The Great Barrier Reef has lost half of its corals

The Great Barrier Reef has lost half of its corals, and fish are left with fewer habitats as the reef structure vanishes. In a new study, scientists have determined that populations of small, medium, and large corals have all declined across the Great Barrier Reef in the last three decades.

Study lead author Dr. Andy Dietzel is an expert at the ARC Centre of Excellence for Coral Reef Studies (CoralCoE). He said that while numerous studies have investigated changes in the structure of other species’ populations over centuries, not much is known about the historical changes that have taken place in coral populations.

“We measured changes in colony sizes because population studies are important for understanding demography and the corals’ capacity to breed,” said Dr. Dietzel.

The researchers examined coral communities across the Great Barrier Reef between 1995 and 2017, and found a drastic depletion of coral populations.

“We found the number of small, medium and large corals on the Great Barrier Reef has declined by more than 50 percent since the 1990s,” said study co-author Professor Terry Hughes.

“The decline occurred in both shallow and deeper water, and across virtually all species – but especially in branching and table-shaped corals. These were the worst affected by record-breaking temperatures that triggered mass bleaching in 2016 and 2017.”

According to Dr. Dietzel, one of the major implications of coral size is its effect on survival and breeding.

“A vibrant coral population has millions of small, baby corals, as well as many large ones – the big mamas who produce most of the larvae,” said Dr. Dietzel. “Our results show the ability of the Great Barrier Reef to recover – its resilience – is compromised compared to the past, because there are fewer babies, and fewer large breeding adults.”

The study authors said that better data on the demographic trends of corals is urgently needed.

“If we want to understand how coral populations are changing and whether or not they can recover between disturbances, we need more detailed demographic data: on recruitment, on reproduction and on colony size structure,” said Dr. Dietzel.

“We used to think the Great Barrier Reef is protected by its sheer size – but our results show that even the world’s largest and relatively well-protected reef system is increasingly compromised and in decline,” said Professor Hughes.

Climate change is driving an increase in the frequency of marine heatwaves, and the study showed steep coral declines in the Northern and Central Great Barrier Reef after two mass coral bleaching events struck the region in 2016 and 2017. In early 2020, the southern part of the reef was exposed to record-breaking temperatures.

The researchers conclude that “there is no time to lose.” If we want to save the corals that remain, greenhouse gas emissions must be drastically decreased as quickly as possible.

The study is published in the Proceedings of the Royal Society B.

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By Chrissy Sexton, Earth.com Staff Writer