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Coral reefs: lessons from the past
A couple of years ago, eight tonnes of rock from Indonesian Borneo were shipped back to the Museum. Although famed for its lush tropical forests, it was the area’s surrounding sea that caught our scientists’ imagination.
The ocean around Borneo is a hotspot of biodiversity, and the collected rocks contain evidence of strange life forms, revealing past environments but also clues to the future of the underwater world.
Making up the bulk of the rocks collected were fossils of coral reef ecosystems that revealed just how different undersea life was 12 million years ago.
‘When you think about coral reefs you imagine turquoise water and beautiful colourful reefs. But these corals were living in muddier places, and maybe deeper places, so there wasn’t as much light,’ says Museum palaeontologist Dr Ken Johnson.
Dr Johnson coordinated the EU-funded Throughflow project, which involved a network of international institutions using the fossil record to understand the origins of marine diversity in Southeast Asia, also known as the Coral Triangle.
The ancient diversity of the Coral Triangle can tell us much about how life has adapted to changing conditions in the past, and how life may well adapt again in the future.
Dark survivors
There are still corals around today that live with less light in deep or murky waters, both in the Coral Triangle and elsewhere, including the turbid beaches closer to the shores next to the Great Barrier Reef in Australia.
Dr Johnson adds:
‘These corals were the way things used to be, possibly before any corals moved into the clear, light water people imagine for modern reefs. Now humans have changed the environment so much that clear water might not work anymore [for corals] in many places.’
The corals found in murkier habitats may be less sensitive to environmental change. ‘As humans impact the oceans more and more, the clear-water reefs might be the ones that die out, and these darker reefs might be the future,’ says Dr Johnson.
Coral reefs are threatened by overfishing, pollution, invasive species and ocean warming and acidification caused by rising carbon dioxide levels. In the future we may therefore see clear-water reefs die out and darker reefs surviving. These darker reefs have a rich biodiversity, but it is very different from the shallow reef ecosystems we know so well.
Reef builders
It’s difficult to know exactly how corals across the world will react to environmental changes, but by looking at how reefs are created today scientists can get some idea.
The most abundant coral in reefs today is Acropora, a branching form with more than 140 species. Common names for Acropora are table coral and staghorn coral. The tiny animal colonies that create the branching solid forms cause them to grow quickly, up to 10 centimetres per year. Any branches breaking off the colony can anchor to the substrate and quickly set up a new colony.
A reef in modern-day Indonesia constructed from Acropora © fenkieandreas/ Shutterstock.com
These corals have been around for millions of years, but they didn’t become dominant reef-builders until about three million years ago. At this time something in the Earth’s climate shifted and a cycle of ice ages began. As ice caps grew and receded on an approximately 100,000-year cycle, sea levels rose and fell by up to 125 metres.
This repeated change in sea level meant that reefs that had taken millennia to form were submerged deeper into the ocean or exposed on dry land. Many coral species failed to cope with these dramatic changes but the fast-growing Acropora thrived.
Losing battle
Although Acropora survived past environmental changes they are not doing so well in the face of today’s challenges. They are easy prey for fish that can chomp their way through corals and they are susceptible to diseases and coral bleaching - the loss of their symbiotic algae that help provide them with energy.
‘Now that we’ve artificially changed the ocean conditions Acropora are sensitive to what we’re doing,’ says Dr Johnson. ‘The corals that are actually building the modern-day reefs are dying. In the Caribbean they’re almost extinct.’
The decline of corals in the Caribbean has been well documented over the last couple of decades, but Dr Johnson has a plan to extend the evidence further back using archives of some of the first underwater photographs.
Watch the audio slideshow to find out more about the project:
Too little, too late?
Coral reefs are declining globally, and while different types of coral might eventually replace them, it will take time and there will be an inevitable loss of biodiversity. Understanding corals, and the effect we’re having on them, may be one way to protect them.
The Museum has a large collection of ancient and modern coral specimens that was first started in the eighteenth century and is still growing in size. The new fossil corals from Borneo have added significant ancient material to the collection, and a huge donation of modern corals from the Chagos Islands in the Indian Ocean will also help us to understand the largest coral atoll in the world - one of the last pristine coral habitats.
Material collected during the Victorian era is also being looked at again using modern technology. CT scanning can examine a specimen without needing to cut it open, revealing details such as growth rings that can infer coral environments and animal burrows that can be indicators of reef health.
The Museum’s collections can support research into corals for centuries, but the global decline in corals may put an end to any future fieldwork and research into these underwater communities. Dr Johnson does not hold much hope for the future of coral reefs.
‘In the next century, maybe all coral reef researchers will be palaeontologists,' he says.
