Researchers at the ARC Coral Reef Research Center of Excellence in Queensland, northeastern Australia, assessed coral communities and colony sizes along the Great Barrier Reef between 1995 and 2017, finding that virtually all coral populations were depleted.
Coral reefs are some of the most vibrant marine ecosystems on the planet, with a quarter to one third of all marine species depending on them at one point or another in their life cycle. The Great Barrier Reef, the largest coral reef in the world, covers an area of almost 133,000 square miles and is home to more than 1,500 fish species, 411 hard coral species and dozens of other species.
“We found that the number of small, medium and large corals in the Great Barrier Reef has declined by more than 50% since the 1990s,” said co-author Terry Hughes, Distinguished Professor at the ARC Coral Reef Research Center of Excellence. Reefs are fundamental to the health of marine ecosystems – without them, ecosystems are destroyed and marine life dies.
Coral population size is also considered vital when it comes to coral reproduction capacity. “There are millions of small young corals in a living coral population, as well as many large ones – the kind of mummies that produce most of the young,” explained Andy Dietzel, a doctoral student at the ARC’s Coral Reef Center of Excellence.
“Our results show that the Great Barrier Reef’s ability to recover – its resilience – is at risk compared to the past because it now has fewer calves as well as large breeding adults,” he added.
The experts found that population decline occurred in both shallow and deep-sea corals, but branched and tabular corals, which provide habitat for fish, were most affected by the massive bleaching in 2016 and 2017.
High ocean temperatures are the main cause of coral bleaching: corals turn white as a result of stressful reactions to too warm water. Bleaching does not kill corals immediately, but if temperatures remain high, the coral will eventually die, destroying natural habitat for many species of marine life.
The Great Barrier Reef has suffered several massive bleaching events over the past five years, and experts say the southern part of the reef was also hit by record high temperatures in early 2020.
“We used to think that the Great Barrier Reef was protected by its enormous size, but our results show that even the largest and relatively well-protected reef system in the world is increasingly threatened and declining,” concluded Hughes.
The authors of the report warned that climate change is increasing the frequency of reef degeneration. “There is no time to waste — we must dramatically reduce greenhouse gas emissions as soon as possible,” they warn in an article published in the Proceedings of the Royal Society.