ISLANDS IN TIME - Dirt: The Erosion of Civilizations - David R. Montgomery

Dirt: The Erosion of Civilizations - David R. Montgomery (2007)


When our soils are gone, we too, must go unless we find some way to feed on raw rock.


ON HIS WAY TO INDONESIA AND THE SPICE ISLANDS, a Dutch admiral discovered a small volcanic island in a remote part of the Pacific Ocean on Easter Sunday 1722. Shocked by apparent cannibalism among the natives, Jakob Roggeveen and his crew barely paused and sailed on across the Pacific. Never attractive for colonization or trade because of its meager resource base, Easter Island was left alone until the Spanish annexed it half a century later. The most interesting thing about the place was a curious collection of hundreds of colossal stone heads littered across the island.

Easter Island presented a world-class puzzle to Europeans who wondered how a few stranded cannibals could have erected all those massive heads. The question mystified visitors until archaeologists pieced together the environmental history of the island to learn how a sophisticated society descended into barbarism. Today Easter Island's story provides a striking historical parable of how environmental degradation can destroy a society.

The tale is not one of catastrophic collapse but of decay occurring over generations as people destroyed their resource base. Easter Island's native civilization did not disappear overnight. It eroded away as environmental degradation reduced the number of people the island could support to fewer than those living there already. Hardly cataclysmic, the outcome was devastating nonetheless.

Pollen preserved in lake sediments records an extensive forest cover when a few dozen people colonized Easter Island. The conventional story is that Polynesians arrived in the fifth century and over the next thousand years cleared the forest for agriculture, fuel, and canoes as the population grew to almost ten thousand in the fifteenth century. Then, within a century of the peak in population, a timber shortage began forcing people to live in caves. Although recent reanalysis of radiocarbon dating suggests colonization may have occurred centuries later, pollen and charcoal from sediment cores indicate the island retained some forest cover through the seventeenth century. The island was virtually treeless by the time the first Europeans arrived. By then the last trees lay out of reach, sheltered at the bottom of the island's deepest extinct volcano.

Soil erosion accelerated once forest clearing laid the land bare. Crop yields began to fall. Fishing became more difficult after the loss of the native palms whose fibers had been used to make nets. As access to food decreased, the islanders built defensive stone enclosures for their chickens-the last food source on the island not directly affected by loss of trees and topsoil. Without the ability to make canoes, they were trapped, reduced to perpetual warfare over a diminishing resource base that ultimately came to include themselves as their society unraveled.

Rapa Nui (the native inhabitants' name for Easter Island) is located at the same latitude as central Florida, but in the Southern Hemisphere. Continually swept by warm Pacific winds, the island consists of three ancient volcanoes occupying less than fifty square miles-a tropical paradise more than a thousand miles from the nearest inhabitable land. Such isolation meant that the island supported few native plants and animals when wayward Polynesians landed after paddling across the Pacific Ocean. The native flora and fauna offered so little to eat that the new arrivals' diet was based on chickens and sweet potatoes they brought with them. Sweet potato cultivation took little effort in the island's hot, humid environment, leaving the islanders with enough free time to develop a complex society centered on carving and erecting gigantic stone heads.

The monstrous statues were carved at a quarry, transported across the island, and then capped by a massive topknot of red stone from a different quarry. The purpose of the statues remains a mystery; how the islanders did it was as much of a mystery for many years. That they transported their immense statues without mechanical devices and using only human power perplexed Europeans viewing the treeless landscape.

When asked how the great stone statues had been transported, the few remaining islanders did not know how their ancestors had done it. They simply replied that the statues walked across the island. For centuries the bare landscape fueled the mystery of the heads. No one, including the sculptors' descendants, imagined that the great stone statues were rolled on logs-it seemed just as likely that they had walked across the island on their own.

Many of the statues were left either unfinished or abandoned near their quarry, implying that their sculptors ignored the impending timber shortage until the very end. As timber became scarce, competition for status and prestige continued to motivate the drive to erect statues. Even though the Easter Islanders knew they were isolated on a world they could walk around in a day or two, cultural imperatives apparently overcame any concern about running out of trees.

European contact finished off what was left of the native culture. In the 1850s most of the island's remaining able-bodied men, including the king and his son, were enslaved and shipped off to Peruvian guano mines. Years later, the fifteen surviving abductees repatriated to their island introduced smallpox to a population with no immunity. Soon thereafter the island's population dropped to just one hundred and eleven, unraveling any remaining cultural continuity.

The story of how Easter Islanders committed ecological suicide is preserved in the island's soil. Derived from weathered volcanic bedrock, thin poorly developed soil, in places only a few inches thick, blankets most of the island. Just as in other subtropical regions, the thin topsoil held most of the available nutrients. Soil fertility declined rapidly once vegetation clearing allowed runoff to carry away the topsoil. After that only a small part of the island remained cultivatable.

Distinctively abbreviated subsoil exposed at the ground surface testifies to erosion of the island's most productive soil. Exposures at the foot of hillslopes reveal that a layer of material brought down from higher on the slopes covers the eroded remnants of the older original soil. These truncated soil profiles are studded with telltale casts of the roots of the now extinct Easter Island palm.

The relationship of soil horizons to archaeological sites reveals that most of the soil erosion occurred after construction of stone dwellings (ahus) associated with the rise of agriculture on the island. These dwellings were built directly on top of the native soil, and younger deposits of material washed off the slopes now bury the ahus'foundations. So the erosion that stripped topsoil from the slopes happened after the ahus were built.

Radiocarbon dating of the slope-wash deposits and soil profiles exposed by erosion, in road cuts, or in hand-dug soil pits record that the top of the island's original soil eroded off between about AD 1200 and 165o. Apparently, vegetation clearing for agriculture triggered widespread erosion of the A horizon upon which soil fertility depended. Easter Island's society faded soon after its topsoil disappeared, less than a century before Admiral Roggeveen's unplanned visit.

A detailed study of the soils on the Poike Peninsula revealed a direct link between changing agricultural practices and soil erosion on Easter Island. Remnants of the original soil still standing on a few tiny hills, flat-topped scraps of the original ground surface, attest to widespread erosion of the native topsoil. Downhill from these remnant soil pedestals, hundreds of thin layers of dirt, each less than half an inch thick, were deposited on top of a cultivated soil studded with the roots of the endemic palm tree. A halfinch thick layer of charcoal immediately above the buried soil attests to extensive forest clearing after a long history of cultivating plots interspersed among the palm trees.

Initial agricultural plots in planting pits dug between the trees protected the ground from strong winds and heavy rainfall, and shielded crops from the tropical sun. Radiocarbon dating of the charcoal layer and material obtained from the overlying layers of sediment indicate that the soil eroded off the upper slopes, and buried the lower slopes, between AD 1280 and 1400. The numerous individual layers of sediment deposited on the lower slopes show that the soil was stripped off storm-by-storm a fraction of an inch at a time. These observations tell the story of how after centuries of little erosion from fields tucked beneath a forest canopy, the forest of the Poike Peninsula was burned and cleared for more intensive agriculture that exposed the soil to accelerated erosion. Agriculture ceased before AD 1500, after just a century or two in which the soil slowly disappeared as runoff from each storm removed just a little more dirt.

The island's birds disappeared too. More than twenty species of seabirds inhabited Easter Island when Polynesians arrived. Just two species survived until historic times. Nesting in the island's closed canopy native forest, these birds fertilized the soil with their guano, bringing marine nutrients ashore to enrich naturally poor volcanic soils. Wiping out the island's native birds eliminated a key source of soil fertility, contributing to the decline of the soil and perhaps even the failure of the forest to regenerate. I doubt the Easter Islanders had any idea that eating all the birds could undermine their ability to grow sweet potatoes.

The story of Easter Island is by no means unique. Catastrophic erosion followed forest clearing by Polynesian farmers on many other-but by no means all-Pacific Islands. Among the last places colonized on earth, South Pacific islands provide relatively simple settings to study the evolution of human societies because they had no land vertebrates before people imported their own fauna of chickens, pigs, dogs, and rats.

The islands of Mangaia and Tikopia provide stark contrasts in human adaptation to the realities of a finite resource base. Sharing many common traits and similar environmental histories until well after people arrived, these societies addressed declining resource abundance in very different ways. As worked out by UC Berkeley anthropologist Patrick Kirch, their stories show how transgenerational trends shaped the fate of entire societies.

Mangaia occupies just twenty square miles-a small dot of land in the South Pacific twenty-one and a half degrees south of the equator. Visited by Captain James Cook in 1777, Mangaia looks like a medieval walled fortress rising from the sea. The deeply weathered basaltic hills of the island's interior climb more than five hundred feet above sea level, surrounded by a gray coral reef lifted out of the ocean. A hundred thousand years ago, growth of the nearby volcanic island of Rarotonga warped Earth's crust enough to pop Mangaia and its fringing reef up out of the sea. Streams flowing off the island's core run into this half-mile-wide wall of razor-sharp coral that rises half the island's height. There they drop their sediment load and sink into caves running down to the island's narrow beach. Radiocarbon-dated sediment cores recovered from the base of the island's interior cliffs tell the story of Mangaia's last seven thousand years.

Covered by forest for five thousand years before Polynesians arrived about 500 BC, Mangaia eroded slowly enough to build up a thick soil in the island's volcanic core. Kirch's sediment cores record sweeping changes between 400 Bc and AD 400, when a rapid increase in the abundance of microscopic charcoal particles records the expansion of slash-and-burn agriculture. Charcoal is virtually absent from sediment older than 2,400 years; dirt deposited less than 2,000 years ago contains millions of tiny carbon fragments per cubic inch of dirt. In the sediment cores, sharp increases in the abundance of iron and aluminum oxides, along with decreased phosphorus content, show that erosion of a thin, nutrient-rich layer of topsoil rapidly exposed nutrient-poor subsoil. The native forest depended on recycling nutrients that the weathered bedrock could not readily resup ply. So topsoil loss retarded forest regeneration. Well adapted to grow on the nutrient-poor subsoil, ferns and scrub vegetation useless for human subsistence now cover more than a quarter of the island.

By about AD I2oo the pattern of shifting slash-and-burn agriculture had stripped so much topsoil from cultivated slopes that Mangaian agriculture shifted to reliance on labor-intensive irrigation of taro fields in the alluvial valley bottoms. Occupying just a few percent of the island's surface area, these fertile bottomlands became strategic objectives in perpetual intertribal warfare. Control of the last fertile soil defined political and military power on the island as population centers grew around these productive oases.

Polynesian colonization changed the ecological makeup of the island, and not only in terms of the soil. Between AD 1000 and 1650 guanoproducing fruit bats vanished as the islanders killed off more than half the native bird species. Historical accounts and changes in the abundance and variety of bones in prehistoric deposits indicate that by the time of Cook's visit Mangaians had eaten all their pigs and dogs, and probably all their chickens too. The Mangaian diet began to change radically-and not for the better.

After most protein sources were gone, charred rat bones became prevalent in deposits excavated from prehistoric rock shelters. Early nineteenthcentury missionary John Williams wrote that rats were a favorite staple on Mangaia. "The natives said they were exceedingly `sweet and good'; indeed a common expression with them, when speaking of any thing delicious, was, `It is as sweet as a rat."" Charred, fractured, and gnawed human bones appear in excavated rock shelter deposits around AD 1500, attesting to intense competition for resources just a few hundred years before European contact. Chronic warfare, rule by force, and a culture of terror characterized the end state of precontact Mangaian society.

Reconstructions of Mangaia's human population mirror those of Easter Island, albeit on a smaller scale. Starting with perhaps a few dozen colonizers around 500 BC, the island's population grew steadily to about five thousand people by AD 15oo. The population fell dramatically over the next two centuries, hitting a low soon after European contact and then rebounding to a modern population of several thousand.

The environmental and cultural history of Tikopia, a British protectorate in the Solomon Islands, provides a striking contrast to Mangaia despite very similar backgrounds. With a total area of less than two square miles, Tikopia is smaller than Mangaia. Even so, the two islands supported comparable populations at the time of European contact. With a population density five times greater, Tikopia sustained a relatively stable and peaceful society for well over a thousand years. This tiny island offers a model for sustainable agriculture and an encouraging example of cultural adaptation to limited resources.

Land use on Tikopia began much as that on Mangaia did. After people arrived about 900 BC, a shifting pattern of forest clearing, burning, and cultivation increased erosion rates and began to deplete the island's native fauna. After seven centuries on the island, the islanders intensified pig production, apparently to compensate for loss of birds, mollusks, and fish. Then instead of following the path taken by the Mangaians and Easter Islanders, Tikopians adopted a very different approach.

In their second millennium on their island, Tikopians began adapting their agricultural strategy. Plant remains found in the island's sediments record the introduction of tree crops. A decline in the abundance of microscopic charcoal records the end of agricultural burning. Over many generations, Tikopians turned their world into a giant garden with an overstory of coconut and breadfruit trees and an understory of yams and giant swamp taro. Around the end of the sixteenth century, the island's chiefs banished pigs from their world because they damaged the all-important gardens.

In addition to their islandwide system of multistory orchards and fields, social adaptations sustained the Tikopian economy. Most important, the islanders' religious ideology preached zero population growth. Under a council of chiefs who monitored the balance between the human population and natural resources, Tikopians practiced draconian population control based on celibacy, contraception, abortion, and infanticide, as well as forced (and almost certainly suicidal) emigration.

Arrival of Western missionaries upset the balance between Tikopia's human population and its food supply. In just two decades the island's population shot up by 40 percent after missionaries outlawed traditional population controls. When cyclones wiped out half the island's crops in two successive years, only a massive relief effort prevented famine. Afterward, the islanders restored the policy of zero population growth, this time based on the more Western practice of sending settlers off to colonize other islands.

Why did Tikopians follow such a radically different path than their counterparts on Mangaia and Easter Island? Despite similar settings and natural resources, the societies that colonized these islands met radically different fates. Tikopia developed into an idyllic island paradise, while Mangaia and Easter Island descended into perpetual warfare. Recalling that Tikopia's utopian system was maintained at the cost of lives prevented or eclipsed in the name of population control, we can justifiably ask which was the higher price. Nonetheless, Tikopian society prospered for thousands of years on a tiny isolated outpost.

An essential difference between the stories of these islands lies in their soils. Deeply weathered soils in Mangaia's sloping volcanic core are nutrient poor. The sharp coral slopes of the uplifted reef hold no soil at all. In contrast, Tikopia hosts young phosphorous-rich volcanic soils. The greater natural resilience of Tikopia's soils-because of rapid weathering of rocks with high nutrient content-enabled Tikopians to sustain key soil nutrients, using them at about the rate that they were replaced from the underlying rocks through intensive, multistory gardening that protected topsoil.

After deciphering the environmental history of both Tikopia and Mangaia, Patrick Kirch suspects that geographic scale also influenced the social choices that shaped these island societies. Tikopia was small enough that everyone knew everyone else. Kirch suggests that the fact that there were no strangers on the island encouraged collective decision making. By contrast, he suggests, Mangaia was just large enough to foster an us versus them dynamic that fueled competition and warfare between people living in neighboring valleys. Easter Island supported a larger and less cohesive society, leading to even more disastrous results. If Kirch is right and larger social systems encourage violent competition over collective compromise, we need to take a sober look at our global prospects for managing our island in space.

The story of dramatic soil loss following human colonization of islands is not restricted to the South Pacific. Viking colonization of Iceland in AD 874 catalyzed an episode of catastrophic soil erosion that continues to consume the country. At first the new colony prospered raising cattle and growing wheat. The population rose to almost eighty thousand people by AD 1100. Yet by the late eighteenth century the island's population had dwindled to half the medieval population. Cooling during the Little Ice Age from about AD 1500 to 1900 certainly influenced the fortunes of the Iceland colony. So did soil erosion.

Iceland had an extensive forest cover when first colonized. In compiling the fslendingabok in the late twelfth century, Ari the Wise described the island as "forested from mountain to sea shore." 2 Since human settlement, more than half of Iceland's vegetation cover has been removed. The native birch forest that covered thousands of square miles now occupies less than 3 percent of its original area.

Over time, herds of sheep increasingly disturbed the landscape. By the start of the eighteenth century more than a quarter of a million sheep roamed the Icelandic countryside. Their numbers more than doubled by the nineteenth century. Visitors began describing Iceland as a bare land devoid of trees. The combination of a deteriorating climate and extensive overgrazing led to severe erosion and abandoned farms. Today, threequarters of Iceland's forty thousand square miles of land are adversely affected by soil erosion; seven thousand square miles are so severely eroded as to be useless.

Once Iceland's slopes were deforested, strong winds blowing off its central ice caps helped strip the soil from roughly half the once forested area of the island. Large herds of sheep broke up the soil, allowing wind and rain to dig their way down to bedrock last exposed by melting glaciers. Soils built up over thousands of years disappeared within centuries. The central part of the island where the soil has been completely removed is now a barren desert where nothing grows and no one lives.

Some areas eroded soon after the Vikings arrived. During the relatively warm period in the eleventh and twelfth centuries, before the Little Ice Age, severe soil erosion caused the abandonment of mostly inland and some coastal farm sites. Later erosion in the lowlands primarily involved farms in marginal locations.

Many theories have been advanced to explain Iceland's abandoned farms. Inland areas have been vacated for centuries, some valleys literally deserted. Until recently, the abandonment was primarily attributed to climate deterioration and associated epidemics. But recent studies have documented the role of severe soil erosion in converting farms and grazing land into barren zones. The history of Icelandic soils can be read through the layers of volcanic ash. Frequent volcanic eruptions imprinted Iceland's dirt with a geologic bar code. Each ash buried the soils onto which they fell. The layers gradually became incorporated into the soil as wind deposited more dirt on top.

In 1638 Bishop Gisli Oddson described layers of volcanic ash in Icelandic soils. The observant bishop noticed that thick layers of ash separated buried soils, some of which contained the rooted stumps of ancient trees. Since Oddson's day, it has been recognized that hundreds of volcanic eruptions after the last glaciation produced fine-grained soils readily eroded if exposed to high winds sweeping across the island. Windblown material accumulates where vegetation stabilizes the ground surface, combining with layers of volcanic ash to build Icelandic soils. Based on ages of the different layers of ash in soil profiles, Icelandic soils accumulated at about half a foot every thousand years, roughly half an inch per century. The loss of vegetation not only accelerates erosion, but keeps soil from accumulating once there is nothing on the surface to trap volcanic ash and windblown silt.

In prehistoric times, relatively loose soil held together by thick native vegetation slowly built up on top of more cohesive lava and glacial till (an unstratified mix of clay, sand, and boulders deposited by glaciers). In areas where the soil sits directly on top of the till, soil accumulated continuously over ten thousand years. In some areas, exposed layers of soil and ash preserve evidence for erosion before the Vikings arrived, during periods when climatic deterioration stressed Iceland's native vegetation. The combination of overgrazing and climatic deterioration during the Little Ice Age triggered the most extensive episode of soil erosion in Iceland's postglacial history.

During the light-filled Icelandic summer, sheep graze twenty-four hours a day, roaming over both heath and wetlands. Trampling generates bare spots up to several feet in diameter. Shorn of a dense root mat, Iceland's volcanic soils offer little resistance to wind, rain, or snowmelt. Patches of bare earth erode rapidly down to hard rock or glacial till, carving little cliffs ranging in height from one to almost ten feet, depending on the local depth of the soil. Once started, these miniature escarpments sweep across the landscape eating away at the remaining pillars of soil and transforming rich grazing lands into windswept plains of volcanic tephra and rock fragments. Soil erosion since Norse settlement removed the original soil from about half the island. Although many factors contribute, overgrazing by sheep is generally acknowledged as the primary cause. Worms may have shaped Darwin's England (once glaciers got through with it), but sheep shaped Iceland.

Rofabards-the Icelandic name for soil escarpments-erode back half an inch to a foot and a half per year. On average, rofabard advance amounts to an annual loss of 0.2 to 0.5 percent of the soil cover from areas across which rofabards presently occur. At this rate it would take just a few hundred more years to finish stripping the soil from the whole island. Since Viking settlement, rofabard erosion has removed the soil from about five square miles per year. Icelandic scientists fear that many areas of the country have already passed a threshold that makes further erosion inevitable. They also know that once stripped of soil the land is pretty much useless.

Figure 25. Professor Ulf Hellden standing on top of a rofabard, the last remnant of soil that formerly covered the surrounding plain, Iceland (courtesy of Professor Hellden, Lund University).

Even though Iceland has lost 6o percent of its vegetative cover and 96 percent of its tree cover, after i,ioo years of inhabitation most Icelanders find it difficult to conceive of their modern desert as having once been forested. Most don't comprehend how severely their landscape has been degraded. Just as at Easter Island, people's conception of what is normal evolves along with the land-if the changes occur slowly enough.

The Caribbean islands of Haiti and Cuba provide another dramatic contrast in how island nations treat their soil. Haiti, which means "green island" in the native language, Arawak, is a modern example of how land degradation can bring a country to its knees. Cuba provides an example of a nation that, out of necessity, transformed a conventional agricultural system into a model for feeding a post-petroleum world.

The history of Haiti, the western third of the island of Hispaniola, shows that small hillside farms can lead to devastating soil loss even without a disastrous hurricane. Within twenty-five years after Columbus discovered Hispaniola in 1492, Spanish settlers had annihilated the island's native inhabitants. Two centuries later, in 1697, the Spanish ceded the western third of the island to the French, who imported African slaves to work timber and sugar plantations serving European markets. The colony's half million slaves revolted in the late eighteenth century, and in 1804 Haiti became the world's first republic of freed citizens to declare independence-from France, Europe's first republic.

Figure 26. Map of Iceland showing the extent of areas considerably to severely eroded, glacial ice, and uneroded soils (created from data provided courtesy of Einar Gretarsson).

Subsequent cultivation on steep slopes converted about a third of the country to bare rocky slopes incapable of supporting agriculture. In colonial times, there were reports of extensive erosion on upland coffee and indigo plantations and plantation owners could count on only three years of productive crops from upland fields. Widespread cultivation of steep slopes began again in the mid-twentieth century when subsistence farmers spread back into the uplands. By 199o, 98 percent of Haiti's native tropical forest was gone. Common erosion control measures such as piling up soil into mounds, or piling up soil against stakes placed along contour to create small terraces, were not very effective in controlling erosion on steep slopes.

Soil loss from the uplands in the rainy season is so severe that bulldozers function as tropical snowplows to clear the streets of the capital, Portau-Prince. The United Nations estimates that topsoil loss over at least half the country is severe enough to preclude farming. The U.S. Agency for International Development reported in 1986 that about a third of Haiti was extremely eroded and practically sterile from soil loss. Farmers worked an area six times larger than the area well suited for cultivation. The UN Food and Agriculture Organization estimated that soil erosion destroyed 6,ooo hectares of arable land a year in the 198os. For the past few decades estimates of the remaining area of "good" farmland showed a long-term decline of several percent a year. With little more than 50 percent of the island's potential farmland still arable, the island's growing population no longer can feed itself.

Prosperity disappeared along with Haiti's topsoil. As subsistence farms literally disappeared many rural families resorted to felling the last remaining trees to sell as charcoal to buy food. Desperate peasants flocking to cities created huge slums that fostered the insurgency that toppled the government in 2004.

Haiti's crippling soil loss is not simply a colonial legacy. Land distribution in Haiti is far more egalitarian than elsewhere in Latin America. After independence the Haitian government confiscated colonial estates and freed slaves began farming unclaimed lands. Early in the nineteenth century, Haiti's president distributed a little more than 15 hectares of land to each of some ten thousand beneficiaries. Since then, land holdings generally were divided upon inheritance and several centuries of population growth gradually reduced the size of the average peasant farm to the point where by 1971, the average farm size was less than 1.5 hectares. With an average of between 5 and 6 people per household, this comes to about 0.25 to 0.3 hectares per person. More than three-fourths of rural households fall below the poverty line and two-thirds of Haitian households fall below the UN Food and Agriculture Organization's minimum nutritional standard. This is Ireland all over again, this time without the landlords.

As the population grew, the land inherited by each successive generation was subdivided into smaller plots that eventually became too small to allow fallow periods. Declining farm income reduced the ability to invest in soil conservation measures. Unable to support themselves, the poorest farmers move on to clear steeper hillsides-the only remaining land not already cultivated-and start the cycle all over on land that can last only a few years. Eventually the shortage of arable land and rising rural poverty pushes peasants from hillside subsistence farms to search for work in Portau-Prince, where the concentration of desperate people in slums contributes to the country's tragic history of civil strife.

In Haiti, the majority of peasants own their own small farms. So small farms per se are not the answer to stopping erosion. When farms become so small that it is hard to make a living from them, it becomes hard to practice soil conservation. In Cuba, fifty miles from Haiti across the Windward Passage, the collapse of the Soviet Union set up a unique agricultural experiment. Before the 1959 Cuban revolution, the handful of people who controlled four-fifths of the land operated large export-oriented plantations, mostly growing sugar. Although small subsistence farms were still common on the remaining fifth of the land, Cuba produced less than half its own food.

After the revolution, in line with its vision of socialist progress, the new government continued sponsoring large-scale, industrial monoculture focused on export crops-primarily sugar, which accounted for threequarters of Cuba's export income. Cuba's sugar plantations were the most mechanized agricultural operations in Latin America, more closely resembling those in California's Central Valley than on Haiti's hillsides. Farm equipment, the oil to run them, fertilizers, pesticides, and more than half of Cuba's food were imported from the island's socialist trading partners. The end of Soviet support and an ongoing U.S. trade embargo plunged Cuba into a food crisis. Unable to import food or fertilizer, Cuba saw the calories and protein in the average diet drop by almost a third, from 3,000 calories a day to i,9oo calories between 1989 and 1994.

The Soviet collapse resulted in an almost 9o percent drop in Cuba's external trade. Fertilizer and pesticide imports fell by 8o percent and oil imports fell by 50 percent. Parts to repair farm machinery were unobtainable. The New York Times editorial page predicted the imminent collapse of Castro's regime. Formerly one of the best-fed nations in Latin America, Cuba was not quite at the level of Haiti-but not much above it. Isolated and facing the loss of a meal a day for everyone on the island, Cuban agriculture needed to double food production using half the inputs required by conventional agriculture.

Faced with this dilemma, Cuba began a remarkable agricultural experiment, the first nation-scale test of alternative agriculture. In the mid-198os, the Cuban government directed state-run research institutions to begin investigating alternative methods to reduce environmental impacts, improve soil fertility, and increase harvests. Within six months of the Soviet collapse, Cuba began privatizing industrialized state farms; staterun farms were divided among former employees, creating a network of small farms. Government-sponsored farmers' markets brought peasant farmers higher profits by cutting out intermediaries. Major government programs encouraged organic agriculture and small-scale farming on vacant city lots. Lacking access to fertilizers and pesticides, the food grown in the new small private farms and thousands of tiny urban market gardens became organic not through choice but through necessity.

Charged with substituting knowledge-intensive agriculture for the embargoed inputs needed for conventional agriculture, the country's research infrastructure built on experiments in alternative agriculture that had languished under the Soviet system but were available for widespread, and immediate, implementation under the new reality.

Cuba adopted more labor-intensive methods to replace heavy machinery and chemical inputs, but Cuba's agricultural revolution was not simply a return to traditional farming. Organic farming is not that simple. You cannot just hand someone a hoe and order them to feed the proletariat. Cuba's agricultural transformation was based as much on science as was the Soviet era's high-input mechanized farming. The difference was that the conventional approach was based on applied chemistry, whereas the new approach was based on applied biology-on agroecology.

In a move pretty much the opposite of the green revolution that transformed global agriculture based on increased use of irrigation, oil, chemical fertilizers and pesticides, the Cuban government adapted agriculture to local conditions and developed biological methods of fertilization and pest control. It created a network of more than two hundred local agricultural extension offices around the country to advise farmers on low-input and no-till farming methods, as well as biological pest control.

Cuba stopped exporting sugar and began to grow its own food again. Within a decade, the Cuban diet rebounded to its former level without food imports or the use of agrochemicals. The Cuban experience shows that agroecology can form a viable basis for agriculture without industrial methods or biotechnology. Unintentionally, the U.S. trade embargo turned Cuba into a nation-scale experiment in alternative agriculture.

Some look to the Cuban example as a model for employing locally adapted ecological insight and knowledge instead of standardized mechanization and agrochemistry to feed the world. They see the solution not simply as producing cheap food, but keeping small farms-and therefore farmers-on the land, and even in cities. Thousands of commercial urban gardens grew up throughout the island, hundreds in Havana alone. Land slated for development was converted to acres of vegetable gardens that supplied markets where local people bought tomatoes, lettuce, potatoes and other crops. By 2004 Havana's formerly vacant lots produced nearly the city's entire vegetable supply.

Cuba's conversion from conventional agriculture to large-scale semiorganic farming demonstrates that such a transformation is possible-in a dictatorship isolated from global market forces. But the results are not entirely enviable; after almost two decades of this inadvertent experiment, meat and milk remain in short supply.

Cuba's labor-intensive agriculture may not produce basic crops as cheaply as American industrial farming, but the average Cuban diet did recover that lost third meal. Still, it is ironic that in retreating from the socialist agenda, this isolated island became the first modern society to adopt widespread organic and biologically intensive farming. Cuba's necessity-driven move toward agricultural self-sufficiency provides a preview of what may come on a larger scale once we burn through the supply of cheap oil that presently drives modern agriculture. And it is somewhat comforting to know that on at least one island the experiment has already been run without social collapse. Less comforting is the question of whether something similar could be pulled off in a society other than a one-party police state.

After Darwin's famous sojourn in the Galapagos, the isolated nature of islands strongly influenced biological theory. But it is only in the last several decades that such thinking reached the realm of anthropology. While people may someday migrate into space to colonize other planets, the vast majority of us remain trapped on our planet for the foreseeable future. Although a global rerun of Haiti, Mangaia, or Easter Island is by no means inevitable, the experiences of societies on islands around the world remind us that Earth is the ultimate island, an oasis in space rendered hospitable by a thin skin of soil that, once lost, rebuilds only over geologic time.