Global warming, overfishing and ocean acidification form a major threat to ocean ecosystems, possibly even leading to an oceanic Holocene mass extinction. We might even have to say goodbye to the planet’s last coral reef by 2050 if we’re not careful.
A new study however shows that the currently most used measure to determine reef health, coral cover, is but the last change a reef undergoes before its final collapse, making the first observed sign of degradation too late in the process to stop it.
Eight ecological light switches
The study appearing in the Proceedings of the National Academy of Sciences shows that a reef undergoes eight big changes as a response to overfishing before it finally collapses. These eight thresholds are like ecological light switches, each time one is flipped the chances of recovery are dimmed.
The last switch, coral cover can remain normal until the reefs ecosystem has already begun collapsing making it in practicality a useless warning system. But luckily the researchers have come up with a new early warning system for ecosystem collapse: fish biomass.
From healthy to point of no return
A healthy well protected coral reef has between 1000 and 1500 kilograms of various species of fish per hectare. At a biomass below 1000 kilograms the first warning signs start to appear in the form of an increased amount of seaweed and sea urchins.
Than at 600 kilograms a new threshold is reached which goes all the way down to 300 kilograms per hectare. This is when the reef is at its maximum sustainable yield. Flip the fourth switch by decreasing the fish biomass even further and you’ve managed to put the reef on a slippery slope.
According to the authors flipping this fourth switch is the turning point for reefs. Once it’s flipped there is practically no going back to a healthy reef.
Obviously it is impossible to put a no fishing sign in all coral reefs, since many people rely on the reefs for their livelihoods. But keeping the fish biomass above let’s say 500 kilograms by implementing the necessary rules and regulations does seem possible.
Although this new find might give reef managers a useful tool to properly monitor and safeguard reef health, it is by no means a total package for protecting the world’s reefs. Like with CCD in bees overfishing isn’t the only thing threatening our reefs.
Just take a look at last years’ coral bleaching record in the Caribbean due to ocean acidification and warming of the world’s oceans. The findings do however provide possibly the easiest to implement way to protect and monitor the world’s coral reefs. Now let’s just hope this information actually gets used.
© Jorn van Dooren | www.bitsofscience.org