Coral Reefs: A Natural History
Coral reefs mean different things to different people. To a sea captain they are potentially hazardous, often poorly marked on the chart, and always lying just beneath the surface of the water. To people living in the tropics beside a reef, or on a small island perched on one, they provide both a home and food. To many divers and snorkelers who visit tropical coasts they are a kaleidoscopic wonderland of colorful fish that circle and dart above the strangest of living structures, and these tourists bring significant revenue and foreign earnings to such regions. The scientist sees something different again, a place of huge diversity in a relatively small space, of apparently chaotic pattern and movement, something to try to make sense of. A reef is all of these things and more.
Coral reefs occupy only about one percent of the Earth’s surface but they contain a high abundance of marine species, and they concentrate huge densities of it too, like an oasis. Not only this, but they also construct the reef itself. A reef is made by corals and other organisms as they extract limestone from the seawater to deposit as solid rock to make their own skeletons. They have been doing this for millennia so that now some reefs are a couple of kilometers thick in parts of the Earth that have been slowly subsiding over the ages, such as in the atolls scattered across tropical parts of the Indian and Pacific oceans. The parts of the reef that are now living form only a veneer on the top. Yet that small quantity has built the structure that supports all the rest, from the thousands of species of simple animals to the swarms of fish above, and to several entire oceanic nations.
Few of us realize that around half a billion to a billion people are now wholly or largely dependent on coral reefs, because they live on them or are dependent on the protein that they produce, or perhaps because they provide sheltered waters for a mainland settlement. That
number does not include many millions more who use them for recreation.
Yet there are fewer than 1,000 species of reef-building coral—a thousandfold fewer than the number of species that live among the structures they have built. Corals are the key animals that deposit the limestone rock, which they do because of help from captive, symbiotic algae, in a close association between animal and plant that has lasted for a couple of hundred million years. The far greater numbers of invertebrates on reefs include many that are hidden, and this is unsurprising given the huge whirls of predatory fish above them. Many animals are gaudily colorful. All are hunting for food and mates, and many use astonishing methods of hunting while avoiding being eaten themselves
Coral reefs have attracted people for generations, and because of that attraction to sheltered sea conditions, food, and space on which to live, they have been facing increasingly damaging pressures. So much so that by about 50 years ago probably about a fifth to a quarter had already been damaged and some destroyed. As human populations have risen steeply over the last couple of generations, this damage has increased, coming from pollution such as sewage, agricultural practices that smother reefs with too much fertilizer and sediment, and from dredging the seabed or industrial pollution because the sea has been a cheap place to dump toxic wastes. Damage has also resulted from overfishing to feed increasing populations, and overfishing is one of the most damaging impacts we inflict, because it unbalances the ecosystem.
Then, around the 1970s, the temperature of the oceans started to rise. It wasn’t noticed much at first because the increase was small and within the general variation. It occurred because for the preceding century people had been burning ever more oil, which, together with the burning of coal and land clearing, added increasing amounts of carbon dioxide (CO2) to the air. This gas traps heat from the sun. Scientists looked at every other possible cause of warming and discounted most: the temperature rise has come mainly from burning fossil fuels. Many species in tropical waters, including corals, are already very close to the maximum temperature they can survive. This warming arrives in pulses, called marine heatwaves, and repeated heatwaves are killing corals and reefs in vast numbers. Today this is the primary threat to their survival. Corals, seemingly robust because of the huge reefs they make, have a rather fragile relationship with their symbiotic algae and will die when this is disrupted. They are now facing an existential threat. This book aims to show and describe the beauty of
this most incredible ecosystem, and to explain some of the problems that it now faces and how we must respond to the threats. It is a natural history of an extraordinary but endangered ecosystem.
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