Part of a lecture about coral reefs — what they are, how they grow,

Section 4. You will hear part of a lecture about coral reefs — what they are, how they grow, and why they are under threat. First, you have some time to look at questions 31 to 35. Now listen carefully and answer questions 31 to 35. WOMAN: Good morning. Today I want to talk about coral reefs, structures that are often called the rainforests of the sea, because although they cover well under one per cent of the ocean floor, they shelter roughly a quarter of all marine species. Let's begin with what a coral actually is, because this surprises people. A coral is not a plant and not a rock, it is an animal, a colony of tiny creatures called polyps, each no bigger than a pinhead. Each polyp draws calcium from the seawater and builds around itself a hard skeleton of limestone, and it is the slow piling-up of these skeletons, over thousands of years, that forms the reef. WOMAN: Now, the reef could not survive on the polyps alone. Living inside the coral's tissue are microscopic algae, and the two depend utterly on each other, a partnership we call symbiosis. The algae photosynthesise and pass sugars to the coral for food, and in return the coral gives the algae shelter, crucially, it is these algae, not the coral itself, that give a healthy reef its brilliant colour. Because the algae need sunlight to photosynthesise, reefs can only grow in shallow, clear water, which is why you find them fringing coastlines rather than out in the deep ocean. Before you hear the rest of the lecture, you have some time to look at questions 36 to 40. WOMAN: This partnership, though, is also the reef's great weakness. The algae are extremely sensitive to temperature. When the sea grows too warm, even by a degree or two for a sustained period, the coral expels the algae, and without them it loses its colour and turns a ghostly white, a phenomenon known as bleaching. A bleached coral is not yet dead, but it is starving, and if the warm conditions persist, it will not recover. Rising sea temperature, driven by climate change, is now the single greatest threat to reefs worldwide. WOMAN: It is not the only one. A second problem is that the ocean is becoming more acidic, as it absorbs carbon dioxide from the atmosphere, and acid makes it harder for the polyps to build their limestone skeletons in the first place. A third, more local, threat is pollution, run-off from farms and towns clouds the water and blocks the sunlight the algae need. And a fourth is simple physical damage, from careless tourism, from anchors, and from destructive fishing. WOMAN: So what can be done? Efforts fall into two kinds. The first is protection, setting up marine reserves where fishing and anchoring are banned, giving stressed reefs a chance to recover. The second, newer and more dramatic, is restoration, scientists are now growing young corals in underwater nurseries and then transplanting them onto damaged reefs, a kind of gardening beneath the waves. Neither is a substitute for tackling the warming itself, but together they buy these extraordinary ecosystems time. And that is where I'll leave it today. That is the end of Section 4. You now have some time to check your answers.