Coral Reefs: A Natural History

Book Preface

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.

INTRODUCTION

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.