(CN) — The iconic Great Barrier Reef, the world's largest collection of coral reefs and a critical habitat for diverse ocean life, has lost at least half of its corals in the past three decades, according to a study released Tuesday.
Researcher Andy Dietzel of the ARC Centre of Excellence for Coral Reef Studies and his colleagues assessed changes in the health of coral communities and their colony size across the massive reef system between 1995 and 2017.
Compared to other ecosystems, the health of the coral populations has not been studied as extensively over time as is needed, according to Dietzel.
"We measured changes in colony sizes because population studies are important for understanding demography and the corals' capacity to breed," Dietzel, who is the lead author of the study, said in a statement.
Study co-author Terry Hughes of ARC Centre said the team of researchers found high levels of coral depletion across the system.
“We found the number of small, medium and large corals on the Great Barrier Reef has declined by more than 50% since the 1990s," Hughes said, adding that decline was seen in all species of coral in both shallow and deep water.
“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.
Branching and table-shaped corals, which saw the largest declines in size, provide critical habitat infrastructure for ocean life, according to the study.
“Declines in the abundance of large colonies thus reduce the productivity of reef ecosystems, and fisheries, both directly, through declines in the availability of coral gametes, larvae and recruits, which constitute important sources of food for fish and other reef organisms including corals, and indirectly, through the loss of structural complexity and habitat,” the study authors wrote.
The coral system, which sits off the northeast coast of Australia, hosts abundant and productive fisheries that could also see declines as the reef diminishes in size, according to the study published Tuesday in the journal Proceedings of the Royal Society B.
Breeding activity will be disrupted as coral size fluctuates, according to Dietzel.
"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," Dietzel 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."
According to the study, more research is needed to determine the cause of changes in populations within the coral habitats, for example whether population declines are a result of low recruitment from species or high mortality after species settle in.
Researchers said more studies are needed examining changes in demography within the coral habitats.
"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.
Coral colonies in the northern and central Great Barrier Reef were affected by record-breaking heat waves in 2016 and 2017, which triggered mass bleaching across the reef system, according to the study.
The southern section of the reef system was impacted by a marine heat wave in early 2020.
Study authors said climate change is frequently the main driver of disturbances, such as marine heat waves, in the reef system.
"There is no time to lose — we must sharply decrease greenhouse gas emissions ASAP," the study authors said in a joint statement.
In an email, Dietzel said the study doesn’t directly link disturbances such as cyclones and mass bleaching to changes in coral colony size but they are “likely drivers” of change in coral populations.