Since the mid-1990s, corals in the Great Barrier Reef have shrunk by more than 50 percent, and this applies to almost any species, at any depth and at any size, according to a new study.
The study covered 2,300 kilometers of the Great Barrier Reef and found alarming losses at almost every level.
“A living population of corals has millions of small, baby corals, as well as very large ones – the big moms that produce most of the larvae,” explains Andy Ditzel of the ARC Center for Excellence in Coral Reef Research.
“Our results show the ability of the Great Barrier Reef to recover – its resilience – is compromised compared to the past, as there are fewer babies and fewer large breeding adults.”
Like the old forests, these larger corals are most troubled by marine scientists.
The loss of older corals like this can have a cascading effect on the entire reef system, as the largest colonies in the population disproportionately affect the reproduction and genes of the next generation, while providing greater habitat and food for fish and other reefs. animals.
“The global decline of large, old trees, for example, means the loss of critical habitat, food and carbon storage,” the authors write. But while the size of terrestrial forests has been closely monitored over the years, trends in coral size are rarely studied; it is traditionally a matter of coverage.
To fill this gap, researchers documented the systematic decline in coral abundance in the Great Barrier Reef in size, habitat, sectors and taxa from 1995 to 2017. During this time, the reef experienced several local cyclones, four massive bleaching events and two major outbreaks. starfish from thorns (not to mention another severe whitening event that happened earlier this year).
Studying the vast area that is the Great Barrier Reef is obviously quite a challenge, and to calculate the size of the colonies, the researchers used the lengths of the intersections of lines as a proxy.
This means that a line is placed on the coral reef to measure the total length of the various organisms below.
Although not a direct measure of coral size, line crossing lengths may indicate a change in the basic size of the colony, and because it has been used for so long, the authors say it is an “indispensable source of historical demographic data” for corals.
The authors found that coral abundance declined sharply in all colony sizes and all coral taxa. These changes were most pronounced in the northern and central regions of the Great Barrier Reef, where most of the last mass coral bleaching has taken place.
“We once thought that the Great Barrier Reef was protected by its enormous size,” says marine biologist Terry Hughes, “but the results show that even the world’s largest and relatively well-protected reef system is increasingly compromised and in decline.”
The loss of medium and large colonies is particularly worrying, as they could possibly delay reproduction and stop older corals from replenishing declining populations. At the same time, the disproportionate loss in the smaller colonies implies a reduction in the distribution of small coral larvae.
“The potential for recovery of older fruit corals is uncertain given the increasing frequency and intensity of disturbing events,” the authors of the present study write.
“The systematic decline in smaller colonies in regions, habitats and taxa suggests that the decline in recruitment further erodes the recovery potential and resilience of coral populations.”
And the recovery window closes quickly. If we do not reduce our emissions by the end of the century, studies show that destructive whitening events such as those that occurred in 2016 and 2017 could occur annually.
“I think if we can control the warming somewhere between 1.5-2 ° C [above pre-industrial levels]”According to the Paris Agreement, then we will have a reef again,” Hughes told The Guardian.
“But if we get to 3-4 ° C due to unlimited emissions, then we will not have a recognizable Great Barrier Reef. ”
The study was published in Notices of the Royal Society B.