Since the mid-1990s, coral in the Great Barrier Reef has declined by more than 50 percent, and that’s true for virtually every species, at every depth and in every dimension, according to a new study.
The search spanned the 2,300km of the Great Barrier Reef and found a worrying leak at virtually all levels.
“A vibrant coral population has millions of tiny little corals, as well as many large ones – the big mothers who produce most of the larvae,” explains Andy Dietzel of the ARC Center of Excellence for Coral Reef Studies.
“Our results show that the resilience of the Great Barrier Reef – its resilience – is compromised compared to the past because there are fewer children and fewer large adults.”
Similar to old-growth forests, it is these larger corals that marine scientists are most concerned about.
The loss of older corals in this way could have a cascading effect on the entire reef system, as larger colonies in a population have a disproportionate impact on next generation reproduction and genes, while also providing greater habitat and food for fish and other coral reef life.
“The global decline of large and old trees, for example, implies a loss of critical habitat, food and carbon deposits,” the authors write. But while the size of terrestrial forests has been closely monitored over the years, trends in coral size are rarely examined; it is traditionally hedging.
To fill this gap, the researchers documented the systematic decline in coral abundance in the Great Barrier Reef in size, habitat, sectors and taxa from 1995 to 2017. During that time, the reef experienced several local cyclones, four events of mass bleaching and two major outbreaks of the crown of thorns starfish (not to mention another serious bleaching event that occurred earlier this year).
Studying the vast expanse that is the Great Barrier Reef is obviously a real challenge, and to estimate the size of the colonies, the researchers used the intercept lengths of the line as a proxy.
This means that a line was placed on the reef to measure the overall length of various underlying organisms.
While not a direct measure of coral size, the intercept lengths of the line may indicate changes in the size of the colony below, and because it has been used for so long, the authors say it is “an irreplaceable source of historical demographics.” on corals.
The authors found that coral abundance had decreased dramatically in all colony sizes and in all coral taxa. These changes were most pronounced in the northern and central regions of the Great Barrier Reef, which is where most of the recent mass coral bleaching has occurred.
“We thought the Great Barrier Reef was protected by its size,” says marine biologist Terry Hughes, “but our results show that even the largest and relatively well-protected reef system in the world is increasingly compromised and in decline.” .
The loss of medium and large colonies is of particular concern, as they could halt reproduction and prevent older corals from replenishing dwindling populations. At the same time, the disproportionate loss in the smaller colonies suggests a reduction in the spread of tiny coral larvae.
“The recovery potential of older fruitful corals is uncertain given the increasing frequency and intensity of disturbance events,” write the authors of the current study.
“The systematic decline of smaller colonies across regions, habitats and taxa suggests that a decline in recruitment has further eroded the recovery potential and resilience of coral populations.”
And the recovery window is closing fast. If we don’t reduce our emissions by the end of the century, studies show that destructive bleaching events like those that occurred in 2016 and 2017 could very well occur on an annual basis.
“I think if we can control the warming somewhere between 1.5-2 ° C [above pre-industrial levels], according to the Paris Agreement, we will still have a coral reef, ”Hughes told The Guardian.
“But if we get to 3-4 ° C because of the rampant emissions, we won’t have a recognizable Great Barrier Reef. ”
The study was published in Proceedings of the Royal Society B.