6—
Between the Devil and the Deep Blue Sea

When the tidal curtain lifts at Pulau Dua, an island in the Java Sea, a bizarre matinee can often be witnessed. The protagonists enter stage from the seaward door in narrow channels on the exposed mud flats that fill with the receding waters. Their costume and makeup artist is truly inspired, for these creatures have red polka dots splashed across their streamlined blue-gray bodies, spiny sails on their backs that can be raised or lowered on demand, bulging eye buds, and gaping frog mouths.

As the stage fills, the action begins. Pairing off with their nearest neighbors, the larger ones raise their fins, rear out of the water, arch their heads, feint right and left, and lunge forward to batter each other's heads. Occasionally, in respite from the battles, they lower their fins and squirm around on the soft mud, where they munch on the abundant crabs and other crustacea lurking there. During these intermissions, periscopic eye buds often protrude above the water, ever watchful for aggressors. In contrast to the nearly two-foot-long fighting grown-ups, the smaller ones, only a few inches long, simply feed the whole time. Whether the splashing and commotion from the battles

scares up food from the mud or warns the crustacea to burrow deeper is unclear.

During the hour of lowest tide, the fighting and feeding continue. A few of these mud skippers, as the creatures are called, emerge as victors, beating back all who contest their little patch of otherwise undistinguished feeding territory. Then, as the curtain begins to fall, they all swim off into the swelling waters.

Pulau Dua means Bird Island in Indonesian, and indeed the treetops are as full of bird life as the mud flats are of aquatic life. The island is a rookery for herons, bitterns, and egrets. During the nesting season, the trees are festooned with dozens of elegantly plumaged adult birds. When the hatchlings emerge, there is a constant daytime din of gibbering young demanding food from their harried elders. The latter cruise back and forth from nests to sea, bringing gulletfuls of fish every half hour or so.

A forty-foot-high platform on a bird observation tower was the roost on which my wife and I elected to spend the nights during our visit there in 1986 with "Chuck" Darsano, an Indonesian naturalist active in local conservation efforts. The platform commands a spectacular view of Pulau Dua's treetops. At sunset, as we sat in our roost, thousands of these huge, graceful birds returned to theirs, the last rounds of the day completed.

The sunset was splendid, but it surely paled in comparison to the sunsets seen at Pulau Dua one hundred years earlier. The island is located off the northern shore of the western tip of Java, within easy earshot of any eruptions in the chain of offshore volcanoes that include Krakatoa. For several years after Krakatoa's giant 1883 eruption, the debris it produced lingered in the air around the world, prismatically displaying the colors of the setting sun.

Besides two observation towers, the only other work of humanity on the island was a stone lean-to for campers. Its stones were carved out of dead coral reef dug from the island's "bedrock." Once a living, submerged reef, Pulau Dua probably met the fate of many reefs that once existed—death by siltation. Corals drown in silt the way we drown in water, or fish in air. Where once a live reef supported a dazzling array of fish, coral, and crustacea, now its corpse catches the mud in which the skippers feed and binds the soil on which grow the trees where the wading birds breed.

In death, then, coral reefs provide for life. But even more intricate and exquisite is the way a living reef provides for its own sur-

vival and growth. Such reefs exemplify the balance of nature. Often misunderstood to mean that nature is unchanging (it never is), the balance of nature has at its core harmonious and productive integration, the working together of the pieces to achieve a synthesis that is fitter and more capable of carrying out more functions than the pieces individually.

Corals are tiny animals. They live colonially in geometrically arrayed spaces embedded in limestone formed from their bodily secretions. These housing projects come in a dazzling variety of shapes and colors and are often fused together into gigantic reefs stretching tens of miles along coastlines and from the surface of the sea down to a hundred feet or more in clear water. When sea level is rising slowly enough, over the millennia, new reef can be built on top of old, so that the dead reef can be several thousand feet thick.

The complexity of a healthy coral reef is a feast for the eyes. To snorkel or dive in its waters is to enter a fantastical world in slow motion. Typically, at any productive reef, hundreds of different species of fish of brilliant colors and bizarre shapes glide by in slow procession, pausing occasionally to nibble at the coralline carpet or to devour smaller fish. Giant clams over a meter wide, electric blue starfish and lavender sea cucumbers, yellow fan-shaped and purple cup-shaped sponges, intricately patterned cone shells, huge rays with bright blue polka dots, shrimps with brilliant red stripes—all share this Seussian universe.

In many species of coral, the individual animals farm the space within their own bodies. In a marvelous example of integration, microscopic algae dwell within the coral. Like other algae, this species is capable of converting sunlight, carbon dioxide, and nutrients in the water, such as nitrogen and phosphorus, into more algae. The coral's digestive products are a rich source of the nitrogen, in the form of ammonia, needed for algal growth. Because ammonia levels are far higher within the coral than they are in the seawater outside, the algae greatly benefit by foregoing the freedom of the open sea. And what do the corals get out of this arrangement? First, the digestive juices of the coral cause glycerol, an energy-rich carbohydrate, to leak from the algae. Thus, for the coral, algal growth is food on the table. Second, like any animal, corals can be poisoned by the ammonia they excrete as a waste product; the algae serve as little waste treatment plants, converting the poison into food. Like the lichens, the integration of the coral and the algae is a splendid symbiosis.

Other reef animals also cultivate algae. The giant clams do it—grow-

ing the same algal species that the coral farm—on their fleshy outer lips. The true pioneers of this approach to life are some evolutionarily ancient sponges, the first multicelled animals, who cultivate the even more ancient blue-green algae within their convoluted interiors.

The types of symbiosis found in reefs are not limited to algal farming. A few species of sponges exert an extraordinary influence on the architecture of their larger reef community. Coral colonies grow by secreting limestone protrusions into available spaces, thereby creating complex shapes and fragile structures, an underseascape of steep convoluted pinnacles and canyons. As the sponges bore into the new coral heads to create protected living space, the loose limestone "tailings" they produce fill up the canyons around the protrusions. Parrot fish and other coral predators also produce sandy limestone debris as they scrape away at the live coral heads in search of juicy coral polyps. This loose material is then bonded with limestone secretions produced by freeliving algae (not those living within the bodies of the coral) to create the large sturdy assemblages we call reefs. It is doubtful that reefs would have the strength to withstand the buffeting of waves if this gradual filling in, this flattening, of the new reef surface did not occur.

The reef structure itself benefits all the reef's denizens, including those that forged it, by passively trapping nutrients carried to the sea in stream waters. These streams, flowing into the lagoons behind the reefs, convey nutrients that would be diluted by the vast open seas were the reefs not effective at trapping their precious cargo.

Because tropical forests are often very effective at retaining soil nutrients, the streams that drain undisturbed tropical watersheds can have lower nutrient concentrations than does the rain. Under those circumstances, the ability of reefs to trap stream nutrients in lagoons is particularly important. Unfortunately, in situations where forest disturbance causes streams to convey nutrient-rich water to the lagoons, the same ability can result in the death of the reef.

Relations among the fish inhabiting coral reefs are complex. Some fish clean the skin, gills, and teeth of other fish. Others school in dense formations that resemble larger fish. Mary Gleason, while a graduate student at the University of California, Berkeley, observed coral with and without the territorial damselfish present; she found that these small fish, which hide out among the nooks and crannies of the reef top, enhance the health of reefs by harassing the crown-of-thorns starfish and other coral predators.

Coral reefs abound in mysteries. Many details of the animal-algae

symbiosis are not understood, such as how the individual algal cells invade new coral polyps. Another mystery concerns the overall productivity of healthy reefs, which, acre for acre, is exceeded by no other ecosystem on the planet. The seawaters that wash over reefs are generally so nutrient-poor that it is hard to understand how reefs can be as productive as they are. Although cultivation of algae by coral and the architecture of reefs do help conserve and trap nutrients within the reef, some scientists do not believe this is sufficient to explain the fertility of reefs. Phosphorus (an essential ingredient in ATP, the substance that allows living cells to store and utilize energy) is so scarce in seawater that this nutrient ought to limit the growth of reefs. Reef productivity is puzzling in the same way as would be a village that thrives solely on tourism even though the tourists bring virtually no money; no matter how thrifty the people, an inflow of resources is needed to sustain wealth.

An insightful observation by the late John Isaacs, an oceanographer at the Scripps Marine Laboratory in San Diego, sheds some light on this puzzle.^[1] Why, he asked, are there no "pelagic trees"? A pelagic tree would be a tree growing in the open waters of the sea (called the pelagic zone). He did not mean trees, literally, but rather organisms like trees that have both roots with which to draw up nutrients and a canopy for purposes of gathering light. In deep-sea water, such an organism would have to be quite tall, of course, for the deep waters of the sea are nutrient-rich while the sunlight only penetrates the surface waters. But if pelagic trees existed, they would grow magnificently. Just like coral reef organisms. Observations of that sort are often the stimulus for new ideas in science.

By looking at the world in a novel way and asking why something does not exist, one often triggers new and useful thoughts. Based on the observation that there are no pelagic trees, two other marine scientists proposed that reefs must benefit from deep ocean nutrients.^[2] To understand their proposal, called the endo-upwelling concept, let us return to South Florida for a moment and take another look at the Florida aquifer, the same underground reservoir of water that played a role in stopping the jetport planned there.

[1] J. Isaacs, "The Nature of Oceanic Life," Scientific American 221 (September 1969):146-162.

[2] F. Rougerie and B. Wauthy, "The Endo-Upwelling Concept: A New Paradigm for Solving an Old Paradox," Proceedings of the Sixth International Coral Reef Symposium 3:21-26, edited by J. Choat et al., Townsville, Australia, 1988.

A mile or so below the everglades, a subterranean "hot plate" creates a mass of slowly rising hot aquifer water. The Florida aquifer comes in contact with the heated crust of the earth, and this heat forces aquifer water to rise through the porous limestone. According to the endoupwelling concept (which not all oceanographers accept), a similar mechanism nourishes reefs: deep nutrient-rich water, forced upward by geothermal heat, penetrates slowly through the porous atolls and brings nutrients to the reef organisms at the surface.

And so it seems that coral reefs are organized to thrive: they are assemblages of life geared to make more and more life the way Bartholomew Cubbins made hats. But all is not as harmonious as it seems. Deep beneath the living crown of the reef, beneath the spectacular profusion of color, form, and symbiotic function, there lies embedded in the coralline bedrock a silent record of a historical drama played out between sea and sky. As with all insightful history, this record may, as we shall see, tell us not only about the history of reefs but about their fate as well.

In winter 1983, strong winds and intense rainfall clobbered the coast of California. Flooding occurred up and down the coast, while huge landslides closed off sections of the coastal roadways. It was one of the wettest California winters in the past century. Around the world, people experienced different but equally unusual conditions. From Tahiti to Indonesia, sea level dropped abnormally. Tahiti experienced one of the severer cyclones in its history. In Indonesia and Central America, drought conditions prevailed. The Galápagos Islands received about ten times the normal amount of rain that year. And fish inhabiting the waters off the coast of Peru were probably hungry that winter, for 1983 was an El Niño year.

El Niño means "the child" in Spanish. This name was conferred by Peruvian fishermen because the first signal of this climatic and climactic event often occurs around Christmastime along the coast of Peru. The signal that the fishermen first notice is an unusual warming of the seawater. In years when this warming is particularly intense, like 1983, it often means hard times for the fishermen, not because of the warming itself but because of the source of the warming.

In normal years (recently designated "La Niña" years), there is in-

tense upwelling of deep water off the South American coast, bringing deep, cool water to the surface. This deep water is a rich source of nourishment for life in the coastal waters because it contains the nutrients released from the sunken corpses of marine plankton. In El Niño years, the upwelling of deep, cold, nutrient-rich water virtually ceases, thus increasing the temperature of the surface water and causing the fish to suffer a scarcity of nutrients.

What alters the upwelling every so often? The answer is a little like the kind of reply you sometimes get when you ask what causes the stock market to drop now and then—loss of investor confidence brought about by excessive rise in stock prices. Then when you ask why stocks recover, the answer is often that prices are low and investors see great bargains in front of them. In other words, there are cyclical forces at work. Like the stock market-investor system, the coupled oceanatmosphere system contains many causal loops, or cyclical sequences of connected events. In rough outline, here is how El Niños and La Niñas alternate in time. The trade winds blow from east to west across the subtropical southern Pacific. These winds rub against the sea surface and cause surface ocean currents to flow the same way. The effect of this would be a piling up of water on the western edge of the Pacific, but of course water does not remain piled up in a mound. The tremendous weight of piled-up water pushes down on the water beneath it, creating a return flow of water, from west to east, below the sea surface. In normal years, then, this deeper and therefore colder water is forced upward as it encounters the South American coast, creating the relatively cool conditions usually found there. To understand El Niño, we next have to ask, what drives the winds that create this oceanic conveyor belt? Here is where the circularity of the system comes in, for the winds are themselves partly driven by sea surface temperatures.

In the equatorial zone, warm temperatures cause rising columns of heated air, some of which then flows north and some south at high elevation. At higher latitudes, this heated air descends over some of the major deserts of the world—those of the southwestern United States and Mexico as well as the Sahara in the north, and the deserts of Australia, southern Africa, and South America. It is no accident that the major deserts of the world lie in the bull's-eye of this descending air, for it is hot and dry, having had all the moisture wrung out of it in the form of tropical rains.

This downwelling creates high-pressure zones, particularly off the west coast of South America because cool seas chill the air there, while

the warmer seas in the western tropical Pacific lead to lower air pressure. Since winds tend to blow from places with high pressure to those with low, that pressure difference helps sustain the east to west airflow. Moreover, as the descending air approaches earth's surface, it tends to flow toward the equator because the rising air created a vacuum. And at that point, it is deflected toward the west because of the earth's rotation.^[3]

But as the upwelling water continues to cool the coastal waters of South America, it also increasingly cools off the air above that region, eventually weakening the rising equatorial airflow. This means weakened equatorward flow of air and reduced pressure difference between the eastern and western Pacific. Thus, the trade winds weaken, and less water is pushed westward. As a result, sea level in the central and western Pacific drops, slowing the eastward push of deep water and its upwelling off the west coast of South America. The atmosphere-ocean system is then in its El Niño phase. La Niña years return when the warm seas of the eastern Pacific, the result of diminished cold-water upwelling in El Niño years, regenerate strong updrafts and hence strong trade winds.

To summarize this jumble of puzzle pieces, let us look at the big picture. Trade winds push surface water to the west, thereby creating a return flow of deeper cooler water that cools the air. As that cooling of the air slows the trade winds, the upwelling of cool water also slows, and the air begins to warm. That warming creates a vertical updraft that jump starts the trade winds—a perfect setup for a system that keeps shutting off and on, or that oscillates, as scientists would say.

Now we can see why El Niño years are likely to be characterized by the weather patterns described earlier. During El Niño years, the warmer air in the eastern Pacific causes more evaporation, and that must mean more rain somewhere. Why California and the Galápagos Islands but not Central America? Hard to say. And, in fact, not all

[3] To see why, picture yourself on a stream of air moving toward the equator from the Northern Hemisphere. The air mass you travel on and the solid earth below you are both rotating toward the east, but as you travel south, you are going at a constant rate east, while the earth is not. Proceeding south, the easterly velocity of the earth below increases (imagine a spinning globe: for it to maintain a constant rotation rate, a spot on the equator has to turn faster than a spot farther north, because the former has farther to go in one rotation). Thus, the earth appears to move away from you toward the east as you move toward the equator. Looked at from the vantage point of an observer at a fixed location on the earth's surface rather than one traveling with the wind, the moving air mass is seen to be curving toward the west.

El Niño years correspond to wet years in California. Although 1983 was a very wet El Niño year in California, 1987 was a terribly dry El Niño year there.

When this interplay between sea and sky produces an El Niño event, biological havoc can result. In the Galápagos Islands, finches (the same species of finches whose beaks Darwin once studied and used as evidence of adaptation in nature) bred profusely in 1983, with some birds laying ten clutches and one female producing twenty-five young. Ornithologists Peter Grant and B. Rosemary Grant observed that some of the young were born, matured, and were ready to breed that same year. Alan Pounds, a scientist at the Monteverde Cloud Forest Reserve in Costa Rica, speculates that the rare and beautiful golden toad disappeared from that reserve in 1987 because of a drought caused by an El Niño event that year. The toad requires wet burrows to survive the dry season; the unusually dry conditions of 1987 may have caused these burrows to become inhospitable.

In Kalimantan, Indonesia, the severe drought helped turn what might have been a routine forest fire into the largest forest fire in recorded history anywhere, destroying up to half the forest and wildlife in an area of over two million square miles. It is nearly certain that the fire was deliberately set by people wanting to clear land for agriculture and settlement. This "slash and burn" (or swidden) style of agriculture can be a perfectly sustainable way to prepare forested land for farming, provided it is carried out in a rotation scheme in which the land has time to recover periodically and the amount of cleared land is not increased each year (and, of course, provided it does not lead to a conflagration because of widespread drought). In societies in which population size did not change much, held in check by one means or another, this method of agriculture was a sensible adaptation to tropical or semitropical conditions. But in Indonesia, vast new areas of forest in Kalimantan are being cleared to resettle the growing population of Java, in some cases without the willing consent of the settlers.

The species loss from the Kalimantan fire is inestimable and the prospects for recovery of the forest uncertain but grim. Measured solely in terms of how much carbon dioxide this one fire emitted to the atmosphere, the damage was high. A reasonable estimate is that nearly two billion tons of this climate-altering gas entered the atmosphere as a result of the fire, an amount equal to that produced from one month of fossil fuel burning at current world consumption rates.

El Niño events can damage coral reefs in a variety of ways. Lowered

sea levels in the central and western Pacific cause damage because the organisms inhabiting coral reefs cannot tolerate prolonged exposure to the air. Intense oceanic storms in the central Pacific often accompany the sea level depression, causing further stress to reefs as strong wave action from each storm physically damages the reef architecture. The 1983 cyclone in Tahiti, for example, caused extensive damage to the already stressed reefs of those islands. The warmer sea surface temperatures that accompany El Niño events in the eastern Pacific can also damage coral, as was observed by marine biologists from the Gulf of Panama to the Galápagos Islands during the 1983 El Niño. The damage was patchy, however, suggesting that there may have been variations in the rate and amount of temperature rise and/or variations in the susceptibility of coral to that rise.

By drilling into a coral reef, beyond the living surface and down into the ancient dead coral bed beneath, a core of coral can be extracted. Probing into a reef is tantamount to going back in time. Examined closely, cores from Indonesian reefs reveal a series of closely spaced, dark bands of varying thickness. Like growth rings in trees, these bands tell us something about yearly events in the life of the coral. The dark bands, it turns out, contain fulvic acid—a common constituent of the partially decomposed organic matter found in soils. In La Niña years, rain falling on the land surface near the reefs erodes soil, and some of this eroded material is washed out to sea where it can be deposited right on the reef. During El Niño years, however, storms in Indonesia are weaker and rarer than in normal years, so less erosion occurs and the bands are paler.^[4] Evidence of this sort tells us that El Niños are not just a recent phenomenon. Life has had to adapt to the environmental stress of El Niños during the past millennium at least, quite likely for much longer than that.

The record of natural soil erosion revealed in coral cores, such as in La Niña years in Indonesia, is also of intrinsic interest. If the erosion is not too severe, reefs are well adapted to this phenomenon and indeed benefit from the nutrient in the eroded soil. If it were not for human disturbance, most of today's living reefs would probably live for five or

[4] Michael Moore, a graduate student in geography at the University of California, Berkeley, has been doing some of the best work of this type in Indonesia. His recent findings suggest that the fulvic acid technique may have to be supplemented with another method, based on analysis of variations in the trace amounts of nuclear isotopes in the reef core, to get reliable reconstructions of past El Niños.

ten more millennia before succumbing either to suffocation from too much sediment deposited on the reef or to air exposure from falling seas that will occur when the next ice age sets in (because a large volume of seawater will be locked up in glacial ice). As we shall see, human disturbance could greatly shorten their lives.

Below the steep and jagged peaks of Mo'orea in French Polynesia lies some of the most fertile soil in the world—a chocolate brown, organically rich treasure that looks and smells good enough to eat. The mixture of clay, which holds in reserve the essential mineral elements for plant growth,^[5] and sand, which drains away the torrential rains that frequently occur, is ideal for pineapple, taro, papaya, and the other crops that grow profusely wherever the native vegetation is cleared. Offshore, on the edge of the lagoon that fringes the island, lies another treasurean abundance of delicious reef fish, available for netting or spearing.

Such is the ideal, the Gauguin vision of tropical paradise. But the reality is sadly askew. After a heavy rainstorm in November 1990, I climbed a ridge high above Pao Pao Bay, on Mo'orea, to witness a ghastly sight. Where just the day before the crystalline waters of the bay hosted a gathering of spinner dolphins in for a rest from their nightly fishing expedition farther out to sea, now there was a huge murky brown clot of earthy water slowly moving out of the bay toward the reef and open sea. It was easy to see that the immediate source of the soil entering the bay was a single stream that drained the valley between two large mountains.

The valley is intensively farmed, primarily for pineapple. Mature pineapple plants are about two feet tall. They grow best in well-tilled soil, in rows a few feet apart. Seen from above, from a raindrop's viewpoint, the pineapple plantations are a mix of about one part green—the pineapple plants—and two parts brown—the bare and well-tilled soil. Intense storms wash away the loose soil, and streams carry it out to the coastal waters. In some parts of Mo'orea, such as the Opunohu val-

[5] These minerals include calcium, potassium, sodium, and magnesium. They are held in chemical bondage within the lattice of the clay, released on demand by plants but not readily dissolved out of the clay unless the rain is acidic.

ley, the soil is less intensively farmed. There, the streams should not be silting up so much in the aftermath of torrential rains, and the reefs offshore from this bay should be less threatened by siltation and overfertilization.

This lush, pristine valley, one of the few in the Tahitian islands that has been spared from the plow, may be converted into a golf course and resort by Japanese developers. Tahitian authorities gave tentative permission in 1991 for the destruction of one of the last remaining wild places on the island despite the protests of numerous environmental groups. Apparently, land is so expensive in Japan that it is cheaper to develop a golf course in Tahiti and fly the golfers there than it is to build it in Japan. Local environmentalists are vigorously opposing this development; as of this writing, it appears they will be successful.

In 1990, Michael Poole, then a graduate student from the University of California, Santa Cruz, studying the behavior of the spinner dolphins in Mo'orea, became concerned about the effects of siltation of coastal waters on his subjects as well as on the reefs that surround the island. He and I have now begun a study that we hope will shed light on the interaction between land use and the health of coral reefs. By contrasting the amount of eroded soil flushing out onto the reef at the mouth of Pao Bay with the amount at the mouth of Opunohu Bay, and at the same time surveying the health of the reefs bathed by these bay waters, we have what is, in effect, a nicely controlled experiment. Of course, if the Japanese resort is built, then the Opunohu valley will no longer be the pristine control system that it now is; erosion will occur from the construction of the resort, and a scientific opportunity, as well as a beautiful valley, will be lost.

Erosion from agriculture, dredging, road building, clear-cutting, and other human activities is probably the major cause of reef degradation throughout the tropical world today, although few studies have actually demonstrated the connection. In one of the best studies to date, a doctoral dissertation by University of Hawaii student George Hodgson, it was shown that erosion from the building of roads used to haul timber out of the clear-cut forests in the Philippines is causing extensive siltation of reef waters and damage to coral. In Australia, massive soil erosion onto the Great Barrier Reef has been shown to be triggered by construction of a roadway in the forest along the coast of northern Queensland, although it has proven difficult to assign a precise share of blame for reef damage there to this erosion. Charles Darwin may have been the first to raise the issue, having observed that reef

development was limited in areas receiving excessive sediments from rivers flowing to the sea.^[6]

Overfertilization and siltation from anthropogenic erosion are probably the major causes of reef damage today, with overfishing a close runner-up. But the future is likely to see more intense damage from a different source and on an even larger geographic scale. This damage will result from global warming, and to understand how climate change might affect reefs, it helps to swing back in time.

Sixty million years ago, Australia was not as close to the equator as it now is. Since then, the Australian tectonic plate has been slowly moving northward toward the equator, causing the climate of Australia to warm gradually. Even a mere twenty million years ago, water temperatures off the coast of that continent were too cool for coral to grow. As Australia moved north, subtropical ocean currents flowing westward from the mid-Pacific were deflected southward from its northeastern tip, which then caused further warming along the eastern coast. By fifteen million years ago, the seawater off the northeastern tip should have been warm enough for coral to thrive, while a couple of million years ago, coral should have been able to grow near what is today the southern tip of the Great Barrier Reef.

Sure enough, when the deep, ancient beds of dead coral beneath the present living crown of the Great Barrier Reef were recently cored and dated, the most northerly part of the old reef was found to be about fifteen million years old and three to four thousand feet thick. And the southern limit of today's reef sits on a dead coral bed only two to three million years old and four to five hundred feet thick.

Coral has both lower and upper temperature limits. Generally, once water temperatures exceed 85 degrees Fahrenheit, corals bleach and eventually die after prolonged exposure. It is the algae living within the coral that apparently are most sensitive to high temperatures, and it is their death that results in the bleached appearance of the coral. The five- to ten-degree warming of eastern Pacific waters that accompanies El Niño events has been observed to damage corals at sites scattered along the coast of equatorial South America and as far north as the Gulf of Panama. But that is a perfectly natural occurrence to which corals have been exposed for millennia. Of greater concern is the impending global warming from the buildup of greenhouse gases in our atmo-

[6] Charles Darwin, 1851, The Structure and Distribution of Coral Reefs (Berkeley and Los Angeles: University of California Press, reprint ed., 1962).

sphere. This warming is projected to raise seawater temperatures in the tropics by 1 to 4 degrees Fahrenheit during the next fifty years. Some of the planet's living coral will still be bathed in water below the 85-degree critical value when this occurs, but not all of it will, and widespread damage is expected. And, of course, when the average temperature of the sea is raised, El Niños will do greater damage as more tropical American reefs are pushed over the critical temperature limit.

In 1991, an unusually severe coral bleaching episode erupted in the reefs surrounding the island of Mo'orea. Rising water temperature was observed then as well, but its cause is unknown; perhaps it was an early symptom of global warming or just a fluctuation resulting from the El Niño event that year. If the latter is true, the bleaching event was part of the natural course (like gap formation in a forest), to which life in coral reefs has adapted through the millennia. If the former is correct, however, then another fuse has blown, with effects that will accumulate over time and lead to a catastrophic loss of biodiversity.

Global warming could also damage coral reefs by inundating them under rising seas. Again, let us look to the past for insight. Twenty thousand years ago, during the last ice age, so much seawater was locked up in glacial ice that sea levels were nearly five hundred feet lower than they are today. Hence, the currently living reefs of the world could not have existed then. They would have been high and dry. The living crown of the Great Barrier Reef today is only about one thousand years old. During this time, the sea level there has varied because of natural causes by only two or three inches per century, while the Caribbean reefs have experienced a sea level rise on the order of six inches per century. Interestingly, the Caribbean reefs have also been accumulating limestone at a rate faster than others around the world, thus keeping pace with the rapidly rising seas there.

At maximum productivity, corals can produce two and a half pounds of limestone per year on each square foot of reef, corresponding to a maximum of four-tenths of an inch of growth per year on the coral bed. Thus, most reefs will not be able to keep up with sea level rise from global warming if the higher projections of four feet or more during the next century are correct. Unfortunately, we know very little about the effect on the species composition and productivity of the top of a living reef when it is under a foot or two more water.

Regardless of what sea level rise from global warming does to reefs, that rise will not go unnoticed in the tropical Pacific. Some very low, flat island nations will lose substantial portions of their territory if seas

rise a mere foot or two. Saltwater intrusion, the same mechanism that we saw destroys underground freshwater supplies in South Florida when swamps are drained, is likely to jeopardize the freshwater supplies of many island nations. Here, rising seas will force salt water into the underground "lenses" of drinking water that sit beneath their land.

While I watched the mud skippers at Pulau Dua, an unmistakable rich, pungent odor of decay, tinged with a hint of rotten eggs, emanated from the surrounding mud flats. The odor brought to mind yet another set of threats to coral reefs around the world. A lengthy detour will explain why.

That odor characteristic of mud flats means that the oxygen content of the muck is low, a result of oxygen consumption by the microorganisms decomposing organic matter there. Shallow coastal waters around the world are often in a similar condition, especially if human sewage is discharged into the waters, but also under perfectly natural conditions.

If you were to place a jar open-end down, just a little bit into the mud, and collect for a few minutes the rising gas bubbles, there would be some nitrous oxide in the jar, along with carbon dioxide, hydrogen sulfide, and other emanations from the rotting muck. Nitrous oxide is odorless and colorless. It is produced by microorganisms who earn a living converting nitrate^[7] into either molecular nitrogen or nitrous oxide, in a process called denitrification. It is a perfectly natural process, although it can be accelerated by overuse of nitrate fertilizers on farmland. We shall see that the microbes, known as denitrifying bacteria, play a major role in determining the fitness of the biosphere for all forms of life.

Nitrous oxide is so unreactive, chemically, that it does not break down or combine with other gases in the lower atmosphere. Slowly but surely it makes its way to the stratosphere, the layer of the atmosphere above about eight miles, where supersonic aircraft fly. In the

[7] Readers may recall from chapter 1 that nitrate, like ammonia, is a form of nitrogen that plants can use for their nitrogen requirements. Nitrate is rich in oxygen compared to nitrous oxide, so the conversion of nitrate to nitrous oxide or to nitrogen can occur in waters or soil low in freely available oxygen, and indeed it is in such places that the reaction is often most vigorous.

stratosphere, it finally reacts^[8] and breaks apart. At this stage, a seemingly implausible link between microorganisms underfoot and the stratospheric ozone shield overhead is joined. To understand this link, let us look at the dynamics of stratospheric ozone.

Ozone in the stratosphere is a gas to which we all owe thanks, for without it, a lethal rain of ultraviolet (UV) radiation from the sun would destroy nearly all of life on earth. Ozone is produced in the stratosphere when high-energy packets or quanta of sunlight strike ordinary oxygen molecules in the stratosphere, splitting these molecules into two oxygen atoms; one of the atoms (0) then combines with molecular oxygen (0₂ ) to produce ozone (0₃ )—a molecule consisting of three oxygen atoms.^[9] If that were the end of it and nothing destroyed the ozone, then ozone production would continue until all the molecular oxygen in the stratosphere was converted to ozone. And for the evolution of life on earth, that would have been almost as bad news as too little ozone, for virtually no mutation-causing UV radiation would have penetrated to the earth's surface. Mutations are often associated with monsters and cancer, but they are also the primary cause of genetic variation among organisms, which, in turn, is a driving force behind the evolution of all the life forms on the planet.

Now back to the broken-down nitrous oxide in the stratosphere. One of the products of its demise is nitric oxide, a gas that triggers a pair of chemical reactions leading to the disintegration of ozone. It is this process that keeps the ozone level in the stratosphere in a balance between too little (which would shower us all with harmful radiation) and too much (which would reduce mutation rates and slow evolutionary change). Ultimately, then, it is the lowly denitrifying bacteria, chomping away on nitrate in the smelly mud flats and the soils around

[8] Most nitrous oxide molecules (N₂ 0 is the molecular formula) are struck by highenergy quanta of sunlight in the stratosphere and break apart into a nitrogen molecule (N₂ ) and an atom of oxygen (O). But about 1 percent of the nitrous oxide molecules react with atomic oxygen to form two molecules of nitric oxide (NO). It is the nitric oxide by-product of nitrous oxide that initiates the destruction of stratospheric ozone.

[9] The reason ozone is created high in the atmosphere by this mechanism, in the region called the stratosphere, and not in the lower atmosphere, the troposphere, is that the high-energy quanta of sunlight do not penetrate to the lower atmosphere because they are nearly all absorbed in the stratosphere. In the troposphere, ozone is created by a different process, and there it is a health hazard, not a blessing. It is created in urban air from hydrocarbons and oxides of nitrogen, emitted largely by automobiles, when those pollutants mix in the presence of sunlight. A major component of smog, ozone causes respiratory damage to people and can also damage vegetation.

the globe, that determine how much mutation-causing UV radiation we receive.

In the 1970s, a few scientists voiced concern that the excess nitrate entering soils, streams, and estuaries as fertilizer runoff from farmland would increase the rate of denitrification, thereby increasing the flow of nitrous oxide to the stratosphere and causing the level of ozone in the stratosphere to decline.^[10] This concern is still a serious one and someday will surely be headline news as our overnitrified planet increasingly denitrifies, further thinning the ozone layer and increasing UV radiation at the earth's surface. But the urgency of this threat has recently been overshadowed by a related, but more immediate, threat to the stratospheric ozone layer.

In the mid-1980s, evidence accumulated showing that the ozone layer was thinning above the antarctic from September through November. Soon thereafter, the cause became apparent; it involved not nitrous oxide but a collection of gases called chlorofluorocarbons (CFCs).^[11] Like nitrous oxide, the CFCs rise inert through the lower atmosphere to the stratosphere, where they are broken down into a product (chlorine in this case) that starts the chemical reaction that destroys ozone. Unlike nitrous oxide, the CFCs are only produced industrially, not by bacteria in soil and water. The CFCs have many applications for human use, including refrigerants in automobile air-conditioners, foaming agents to create insulated packing material, cleansers in the electronics industries, and propellants in spray cans (a use that is virtually eliminated in the United States but occurs elsewhere).

Just a few years after it was first discovered, the hole in the antarctic ozone layer was found to be big enough to warrant immediate action. In 1987, many nations meeting in Montreal formally agreed to gradually reduce the use of CFCs. In early 1989, with the problem looking even more serious, the European nations and the United States met again and agreed on the complete elimination of CFC production and use by the turn of the century. Although not all nations have signed the international agreements, the major producers and users of these

[10] Harold Johnston, a chemistry professor at the University of California, Berkeley, first shed light on much of the science of the preceding paragraphs and played a key role in warning the public about the hazard of excess oxides of nitrogen in the stratosphere.

[11] Two scientists, Mario Molino and Sherwood Rowland at the University of California, Irvine, had figured out a decade earlier that CFCs had the ability to destroy ozone and, like Johnston, warned the public of the menace.

destructive gases have, and the prospects are good that production and use will be virtually eliminated in a decade or less. Unfortunately, closing the CFC pipeline to the atmosphere will not immediately stop the ozone loss, because the CFCs emitted over the past several decades will continue to destroy ozone well into the next century. As a result, further ozone thinning and increased UV radiation are expected.

We know already that the problem is no longer confined to the antarctic, where it was first discovered. Over the midlatitudes of the Northern Hemisphere, the ozone layer has thinned by about 3 percent during the past decade, less than the thinning in the antarctic but enough to cause around half a million new cataracts and a hundred thousand new cases of skin cancer worldwide each year because of increased UV radiation. While nearly all these skin cancers are treatable, several thousand new cases per year of melanoma, a much more serious cancer that is fatal in about 25 percent of the cases, are also predicted.

What does the ozone problem have to do with coral reefs? Aren't they protected underwater? Unfortunately, the few feet of seawater above life in the shallow seas offers little protection from UV radiation, for the clear waters of reefs are fairly transparent to this radiation, down to depths of tens of feet. Tropical UV radiation is projected to increase by up to 10 percent over the next few decades, even with the international agreements in force (the increase would be far greater with no agreements). What will this do to life in the coral reefs? The sorry fact is we do not really know. Some minute forms of marine life, the plankton, are known to be very sensitive to even smaller increases in UV radiation than those predicted, and it is virtually certain that mortality of some individual organisms will increase as a result of higher radiation levels. It is less clear whether that will lead to systemwide effects, such as an overall decline in reef productivity. Speculation exists that it could lead to the outright extinction of some organisms, perhaps including participants in one of the many symbioses discussed earlier, which would, of course, mean dragging down other organisms as well.

Living coral reefs occupy but a tiny fraction of the ocean, yet within them lies a treasure trove of biodiversity. The marine environment, in general, and coral reefs, in particular, are unusual in that, compared to terrestrial environments, they contain a much higher amount of taxo-

nomic diversity at the phylum level and other inclusive levels in the classification hierarchy. In terms of numbers of species, marine habitats contain less than 15 percent of the world's life forms; most of the species on earth are terrestrial insects, all belonging to only one of the many classes. But the oceans contain about two-thirds of all the phyla and classes. This means they contain two-thirds of all the highly and broadly distinct types of life. And within the oceans, most of that diversity is found in the coral reefs. Only about 1 percent of all marine taxa are found in the vast, open, deep waters of the oceans.

If reefs degrade, we could lose all the organisms in some of the higher levels of the taxonomic hierarchy. The extinction of an entire class or phylum is a much more serious loss than the loss of the same number of species in a much larger class or phylum that continues to exist. The reason biologists consider it to be more serious is that we are interested in the quality as well as the quantity of the information in the genetic "library" of life. Species in distinct phyla or classes differ much more dramatically from one another with respect to biochemistry, strategies of adaptation, and so forth, than do species within the same class or phylum. Loss of a class or phylum is like loss of all artworks in a unique historical period; loss of the same number of paintings representing all the periods in art history would certainly be a loss but not as great a loss.

The reefs of Tahiti contain fewer species than the reefs of Fiji to the west; the reefs of Australia are richer still in species, while those of the Philippines farther to the west contain the most diverse collection of species of any reefs in the world. The reason has to do with the westblowing trade winds, those key actors in the El Niño cycle. These winds cause the equatorial ocean currents at the sea surface to also flow from east to west, and thus the natural trajectory of drifting organisms in the equatorial waters is from east to west. This replenishes the populations in the west and maintains their diversity at a higher level than in the east, much as we saw in chapter 5 how periodic movement of organisms to the tropics during ice ages maintains higher species diversity in the tropics than in the higher latitudes.

Cultural diversity ofHomo sapiens, like that of wild species, is remarkably rich in the tropics. Linguists estimate that over one-third of the world's languages are spoken in the South Pacific islands. Very likely, a process similar to that by which species diversity became enriched in the waters around these islands is responsible: isolation interrupted by occasional cultural transplantations. But the interruptions have in-

creased in recent years, to the point where they threaten to destroy rather than enhance this diversity (and that of the wild species as well). And thus I am reminded of Bill.

As a Fijian youngster growing up on a small islet off the island of Taveuni, Bill and his playmates would catch venomous sea snakes and drape them around their necks, pretending the shiny black and white banded creatures were jewelry. Years later, Bill learned from tourists, armed with facts and fears, how close to death they played.

When Bill was a teenager, in the 1950s, his family, along with the extended family that comprised his village, left their islet home and settled on Taveuni. He explained to me, as I accompanied him on a return visit to the islet, that the mud flats and waters surrounding their old home had once provided adequate food for the small tribe. But as their numbers grew and as they sought cash income, a move to Taveuni, with more arable land, became irresistible.

Bill often carries a long, light spear—a cluster of sharp metal prongs on a ten-foot bamboo pole. Seeing it continually unused, I had begun to wonder if its purpose was more mnemonic than predatory. One afternoon, however, as we were preparing to return to Taveuni after visiting his ancestral island, my doubts were erased. Bill was starting the outboard motor on his skiff while recounting some tribal history. I must have blinked when he threw the spear, because the next thing I saw was a quivering pole impaled in the side of a frantic fish. He had hit a two-foot-long, black, fast-swimming trevally at a distance of fifty feet!

Earlier that day, he demonstrated how to catch stomatopods, lobsters with a menacing talon in their front claws used to impale and clasp their prey. The species on Fiji grow about a foot long and inhabit mud flats. At low tide, their burrow openings are easy to spot at the edge of the sea, but the trick is to get them out of the burrows, which extend down a foot or so and then back as much as ten feet toward open water. Bill rammed a heavy, sharp stick into the mud ten feet seaward of the opening, wiggled it, and asked if I saw anything peculiar at the burrow opening ten feet away. I did not, so he moved down the coast a few feet and tried again. This time I saw undulating water at the burrow hole. He moved a foot or so closer to the opening and again

rammed the stick in. This time water burbled even more emphatically at the opening. Gradually he worked the creature toward the daylight end of its burrow, until it was within a pace of the opening. A plunge with his spear, followed by an arm's-length grab into the burrow, and he had caught dinner.^[12] This last step requires nerve and skill, because even when the spear hits home, the formidable pincers can still function.

Today, a single village contains most of the former inhabitants of Bill's childhood island home. They raise a variety of cash crops and grow most of their own fruits and vegetables. But Bill does not live in the village. His family and several of his close relatives own land on a hillside about ten miles away. Bill and I walked one day along most of the trails that snake through their nearly 250 acres, from the coast to the dividing ridge down the center of Taveuni. Much of the land is covered with old-growth forest, but here and there are small clearings planted with coconut palm, papaya, taro, kava,^[13] and other crops.

On this walk, the full cause, extent, and implications of Bill's ambitions became clear. Indians on Fiji now outnumber native Fijians by a small margin. They are also the more entrepreneurial and own most of the businesses and tourist facilities. Many Fijians, including Bill, resent this. Along with his wife and daughter, he worked for many years at an expensive Taveuni resort run by an Indian. Pay was meager and the workday long. Worse yet, it was demeaning for Bill in a way that even subsistence living would not be. When Bill and his family announced they were quitting, in 1988, the boss told him he would be crawling back starving in six months.

[12] Although I enjoyed dinner that evening, I might not have if I had known then what I learned about a month later from Rick Steger, a marine biologist who was studying stomatopods at the University of California's R. C. Gump Marine Biological Station on the French Polynesian island of Mo'orea. While many stomatopod mysteries remain, these feisty little hook-clawed lobsters are known to do something that is quite rare in the animal kingdom: they mate for life. Well, almost. It turns out that even they make exceptions. A single male and female, plus any youngsters, live together in each burrow, and it is the male's responsibility to sally forth to the sea in search of food for the family. Occasionally, however, he ends up as dinner for a shark or a person, or whatever. And in that event, a neighboring male may just abandon his own mate and burrow up with the widow. It appears that this will only occur, however, if the widow is larger than the current mate.

[13] Kava root is boiled to prepare a frequently and ritually drunk soupy tea. It is nonalcoholic but induces a sleepy, tingly euphoria. Kava is a kind of currency among Fijians and their guests as well. Visitors bring it as a gift much the way Westerners bring flowers or wine to dinner parties; in Fiji, it is considered offensive to show up at a village without some.

A year later, Bill had created on his land, down by the waterfront, a beautiful and ecologically nonobtrusive campground for low-budget travelers. His wife, a talented dancer and organizer of the village dance troop, is also a superb cook and prepares delicious Fijian meals for the campers. They keep the campsite immaculately clean, and to top it off, Bill's knowledge of where rare birds, such as the silktail and the orange dove, lurk on Taveuni is at the service of his guests.

The campsite is only a first step beyond "crawling" in Bill's mind, however. Bill wants to show the Indians of Fiji that he can do better than they. In that game, sadly, better means bigger. As we strode over his land, he showed me his planned site for a future resort for the wealthy, right on the ridge top with commanding views of both the east and west coast waters. And midway down the slope he had another development planned to accommodate middle-income tourists.

Road access will be needed, where now only trails lace the forested slope. Construction of roads and buildings will surely cause erosion in the rainy season. The splendid coral reef that lies just one hundred feet offshore from Bill's coastal campground will suffer as eroding soil spills into the sea. Tourism will probably boom, as the initially lush forests, diverse reef life, and exotic birds attract tourists, and then bust, as the ecological damage eventually sends the tourists off to other less-spoiled South Pacific islands, if any are left. Like the stomatopod, Bill and his island paradise are being driven toward destruction.

On one rainy night on Pulau Dua, we slept in the coral lean-to. In our mummy bags with their gaudy, Day-Glo-colored nylon cases, all snug within the limestone chamber of the lean-to, we might have looked a little like a gigantically magnified fragment of the living reef that once fringed Pulau Dua.

That image does not do justice, however, to the true extent to which humanity is a component of coral reefs. Whole nations are built on the skeletons of once-thriving reefs. Many tens of millions of people derive a substantial amount of their protein from the fish attracted to healthy coral reefs. Tourists spend large sums of money for the privilege of a few days of snorkeling or diving in reefs. Mysteries of coral reef ecology will grip the curiosity of scientists as long as reefs and science exist. And to those who never study, visit, eat the products of, or live on, reefs,

the gift is no less desirable; like reception of a card announcing that a distant relative gave birth to a healthy child, knowledge that healthy reefs exist is a reminder of the perfection and the mystery of life.

And what is humanity's gift in return to these splendid living treasures? To date it is overfishing, siltation and overfertilization from erosion on mismanaged land, enhanced UV radiation because of our destruction of the stratospheric ozone layer, and the promise of slow cooking from global warming—a multitude of threats that jeopardize the health of coral reefs around the world.^[14]

We could bear better gifts—gifts like family planning, wise management of marine and soil resources, use of substitutes for CFCs, energy conservation, and development of solar technologies. Much healthy reef remains, and our actions to date have by and large not led to irreversible damage. The burning fuse can still be doused.

[14] Readers interested in a detailed look at the status of individual reefs around the world should consult the encyclopedic work, Coral Reefs of the World, 3 vols., edited by Susan Wells, United Nations Environmental Program Regional Seas Directories and Bibliographies (Nairobi, Kenya, and Gland, Switzerland: UNEP and IUCN, 1988).