The Great Barrier Reef in Australia, which is the largest reef system in the world, has lost more than half of its coral population in the past three decades, according to a new study.
The research, published in the journal Proceedings of the Royal Society B, assessed coral communities and their colony size along the length of the Great Barrier Reef between 1995 and 2017, and found that small, medium, and large corals have all declined in the period.
"We measured changes in colony sizes because population studies are important for understanding demography and the corals' capacity to breed," said Andy Dietzel, a co-author of the study from the ARC Centre of Excellence for Coral Reef Studies (CoralCoE) in Australia.
"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 Terry Hughes, another co-author of the study from CoralCoE.
According to the study, 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," Hughes said.
Since the reefs are underwater ecosystems home to several interdependent species, the scientists said the loss of these corals means a loss of habitat, which in turn diminish fish abundance and the productivity of coral reef fisheries.
Dietzel said one of the major implications of coral size is its effect on survival and breeding.
A vibrant coral population has millions of small, younger corals, as well as several large ones, he said.
"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," Dietzel said.
The scientists cautioned 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," Dietzel said.
"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," Hughes added.
According to the researchers, climate change is driving an increase in the frequency of reef disturbances such as marine heatwaves.
The study recorded steeper deteriorations of coral colonies in the Northern and Central Great Barrier Reef after the mass coral bleaching events in 2016 and 2017.
The southern part of the reef, the scientists said, was also exposed to record-breaking temperatures in early 2020.
"There is no time to lose -- we must sharply decrease greenhouse gas emissions ASAP," they concluded.