“It’s one of the most gripping things about the reef. But while we’ve lost 50 per cent on average, in some places we’ve lost all the corals and there’s just nothing,” Professor Baird said.
Like a reduction of trees within a forest, a drop in the number of corals thins out the reef’s canopy, which provides habitat for fish and other species crucial to a vibrant ecosystem, said Centre of Excellence senior research fellow Tom Bridge.
“It’s not just that the corals are living themselves, they provide the structure and the three dimensional habitat that supports all the other biodiversity that inhabits coral reefs,” Dr Bridge said.
“If you get a decline in corals, then you will get feedbacks which will result in the loss everything else. One example is with the schools of small fish that live around corals, when corals become too sparse they have to travel large distances from coral to coral and fall victim to predators.”
Eminent forest ecologist David Lindenmayer explained the impact when keystone species like old growth trees and corals are lost from an ecosystem.
“It has an enormous effect. With trees those fundamental ecosystem services like hollows for animals, lots of pollen, nectar and carbon into the soil are lost which cannot be replaced even if smaller trees start to grow back,” Professor Lindenmayer said.
Marine heatwaves are hotter and more frequent due to climate change. A run of clear days coincides with a spike in sea temperature causing mass bleachings where corals are baked to death. Large scale bleaching events hit in 2016 and 2017, and others occurred early this year.
Lead author of the Centre of Excellence for Coral Studies report Andy Dietzel said despite the widespread loss visitors to the reef would continue to marvel at colourful arrays of underwater life for years to come, in certain locations. But the pace of climate change would determine the fate of the reef in coming decades.
“I don’t think the tourism industry will run out of semi-healthy reef anytime soon,” Dr Dietzel said. “There is some hope that some coral species can adapt to heat stress. But the current trajectory of climate change is impacting faster than we would have expected and we’re not sure if the corals can keep track with more extreme heatwaves.”
Professor Baird said while some fish species may be able to migrate to deeper or more southern waters to survive, but “where it’s already bleached, the fish are gone forever”.
Mike is the climate and energy correspondent for The Age and The Sydney Morning Herald.