Humanity leaves a great deal of collateral damage in its wake. From Fukushima Daiichi to climate change, destruction is often the common denominator of our footprints. With extinctions on the rise and ecological communities threatened from the Arctic to the Amazon, conservation biologists openly fret that some ecosystems may be too fragile to recover from humanity’s short-sightedness. But, predicting ecosystem resiliency is about as accurate as reading a crystal ball, thanks largely to the dearth of data on the complex species interactions that define ecosystems.
Coral reefs are particularly complex and delicate marine ecosystems that appear particularly vulnerable to disruption. As ecological communities go, they have cast their lot in a peculiar spot: the vast but nutrient-poor waters of tropical seas. Oases in a desert, these reefs shelter fish, algae, seaweed, sponges, crustaceans, mollusks, worms, and starfish, not to mention the corals themselves (odd cousins of anemones and jellyfish). Reefs form when adult corals aggregate in immobile colonies, clustering on shallow rocks and seabeds. Their hard calcareous shells protect them and provide a surface on which new corals and other reef creatures can grow and thrive. Thus, though coral reefs cover less than 0.1% of the ocean’s surface area (all of them could fit inside Nevada), they house over 25% of marine species.
The real core of coral reefs is not the corals themselves; it is the zooxanthellae, microscopic algae that live symbiotically within the corals. Protected and insulated by their coral hosts and caretakers, in return zooxanthellae share with the corals the nutrient bounty they derive from the sun’s rays via photosynthesis. This partnership, the foundation of coral reef ecology, explains why coral reefs prefer shallow seas lit by the sun’s rays.
The invaluable zooxanthellae are also a canary in the coal mine for reef health. When stressed, coral expel their zooxanthellae. This process, known as “bleaching,” leaves corals ghostly white and deprives them of a major source of nutrients. If the stress is not resolved and the coral do not again take up their zooxanthellae, they will perish and the coral reef ecosystems will degrade. Calcareous reefs will break down. Fish and other creatures leave or die. Coral bleaching is on the rise, with uncertain potential for recovery.
New research published in the journal Science sheds light on the resilience of at least one coral reef from a mass-bleaching event. The isolate Scott Reef lies off Australia’s northwestern coast, over 150 miles from other reefs and 600 miles north of the nearest major metropolitan area. Fifteen years ago, scientists from the University of Western Australia and James Cook University were presented with a rare opportunity to study how this lonesome reef system recovered from a catastrophic bleaching event. In 1998, months of high ocean temperatures triggered coral bleaching globally, and Scott Reef was no exception. Within six months, over 80% of corals in Scott Reef perished after expelling their zooxanthellae. For the next six years, coral coverage across the reef hovered at a meager 10-15%, presumably since the reef’s isolation kept it from recruiting juvenile corals (which can swim) from healthy reefs. However, a decade after the catastrophic bleaching event, juvenile corals began to arrive from distant reefs. By 2010, coral coverage in Scott Reef reached pre-bleaching levels, and brought with it the marine animals and plants typical of a healthy reef ecosystem.
The lesson of Scott Reef is that coral reefs, while fragile, are also resilient. A community that lost 80% of its coral in a bleaching event recovered in less than two decades. But this observation may not hold true for other reefs. During recovery, Scott Reef’s isolation kept it free from human disturbances. In addition, this reef entered the bleaching period with a healthy fish stock, which likely saved it from post-bleaching invasion by the seaweed and sponges that often hamper coral recovery. But many other reefs lack healthy fish stocks, thanks to pollution, the exotic pet trade, and overfishing. Those reefs would kill for Scott Reef’s exile.
Caveats aside, Scott Reef’s short recovery demonstrates that the best way to aid a reef ecosystem after a bleaching and coral die-off may be to get out of the way. Reduce or eliminate as many direct and indirect human disturbances as possible, allow coral survivors to stabilize the reef, and wait for juvenile coral migrants to arrive. Sadly, bleaching events worldwide are on the rise, thanks to warmer temperatures, rising ocean acidity, and extreme weather events. These effects of climate change could combine with direct human meddling to eradicate coral reefs within this century. Species protection laws, new marine sanctuaries, and curbs on pollution could lessen the stresses and slow the damage. But, political support for these measures is tepid. Meanwhile, isolated and resilient Scott Reef will soon have company; it sits above a large natural gas formation that is ripe for harvesting.
Like humans, the end of the last ice age some 10,000 years ago was a watershed moment for coral reefs. While retreating ice sheets exposed new habitats for ancient humans on land, coral reefs colonized the new shallow seas created by a rise in sea level. Yet today, one-third of coral species are at elevated risk of extinction due to climate change and other human activities. It remains to be seen whether this planet is big enough for both of us.
- Gilmour et al. 2013. “Recovery of an Isolated Coral Reef System Following Severe Disturbance.” Science 340: 69-71 and supplementary materials.
- Australian Broadcasting Corporation.