Since the mid-1990s, corals in the Great Barrier Reef have fallen by more than 50 percent, and this applies to virtually every species, at any depth and in any size, according to a new study.
The research spanned the entire 2,300 kilometers of the Great Barrier Reef and found a disturbing loss at virtually every level.
“A living coral population has millions of small baby corals as well as many large ones – the large mamas that produce most of the larvae,” explains Andy Dietzel of the ARC Center of Excellence for Coral Reef Studies.
“Our results show the Great Barrier Reef’s ability to recover – its resilience – is compromised over the past because there are fewer babies and fewer large breeding adults.”
Similar to ancient growth forests, it is these larger corals that marine scientists are most concerned about.
Losing older corals like this can have a cascading effect on the entire reef system, as the largest colonies in a population disproportionately affect reproduction and the next generation of genes, while providing greater habitat and food for fish and other reef life.
“The global decline in large, old trees, for example, entails a loss of critical habitat, food and carbon storage,” the authors write. However, while the size of terrestrial forests has been closely traced over the years, trends in coral size are rarely investigated; it is traditionally about coverage.
To fill this gap, researchers documented the systematic decline in coral abundance in the Great Barrier Reef across size, habitats, sectors and taxis from 1995 to 2017. During that time, the reef experienced several local cyclones, four mass bleaching events and two major eruptions of the crown of thorns. starfish (not to mention another serious bleaching incident that occurred earlier in the year).
Studying the Great Barrier Reef is, of course, quite a challenge, and to estimate the size of the colonies, researchers used line-intercept lengths as a proxy.
This means that a line was placed on the coral reef to measure the overall length of various organisms below.
Although not a direct measure of coral size, line cut lengths may indicate shifts in the underlying colony size, and because it has been used for so long, the authors say it is “an irreplaceable source of historical demographic data” about corals.
The authors found that an abundance of corals had fallen sharply across 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 recent mass of coral bleaching took place.
“We used to think that the Great Barrier Reef is protected by its large size,” says marine biologist Terry Hughes, “but our 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 of particular concern as they may impede reproduction and stop older corals from replenishing shrinking populations. At the same time, the disproportionate loss in smaller colonies suggests a reduction in small coral larvae spreading around.
“The potential for recovery of older fecund corals is uncertain given the increasing frequency and intensity of disturbance events,” the authors of the current study write.
“The systematic decline in smaller colonies across regions, habitats and taxis suggests that a decline in recruitment further eroded 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 bleaching events such as 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 reef, ”Hughes told The Guardian.
“But if we get to 3-4 ° C due to unrestrained emissions, we do not have a recognizable Great Barrier Reef. ”
The study was published in Procedures of the Royal Society B.