How humans affect coral reef recovery from natural disasters

The world’s coral reefs are both stunningly beautiful and vital to ocean health, hosting a huge diversity of fish and marine life. And they are, as they always have been, under pressure from periodic natural disasters. However, a coral reef’s ability to recover from unavoidable and often unpredictable natural disasters, like hurricanes and tsunamis, may depend on human activities including fishing and pollution. UC Davis marine biologist Mike Gil is one of the scientists working to understand how reefs recover from natural disturbances in the presence of unnatural, man-made stressors.

Recently, Gil worked with an international team of scientists to carry out an unusual experiment where they could reproduce the effects of overfishing, sedimentation and pollution on a reef already recovering from natural disasters.

The team worked from the French research station on the island of Moorea in French Polynesia, where they could study a reef that had already been impacted by two natural events: an outbreak of the “Crown of Thorns” starfish, which eat the tiny animals that build coral reefs leaving empty coral skeletons, and a cyclone that broke up much of the dead coral skeleton.

“It’s a system that was hit pretty hard,” Gil said.

On the other hand, the remote reef is not yet overwhelmed by human activity. That made it a good place to study the effects of humans on a recovering reef.

Collecting coral “nubbins”

To reproduce a recovering piece of reef, Gil and his colleagues collected small pieces of coral, or “nubbins” and planted them, along with terracotta tiles, onto cinder blocks on the reef.

Gil talks about collecting coral in this video.

The experiments needed to be in the open ocean, and not in a lab, because much of reef recovery happens as tiny organisms floating in the ocean settle out and start new colonies.

Pollution from sewage or agricultural runoff contains nutrients, so to replicate pollution they put added fertilizer on the tiles. To reproduce sediment runoff – a consequence of developing coastlines with concrete and roadways – they dumped sediment on top of the experimental tiles. And to mimic the effects of overfishing, they put cages around their test corals to keep out the bigger fish.

If you think a research gig in the South Seas sounds pretty nice, moving approximately a ton of rough cinder blocks on and off a small boat in bad weather sounds less so. Gil tells you what that was like in this video.

What makes understanding reefs complicated is that these different effects – overfishing, pollution, sediments – all play into each other with effects that are hard to predict, Gil said. For example, nutrients from pollution can boost the growth of algae that smother coral. But overfishing can also remove herbivorous fish, leading to a further boom in algae on the reef.

Recovering reef community affected

All these factors can affect the community that emerges as the reef recovers, Gil said.

“Not only can humans kick a reef while it’s down, but the emergent community depends on the mix of stressors,” he said. “It’s difficult to say how the system will respond – sometimes the combination of two stressors can be worse than the sum of them both, sometimes they can weaken one another.”

The team’s results were published recently in the journal Coral Reefs, and add to the body of evidence about how human interference impacts reefs. That knowledge may be useful in helping to save the world’s remaining reefs.

Coauthors on the study, which was supported by grants from the French Embassy in the U.S. and the National Science Foundation, are: Silvan Goldenberg of the University of Adelaide, Australia; Anne Ly Thai Bach, National Center for Scientific Research (CNRS), France; Suzanne C. Mills, Laboratoire d’Excellence CORAIL, Tahiti, France; and Joachim Claudet, CNRS and CORAIL, France.

For more about Mike Gil’s work on coral reefs, subscribe to his Youtube channel or go to



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