The Biology of Coral Reefs . Sheppard, C. R. C., S. K. Davy, and G. M. Pilling . 2009 . Oxford University Press , Oxford , United Kingdom . 339 pp. $120.00 (hardcover) . ISBN 978-0-19-856635-9 . A Reef in Time . Veron, J. E. N. 2008 . The Belknap Press of Harvard University Press , Cambridge , MA . 289 pp. $18.95 (paperback) . ISBN 978-0-674-03497-6 .

What is the future of coral reefs, iconic ecosystems of the world's tropical and subtropical oceans? They are still viewed by the general public as sublime underwater gardens that harbor a diversity of species unparalleled on Earth. For centuries, and probably millennia in some locations, they have supported coastal inhabitants. It appears these wondrous natural resources are facing immediate extinction, but, in fact, the extinction process has been underway for decades in some parts of the world. Unless humans take swift and decisive action, coral reefs in their present form may disappear totally in the coming millennia and be replaced by a different ecosystem that lacks the physical structure and species richness characteristic of coral reefs.

This is the theme of two recent reference works on coral reefs. Both books are written by well-respected scientists who are long-time practitioners with worldwide experience in the field. The first, The Biology of Coral Reefs, is by Charles Sheppard and co-authors Simon K. Davy and Graham M. Pilling. The second, A Reef in Time, is by J. E. N. Veron, a coral taxonomist. These authors weave together an intriguing account of the history and ecology of present-day reefs, most of which is based on solid fact or well-accepted theories. They apply standard knowledge of coral physiology and carbonate biochemistry, scaled up to ecosystem-level processes and geological time scales, to expertly lay the groundwork for their expositions. Then, they speculate on the future of coral reef ecosystems, given present anthropogenic effects and anticipated climate change.

The work by Sheppard et al., The Biology of Coral Reefs, is organized in a convenient manner for easy reference and contains updated treatments of standard topics, such as the distribution and types of coral reefs, levels of species richness and abundance, diurnal vertical migration of plankton, camouflage and mimicry in reef fishes, and the recurrent theme of human effects on coral reefs. It also features a number of relatively new topics such as reef microbiology (e.g., fungi, protozoans, microalgae, bacteria, and viruses), the study of which has become possible due to the development of new techniques, such as cultivation-free methods for characterizing bacteria and DNA barcoding.

This is an authoritative reference work on coral reefs and contains an expanded treatment of the coral-algal symbiosis that extends into the now-recognized molecular diversity of the zooxanthellae. This discussion flows seamlessly into the biochemistry of coral calcification and the phenomenon of coral bleaching, both of which underlie responses of coral reef communities to the twin effects of anthropogenic climate change, warming of oceanic waters, and acidification. Concluding sections dwell on “phase shifts, alternative stable states and hysteresis,” which is again relatively new territory in the theoretical treatment of possible future trajectories of the status of coral reefs under different scenarios of human activities and climate change. Sheppard et al. emphasize the importance of controlling direct, negative human effects first to secure the health of coral reefs. This would confer on coral reefs increased resilience to climate change.

A Reef in Time, by Charlie Veron, is also an authoritative treatise on coral reefs, but it is written with a distinctly more personal tone. Coral reefs are clearly the author's passion, with the Great Barrier Reef being the main focus of his life's work. His narrative begins with an overview of the types of reefs (with a focus on those comprising the Great Barrier Reef) and the known controls and processes underlying the structure and function of coral reefs. The greatest focus is on geological aspects of coral reefs—their geological history, controls on reef formation, and possible effects of future climate change. The book ends with a plea for prompt management action focusing on reducing carbon dioxide emissions. Veron is an engaging writer, and his story of reefs over the hundreds of millions of years often is a riveting tale. Although many aspects are well-researched and documented, the author concedes that factual information has to be spiked here and there with a little imagination.

Veron emphasizes that the great mass extinctions throughout Earth's history, in which a majority of species of an era disappeared, were generally followed by millions of years when reef building was absent from the geological record. In other words, the causes of the eradication of numerous life forms are probably also linked to reef demise. On the particular time scales involved, Veron explains that change is ultimately traced to plate tectonics (together with seafloor spreading) and associated climatic repercussions, notably the release of carbon dioxide from volcanoes and methane from the ocean at the margins of continental plates.

The author rules out a major role in mass extinctions of factors such as acid rain, hydrogen sulfide buildup, and anoxia. Although these phenomena may have been significant in particular cases, Veron argues that the main culprits of mass extinctions and subsequent gaps in reef building are related to the global carbon cycle. This leaves, as candidates of causation, high levels of methane and carbon dioxide, which echoes ongoing debates on mass extinction of coral reefs.

As with current treatments of coral reefs, a substantial amount of the discussion is devoted to the greenhouse effects of methane and carbon dioxide. Although the geological record reveals dramatic warming episodes in Earth's history, the current rate of warming in an already warm world appears unique. This definitely does not bode well for present-day reefs, especially because incidents of mass bleaching are already becoming more frequent, but Veron believes the real danger to the survival of coral reefs is ocean acidification. The mechanisms by which increasing carbon dioxide levels in the atmosphere lead to reduced pH, which affects aragonite saturation and, hence, calcification by living organisms, are well described by both Sheppard et al. and Veron. In the business-as-usual scenario (i.e., do nothing to substantially reduce carbon dioxide emissions), the prognosis for coral reefs is alarming—reef building may cease and the forces of erosion take over, leading to the gradual physical disappearance of these ecosystems.

In the end, all authors stress there is still hope and time for decisive action. But time is running short. Human society is faced with a range of complex choices. The authors ask readers to answer perhaps the most challenging question: Do we value coral reefs sufficiently to accept even difficult and painful lifestyle changes?