CASE 8 BIODIVERSITY HOTSPOTS: RAIN FORESTS AND CORAL REEFS

Today, coral reefs are among the most diverse ecosystems in the ocean, but they are under threat from human activities (Chapter 49). Is this unusual, or have reef systems bloomed and collapsed in the past? Reefs have existed through most of Earth’s recorded history, but the kinds of creatures that compose them have changed through time because of both evolutionary innovation and mass extinction. Ecological opportunities are created by the appearance of a new habitat or place, or the origin of a novel innovation that grants new access to resources, or by the extinction of earlier occupants of the habitat or place. Diversification of one group of organisms often spurs the diversification of others that depend on them, and so the history of life is as much about new opportunities created by building on adaptive radiations as by their extinctions.

Bacteria built the first reefs, layered mounds of limestone that rose above the seafloor. With the Cambrian diversification of animals, calcified sponges rose to ecological prominence as participants in reef construction, but these communities were short lived, as they were wiped out later in the Cambrian Period, perhaps 515 million years ago. Subsequent renewed diversification of sponges, bryozoans, and cnidarians with massive calcium carbonate skeletons resulted in new and different reef communities beginning in the Ordovician Period, but these, too, were largely eliminated by renewed mass extinction 375 million years ago, during the Devonian Period. Reefs were widespread again by the Permian Period (300–252 million years ago), dominated this time by different groups of sponges and bryozoans, along with skeleton-forming green algae. The devastating extinction at the end of the Permian Period removed all reef-building animals, and only later did modern corals begin to build reefs. Indeed, reefs like those of the present day have developed only over 40 million years of Earth’s history.

Reefs illustrate how evolution and extinction have repeatedly reshaped animal diversity through time. As discussed in Chapter 49, we may be nearing another great change in reef biology as global climate change threatens reef communities in many parts of the world. Today’s coral species are at their tolerance limits of ocean temperature and pH, and both conditions may be changing more rapidly than species can adapt.

Humans, then, are influencing reefs and the biological diversity they sustain in ways matched only by massive volcanism and meteorite impact in the past. From the perspective of Earth history, the loss of coral reefs would simply be one more in a long series of extinction episodes. For humans, however, the commercial, aesthetic, and ecological consequences would be substantial and permanent. A similar, if simpler, history of evolution and extinction has shaped plant diversity in rain forests and other terrestrial ecosystems, which are also threatened today by the pace of habitat destruction and changing climate.

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