IELTS reading practice test 4

Gardening under the sea

Coral reefs cover less than one per cent of the ocean floor, yet they support around a quarter of all marine species. They also protect coastlines from waves and provide food and income for hundreds of millions of people. Over recent decades, however, many reefs have been damaged by rising sea temperatures, pollution and storms. When water becomes too warm, corals expel the tiny algae that live inside them and give them their colour. This process, known as bleaching, does not kill the coral immediately, but if high temperatures continue for several weeks, the coral may starve.

In response, scientists and volunteers have begun to restore reefs in much the same way that foresters replant trees. The most common method is 'coral gardening'. Small fragments of healthy coral are attached to underwater frames or ropes, where they grow in nurseries for between six and twelve months. They are then fixed onto damaged parts of the reef using cement or cable ties. Marine biologist Dr Rafael Ortiz, who has run nurseries in the Caribbean for fifteen years, says that well-managed nurseries can grow fragments several times faster than corals grow in the wild.

Other teams are experimenting with different approaches. In Australia, researchers led by Professor Amy Chen collect coral spawn, the eggs and sperm released by corals during a few nights each year, and raise millions of baby corals in floating pools before releasing them onto damaged reefs. Professor Chen points out that this method allows restoration at a much larger scale than planting fragments by hand. Meanwhile, engineers in Indonesia have installed thousands of steel structures, nicknamed 'reef stars', which provide a stable base on loose rubble where young coral would otherwise be swept away.

Some scientists are trying to help corals cope with future conditions. Dr Kofi Mensah's laboratory breeds corals from colonies that survived previous heatwaves, in the hope that their offspring will be more heat-tolerant. Dr Mensah stresses, though, that such 'assisted evolution' is still at an early stage and that the long-term effects on reef ecosystems are not yet understood.

Restoration has clear limits. Even the largest projects cover only a few hectares, while the reefs being lost are measured in thousands of square kilometres. Restored areas can also be destroyed by a single severe heatwave. For this reason, most experts agree that restoration must go hand in hand with reducing greenhouse gas emissions and improving water quality. As Dr Ortiz puts it, 'Restoration buys time. It does not solve the problem.'