibly diverse ecosystems that are home to many different species of marine animals and plants. However, these ecosystems are under threat from a phenomenon known as coral bleaching. Coral bleaching occurs when the coral’s symbiotic algae, which provide them with food and color, are expelled due to changes in temperature, light, or nutrients.

When coral bleaches, it turns white, loses its color and may eventually die. The loss of coral has serious implications for the ocean’s biodiversity, as well as the millions of people who depend on these ecosystems for food and income.

Climate change is the primary cause of coral bleaching. As ocean temperatures rise, the coral becomes stressed, which can cause it to expel its algae. Other factors that can cause bleaching include pollution, overfishing, and nutrient runoff.

Scientists are working to understand how to protect coral reefs from bleaching. One approach is to reduce greenhouse gas emissions, which would help slow the rate of warming oceans. Another is to reduce pollution and overfishing in coral reef areas.

In summary, coral bleaching is a serious problem that threatens the health of coral reefs and the many species that depend on them. While there is no easy solution to this problem, scientists are working hard to find ways to protect these important ecosystems from further damage. It’s up to all of us to take action to protect the ocean’s biodiversity and the millions of people who rely on it.

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