The research spanned the entire 2,300 kilometers of the Great Barrier Reef and found a worrying loss at nearly every level.
“A vibrant coral population has millions of small and many large baby corals – the large mums that produce the most larvae,” explains Andy Dietzel from the ARC Center of Excellence for Coral Reef Studies.
“Our results show that the Great Barrier Reef’s ability to recover – its resilience – is compromised compared to the past with fewer babies and fewer large adult breeders.”
Similar to old growth forests, it is these larger corals that marine scientists are most concerned about.
The loss of older corals like this one could have cascading effects on the entire reef system, as the largest colonies in a population disproportionately affect reproduction and the genes of the next generation while providing more habitat and food for fish and other reef life.
“The worldwide decline of large old trees implies, for example, the loss of critical habitats, food and carbon stores,” the authors write. While the size of the land forests has been carefully tracked over the years, trends in coral size are rarely studied. It’s traditionally about reporting.
To fill this gap, the researchers documented the systematic decline in coral abundance in the Great Barrier Reef in terms of size, habitats, sectors, and taxa from 1995 to 2017. During this time, the reef saw several local cyclones, four mass bleaching events, and two major crown thorns eruptions – starfish (not to mention another major bleaching event that occurred earlier this year).
Obviously, studying the vastness of the Great Barrier Reef is quite a challenge. To estimate the size of the colonies, the researchers used line segment lengths as a proxy.
This means that a line has been drawn across the coral reef to measure the overall length of various organisms below.
While this is not a direct measure of coral size, line intersection lengths can indicate shifts in underlying colony size. Because it has been used for so long, the authors say it is “an irreplaceable source of historical demographic data” on corals.
The authors found that coral abundance had decreased sharply in all colony sizes and coral taxa. These changes were most pronounced in the northern and central regions of the Great Barrier Reef, where most of the recent mass coral bleaching has occurred.
“We used to think that the Great Barrier Reef was protected by its sheer size,” says marine biologist Terry Hughes. “However, our results show that even the largest and relatively well-protected reef system in the world is increasingly endangered and in decline.”
The loss of medium and large colonies is of particular concern as it could potentially affect reproduction and prevent older corals from replenishing shrinking populations. At the same time, the disproportionate loss in smaller colonies suggests a reduction in the spread of tiny coral larvae.
“The potential for restoring older fertile 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 in regions, habitats and taxa suggests that a decline in recruitment has further undermined the recreational potential and resilience of coral populations.”
And the restore window closes quickly. If we don’t cut our emissions by the end of the century, studies show that destructive bleaching events such as those that occurred in 2016 and 2017 can very well happen annually.
“I think if we can control the warming anywhere between 1.5 and 2 ° C. [above pre-industrial levels]Then after the Paris Agreement we still have a riff, “Hughes told The Guardian.
“But when we get to 3-4 ° C. Due to the uninhibited emissions, we then have no recognizable Great Barrier Reef. ”
The study was published in the Royal Society procedure B..
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