9
HUMBOLDT’S GIFTS

ONE MORNING A FEW WEEKS after my return from Costa Rica, I caught the bus to Oxleas Wood, one of the few remaining scraps in London of the forest that once covered southern England. It was a weekday morning, so thankfully there were only a few joggers and dog walkers about to see what I did next. I stretched out my arms—it’s almost exactly a meter from the tip of my nose to the tip of my out-stretched fingers—and, taking aim at a likely looking patch of trees, paced out 50 meters, thus completing one-tenth of an amateur’s gentraso.

On my way I bumped into 22 stems broad enough to be measured—not many less than you would find in a cloud forest. But 17 of them were oaks. Of the remainder, there was one each of silver birch, holly, hawthorn, sweet chestnut, and sycamore. The last two of these are not even native: The first was introduced by the Romans, the second from France in the middle ages. Strolling around, I guessed that another nine lines would have added beech, ash, rowan, and hazel to my species list, but I would have been lucky to get much beyond low double figures.

I would have gotten much the same result had I compared any European woodland with any patch of Costa Rican forest. Life is more



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In the Beat of a Heart: Life, Energy, and the Unity of Nature 9 HUMBOLDT’S GIFTS ONE MORNING A FEW WEEKS after my return from Costa Rica, I caught the bus to Oxleas Wood, one of the few remaining scraps in London of the forest that once covered southern England. It was a weekday morning, so thankfully there were only a few joggers and dog walkers about to see what I did next. I stretched out my arms—it’s almost exactly a meter from the tip of my nose to the tip of my out-stretched fingers—and, taking aim at a likely looking patch of trees, paced out 50 meters, thus completing one-tenth of an amateur’s gentraso. On my way I bumped into 22 stems broad enough to be measured—not many less than you would find in a cloud forest. But 17 of them were oaks. Of the remainder, there was one each of silver birch, holly, hawthorn, sweet chestnut, and sycamore. The last two of these are not even native: The first was introduced by the Romans, the second from France in the middle ages. Strolling around, I guessed that another nine lines would have added beech, ash, rowan, and hazel to my species list, but I would have been lucky to get much beyond low double figures. I would have gotten much the same result had I compared any European woodland with any patch of Costa Rican forest. Life is more

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In the Beat of a Heart: Life, Energy, and the Unity of Nature varied in the tropics than in temperate regions and more varied in temperate regions than at the poles. This is one of the earth’s most obvious features. The possible causes are simple. Either species are more likely to evolve in the tropics or they are less likely to go extinct—or both. But working out what might speed up the birth of species or slow down their death has proved much more complicated. Two centuries after the first hypothesis was proposed, there is still no accepted explanation for what causes biodiversity to peak in the tropics. Besides being the oldest question in ecology, it is one of the toughest. Discovering Diversity The greater variety of tropical life must have been obvious to European travelers from the start. Science began to come to grips with tropical diversity in the eighteenth century, when expeditions added scientific inquiry to their missions of discovery and conquest. The model was the voyages of James Cook, whose ships observed, measured, and collected everywhere they went. In the process Joseph Banks, the naturalist on Cook’s first voyage, acquired the world’s largest herbarium. Of course, besides containing more species, the tropics have a very different climate than the temperate regions. Natural philosophers immediately fastened on to this feature as the cause of tropical regions’ lushness. German botanist Carl Ludwig Willdenow discussed this issue in his book, The Principles of Botany and of Vegetable Physiology, published in 1792 and translated into English in 1805. Willdenow noted that warmer places had more plant species. He also gave a detailed measurement—it might be the original piece of macroecology—of what is now called the latitudinal gradient in diversity: The Florae of different parts of the globe, with which botanists have favoured us, show indeed that vegetation increases with the degree of warmth. In Southern Georgia, according to credible accounts, only 2 wild growing plants are found; in Spitzbergen, 30; in Lapland, 534; in Iceland, 553; in Sweden, 1,299; in Brandenburg, 2,000; in Piedmont, 2,800; on the coast of Coromandel [the region of southeast India around Madras], about 4,000; in Jamaica as many; and in Madagascar nearly 5,000.

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In the Beat of a Heart: Life, Energy, and the Unity of Nature Current estimates stand at 25 plant species for the South Atlantic island of South Georgia and 10,000 to 12,000 for Madagascar. Willdenow did not suggest any explanations of why warmer temperatures led to more diverse plant life. But one of his students, an aristocratic teenager who went on to be one of the greatest scientists of the era, did. In his old age, Baron Friedrich Heinrich Alexander von Humboldt tried to destroy all records of his childhood, to prevent future biographers uncovering anything unflattering and to prevent any revelations about his personal life obscuring history’s view of his science. No matter: Alexander von Humboldt’s scientific achievements alone are more than enough to chew on. As a biologist he demonstrated the influence of electricity on nerve and muscle tissue and showed how the electric eel has its shocking effect. As a natural historian, he collected 60,000 specimens and described 3,500 new species. As an anthropologist, he was the first European to study the Incan, Aztec, and Mayan civilizations and deciphered the Aztec calendar. As a geologist he was the first to note that volcanoes come in chains and suggested that this had to do with lines of weakness in the earth’s crust. As an oceanographer, despite never sailing on the Pacific, he predicted the existence of that ocean’s Peru Current (sometimes called the Humboldt current) from observations of the South American climate. His friends included Goethe, Thomas Jefferson, and Tsar Nicholas I of Russia. He founded the first international scientific organization, to pool and share data on the study of the earth’s magnetic field. His last work was a book, titled Cosmos, which attempted to describe and unite all scientific knowledge. It was unfinished on his death in 1859, at age 90. The German government gave him a state funeral. Born into a family of slightly down-at-the-heel Prussian nobility, Humboldt followed his time under Willdenow with study at Göttingen University, then Germany’s leading scientific school. After this he studied geology at the Freiberg School of Mines. In 1792 he took up a job as an inspector in the Prussian Department of Mines. In 1796 his mother died and left him a large fortune. Humboldt could now pursue his greatest ambition—to travel. As a child he had read of Cook’s

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In the Beat of a Heart: Life, Energy, and the Unity of Nature voyages and dreamed of emulating them. At Göttingen, Georg Forster, the naturalist on Cook’s second voyage, became his mentor. Quitting the Department of Mines, Humboldt spent the next winter in the Alps, surveying the land, taking weather readings, and practicing to be an explorer. His first plan was to join a British expedition to Egypt, but Napoleon’s invasion of Egypt killed the project. So Humboldt went to Paris, then the center of the intellectual world. There he met scientists such as the zoologist Jean-Baptiste Lamarck, the anatomist Georges Cuvier, and Lavoisier’s former colleague, the mathematician Pierre Simon Laplace. Humboldt made such a favorable impression on the grandees of French science that he won a place on a planned expedition to the Pacific, a five-year journey over land and sea. Napoleon, however, intervened again, canceling the expedition and diverting its funds into his military campaigns. But if Humboldt had no voyage, he did find a traveling companion, in the shape of the abortive expedition’s botanist. Aimé Bonpland, a Frenchman born near Bordeaux in 1773, had qualified as a physician but had little interest in medicine. His true passion was plants, particularly roses. Humboldt was stiff and formal; Bonpland was a charmer. Humboldt was meticulous and hard working; Bonpland made up his life as he went along. Humboldt never married and seems to have met his emotional needs through work; Bonpland was a womanizer. Humboldt was rich; Bonpland was usually broke. But they shared a love of nature, a belief in the glory of science, and a yearning for travel and adventure. The two resolved to see the world together. Their plan was to travel to North Africa, but the ship that was to carry them from Marseille to Algiers was wrecked before it could reach them, and so in the winter of 1798 they went instead to Spain, studying Iberian geography and nature en route. In Madrid their luck improved. The Saxon ambassador to Spain was enthusiastic about science, and helped the would-be explorers win the ear of one of the Spanish ministers. This acquaintance led in turn to an audience with King Carlos IV. At the time, Spain’s colonies were closed to foreigners and were permitted to trade only with Spain. No foreign scientist had visited South America for more than 60 years, and the continent’s

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In the Beat of a Heart: Life, Energy, and the Unity of Nature interior was essentially uncharted. Humboldt persuaded the king that Spain’s dominions should be studied. With his geological expertise, he could assess the empire’s mineral wealth, advise on how to get better returns from existing mines, and suggest where to look for new deposits. As biologists, Humboldt and Bonpland could collect specimens for the royal museum and gardens. Carlos IV agreed—Humboldt and Bonpland had to travel at their own expense, but were given documents that allowed them to go where they liked and instructed Spanish ships to give them passage. The king’s decision was to backfire. After his South American voyage, Humboldt returned to Paris and published accounts of his journeys. Another temporary Parisian, Simon Bolivar, read of the wonders of Incan and Aztec civilizations and, between the lines, of the corruption, injustice, and neglect of Spanish colonial rule, particularly slavery. Bolivar sought out Humboldt, himself a republican and fierce abolitionist, and the two met several times. A few years later Bolivar returned home and liberated most of Latin America from Spanish rule. The Greatest Scientific Traveler Who Ever Lived On June 5, 1799, Humboldt and Bonpland set sail in the Pizarro from La Coruña at the northwest tip of Spain. As he sat in his cabin waiting to depart, Humboldt wrote a letter to a friend explaining that, rather than just cataloguing what he saw, he wanted to explain it: I shall collect plants and fossils and make astronomical observations. But that’s not the main purpose of my expedition—I shall try to find out how the forces of nature interact upon one another, and how the geographic environment influences plant and animal life. In other words, I must find out about the unity of nature. Humboldt and Bonpland spent the next five years exploring the Americas. They explored coastal Venezuela and journeyed 1,700 miles up the Orinoco River, mapping it as they went and showing that its upper reaches connect with the Amazon. They studied the tribes living along the river’s banks, and there is an apocryphal story that Humboldt reconstructed the language of an extinct tribe from the vocabulary of a parrot he bought from a neighboring tribe. They went to the Andes, Mexico, Cuba, and the United States. The journey has been described

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In the Beat of a Heart: Life, Energy, and the Unity of Nature Alexander von Humboldt (1769–1859). Credit: Bildarchiv Preussischer Kulturbesitz, Berlin.

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In the Beat of a Heart: Life, Energy, and the Unity of Nature as the scientific discovery of America, and it became the foundation of Humboldt’s life’s work. The accounts of his voyage inspired a generation of nineteenth-century naturalists. He was a hero to Darwin, who described him as “the greatest scientific traveler who ever lived” and joined the crew of the Beagle to follow in Humboldt’s footsteps. He recommended Humboldt’s writings to his friends and once wrote: “I shall never forget that my whole course of life is due to having read and re-read as a youth his Personal Narrative [of the American expedition].” One twentieth-century biographer wrote that Humboldt “combined meteorology, geography, geology, botany and zoology and, single-handed, created the science of ecology.” Back in Paris, Humboldt spent his inheritance—within a few years he was nearly as poor as Bonpland—on publishing his observations and ideas from the voyage. The core of his thinking about tropical diversity is contained in an 1807 essay, Ideas for a Physiognomy of Plants. Like Willdenow, he equated warmer climes with more species: The verdant carpet which a luxuriant Flora spreads over the surface of the earth is not woven equally in all parts; for while it is most rich and full where, under an ever-cloudless sky, the sun attains its greatest height, it is thin and scanty near the torpid poles, where the quickly-recurring frosts too speedily blight the opening bud or destroy the ripening fruit…. Those who are capable of surveying nature with a comprehensive glance, and abstract their attention from local phenomena, cannot fail to observe that organic development and abundance of vitality gradually increase from the poles to the equator, in proportion to the increase of animating heat…. It is beneath the glowing rays of the tropical sun that the noblest forms of vegetation are developed. Tropical Wonders Discoveries made since Humboldt marveled at the Orinoco have only reinforced our view of tropical life. There are not just more plant species in the tropics, there are more woodpeckers, ants, and monkeys—and birds, insects, and mammals in general. The same goes for parasites: Most human diseases are tropical diseases, and the tropics harbor more pathogens of all kinds. Even cultural diversity follows a similar pattern. In sub-Saharan Africa, the areas that contain the most species also contain the most linguistic groups.

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In the Beat of a Heart: Life, Energy, and the Unity of Nature Forests are not the only environment where diversity peaks in the tropics. Grasslands contain more species nearest the equator and so do deserts. More species dwell in tropical lakes and rivers than in temperate fresh water. In the sea, diversity in fish, molluscs, and plankton peaks at the equator. Tropical seas contain more species both along coasts and in the open ocean. Nor is the diversity gradient a recent thing. Fossils, including those of trees and foraminifera, the planktonic creatures beloved by D’Arcy Thompson, show that the tropics have contained more species than the temperate zones for at least the past 250 million years. The diversity gradient is pervasive and incontrovertible. But it is general and qualitative, and there are many complications and exceptions to the general pattern. In some groups—such as aphids, seabirds, and marine mammals—diversity peaks in the temperate regions. Points at the same latitude in different continents have different numbers of species. The steepness of the diversity gradient varies between groups and between the northern and southern hemispheres. And although diversity gradients have probably been constant throughout the earth’s history, the fossils suggest that they are steeper now than at any point in the past 65 million years. Nevertheless, such a widespread phenomenon cries out for a general explanation. Humboldt saw a clue as to what this might be in another pattern in diversity. Humboldt could never see a mountain without needing to stand on its summit, and in the Andes this obsession led to an attempt on the extinct volcano Chimborazo, in Ecuador. The mountain is 20,700 feet high; at the time it was thought to be the world’s highest. Bonpland and Humboldt made it up past 19,000 feet, higher than any human had gone before, until a wall of ice and snow blocked their route to the summit. When news of the explorers’ feat reached Europe, it knocked Napoleon’s conquests off the front pages. Napoleon was not impressed. (Back in Paris, Humboldt was presented to the emperor: “I understand you collect plants, monsieur,” he said. “Yes, sire,” Humboldt replied. “So does my wife,” Napoleon responded.) As he ascended Chimborazo, Humboldt decided that the bleeding gums and other unpleasant symptoms he was suffering must be due to

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In the Beat of a Heart: Life, Energy, and the Unity of Nature lack of oxygen—making him the first person to diagnose the cause of altitude sickness. He also noticed that gaining altitude had the same effect on the vegetation as traveling north. As he climbed, so the tropical forest gave way to cypresses, oaks, and pines, much like the trees in European forests: The extraordinary height to which not only individual mountains but even whole districts rise in tropical regions, and the consequent cold of such elevations, affords the inhabitant of the tropics a singular spectacle. For besides his own palms and bananas he is surrounded by those vegetable forms which would seem to belong solely to northern latitudes…. Thus nature has permitted the native of the torrid zone to behold all the vegetable forms of the earth without quitting his own clime. The key to this changing vegetation, and therefore the explanation for tropical diversity, Humboldt suggested, was water. As you move away from the tropics, or gain altitude, freezing temperatures became steadily more common. In such an environment, plant life is suspended for much of the time: “Nature undergoes a periodic stagnation in the frigid zones: for fluidity is essential to life.” Humboldt believed that freezing was a stricture that relatively few forms of plant life could adapt to. Only those that could either withstand the cold or shed their leaves and wait for spring would survive. In the more permissive equatorial environment, life ran riot. “The nearer we approach the tropics, the greater the increase in the variations of structure, grace of form, and mixture of colours.” A Harsh World? The idea that the tropical climate is more conducive to life, and so leads to a greater variety of it, is a persistent theme of explanations for tropical diversity. But it is a more slippery notion than it first appears. The idea carries a powerful whiff of circularity. The tropics have lots of species because they are a benign environment. How do we know they are a benign environment? Because they have lots of species. We usually distinguish an environment as harsh or lush by its abundance of life, but this says nothing about what in that environment might cause that abundance or scarcity. Anyway, who’s to say what’s harsh? True, environments that seem

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In the Beat of a Heart: Life, Energy, and the Unity of Nature ill suited to our notion of an easy life—hot springs, salt lakes, deserts—have few other species living in them. Yet to a thermophilic bacterium, a hot spring is home. To a Stenocara beetle, which can collect the water from fog on its wings, the Namib Desert is just fine. If these species can adapt to their environment, why haven’t more? Hot springs are rich in nutrients, after all, and there is no shortage of solar energy in the Namib. There are ways to save arguments about environmental quality from circularity. One is to replace harshness with commonness. Some environments, such as the climates and conditions that support forests, are widespread. Others, such as hot springs, are usually small and isolated. We would expect lots of species to adapt to life in a common environment, but few of them would then be equipped to colonize a very different one. Yet some environments with relatively few species, such as deserts and tundra, are neither small nor isolated. Their environments, however, are hostile to life by other measures than the number of species found there. Here, it might be the physical and chemical limits on living processes that limit diversity. Life’s chemistry needs water, so living in places where water is absent or frozen will be difficult and expensive, requiring resources to be diverted from growth and reproduction. But the question of why, given that a few species can adapt to life on the tundra, more cannot still applies. Evelyn Hutchinson addressed this problem in his Homage to Santa Rosalia. Maybe, he suggested, as well as placing physical limits on what can live where, climate controls diversity by controlling life’s fuel supply. Hot and sunny—high-energy—environments contain more species than cold, gray places. Tropical forests also get more rain, another essential ingredient for plant life. In fact, a place’s temperature and rainfall more accurately reflect its species diversity than does its latitude—the diversity gradient is climatic. The match between energy and diversity stares us in the face. Perhaps this is why more species survive in the tropics than temperate or cold climates. In other words, the reason there is only one species of polar bear is not that only one bear species has evolved to hunt on ice floes and swim in the Arctic Ocean, but that there isn’t enough food in the Arctic to support two bear species. Every species must maintain a certain

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In the Beat of a Heart: Life, Energy, and the Unity of Nature number of individuals to survive. If its numbers fall too low, life’s cruel lottery will finish it off. Perhaps environments with more energy can support a greater quantity of life and so allow more species to keep their numbers on the right side of the threshold for survival. But the idea that energy promotes diversity by allowing greater numbers of organisms to live together also has its problems. Tropical seas and soils are both low in nutrients and rich in species. More resources do not necessarily mean more diversity. Adding nutrients to lakes, rivers, or soil, in the form of fertilizer or sewage, leads to a drop in the number of species. A few fast-growing species, good at sucking up abundant nutrients, come to dominate. On the other hand, this might be because few of the local species are adapted to such conditions. Over evolutionary timescales, one might expect diversity to rise as more species evolved to deal with the richer environment. And in reality it is the quality, not the quantity, of tropical life that is outstanding. As Al Gentry found, and Brian Enquist and Karl Niklas confirmed, there aren’t more trees, or a greater mass of wood, in a hectare of Amazonian rain forest than a hectare of Alaskan conifer woods—just vastly more species. Other researchers have found that the same goes for North American birds and butterflies. There is a slight trend toward increased numbers of individuals at lower latitudes, but the number of species rises far more quickly. Diversity Through Stability Perhaps, then, it’s the quality of the climate, not the quantity of energy, that’s important. As well as being warmer and wetter, the climate in the tropics is, as Humboldt noted, more stable, lacking the seasonal fluctuations in temperature found in temperate regions. This stability might encourage diversity by letting species be more specialized, making their niches narrower and allowing a finer division of natural resources. If an animal in a temperate forest lives on fruit, or leaves, or insects, it has a problem. These food sources disappear for some of the year. The animal must either broaden its diet, hibernate, or migrate. But in the tropics it can eat leaves, fruit, or bugs all year round, leaving more resources for other species. Perhaps stability promotes

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In the Beat of a Heart: Life, Energy, and the Unity of Nature support his brother. Alfred became a schoolmaster in Leicester, a market town in the English midlands. In the town library he read Humboldt’s story of his South American journey and met a fellow naturalist, Henry Bates. The two became friends and, like Humboldt and Bonpland half a century earlier, began looking for expeditions. They settled on a journey up the Amazon. Amazonian nature was unexplored, and they could finance the trip by selling the specimens they bagged to museums and collectors in London. The two left England in April 1848. Wallace and Bates did make epic journeys in South America, but separately. No one knows why, but they parted company a few months after reaching Brazil. Wallace spent the next 4 years (Bates stayed for 11) traveling throughout Amazonia, going farther upriver than any previous naturalist, living with native tribes, and, with gun and collecting jar always at the ready, snaffling every species that crossed his path. He sent back regular shipments of beetles, butterflies, birds, alligators, monkeys, plants, and fish, to be sold by his London-based agent. The trip, however, ended in a series of disasters. Alfred’s younger brother Herbert, who had come to Brazil in 1849 to join Alfred and pursue his own career as a collector, died of yellow fever in June 1851. Alfred suffered recurrent bouts of fever, some so bad he feared they would kill him (he would carry malaria for the rest of his life). And three weeks into the voyage home, after leaving Brazil in July 1852, his ship, the Helen, caught fire and sank. Wallace and the crew spent 10 days in a small boat, eking out what little food and water they had, before a passing ship came to the rescue. Alfred had lost his journals, his drawings, a small menagerie of live animals, a commercial collection that he estimated to be worth £500, and his personal collection, containing “hundreds of new and beautiful species, which would have rendered my cabinet … one of the finest in Europe.” All that survived was one parrot that had made it off the burning ship. Wallace had hoped to return to London with a collection that would secure his financial future and make his scientific name. As it was, he could only be grateful that his agent had insured the lost specimens. There was nothing to be done except set off for the tropics again. Eighteen months later, in January 1854, he went east. Wallace spent the

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In the Beat of a Heart: Life, Energy, and the Unity of Nature next eight years wandering throughout the Malay Archipelago, the region that is now Indonesia and Malaysia, from Singapore to New Guinea. Again, he aimed to fund the trip through commercial collecting. It was the basis of all his future biology and, he said, “the central and controling incident” of his life. Wallace did not give a full account of his ideas about diversity gradients until much later, in an 1878 book called Tropical Nature. In it he gives a beautiful description of what it feels like to be a temperate naturalist in a tropical forest: Instead of endless repetitions of the same forms of trunk such as are to be seen in our pine, oak or beechwoods, the eye wanders from one tree to another and rarely detects two columns of the same species…. If the traveler notices a particular species and wishes to find more like it, he may often turn his eyes in vain in every direction. Trees of varied forms, dimensions and colors are around him, but he rarely sees any one of them repeated. Time after time he goes towards a tree which looks like the one he seeks, but a closer examination proves it to be distinct. He may at length, perhaps, meet with a second specimen half a mile off, or may fail altogether, till on another occasion he stumbles on one by accident. Wallace subscribed to Humboldt’s view that climate limited the spread of plant species and that the tropics harbored those too delicate to survive harsher climes. But a great deal else had happened since Humboldt had seen the rain forest. Geologists had realized that the world was vastly older than the biblical chronology could account for. And Darwin and Wallace had separately realized that all organisms were locked in a perpetual struggle that, through the mechanism of natural selection, could create and modify species. Even in Leicester, Wallace had been thinking about how species might give rise to new species. He continued thinking about the topic in the Amazon, discussing the subject with Bates and other Western naturalists he encountered on the river. Full revelation came in early 1858. Collecting on the island of Gilolo (now Halmahera), which lies between Sulawesi and New Guinea, Wallace fell sick with malaria. Lying in bed with a fever, his mind turned to Thomas Malthus’s theories of how war, famine, disease, and disasters kept human numbers in check. Something similar, he thought, must apply to other animals. He recalled his next thought in his autobiography, published in 1905:

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In the Beat of a Heart: Life, Energy, and the Unity of Nature Then it suddenly flashed upon me that this self-acting process would necessarily improve the race, because in every generation the inferior would inevitably be killed off and the superior would remain—that is, the fittest would survive. When his fever eased, Wallace wrote up his ideas in a short paper, “On the Tendency of Varieties to Depart Indefinitely from the Original Type,” and sent it to Charles Darwin for comment. Unbeknown to Wallace, Darwin had had a similar insight 20 years previously, also inspired by Malthus, but had never published (one of science’s great what-ifs is how biology would have developed had Wallace perished in the mid-Atlantic with the Helen). Darwin, poleaxed by Wallace’s missive, hurried to secure his claim to the idea. On July 1, 1858, at a meeting of the Linnean Society in London, (he was not consulted) Wallace’s paper and a previously unpublished 1844 essay by Darwin were presented jointly, and natural selection was unveiled. Guided by his evolutionary thinking, in Tropical Nature Wallace suggested several reasons for the diversity gradient. In a uniform climate, he reasoned, competition between species would be stronger, resulting “in a nice balance of organic forces, which gives the advantage now to one, now to another species, and prevents any one type of vegetation from monopolising territory.” And species could also be more specialized, to shade or sun, or to grow on other plants as epiphytes or parasites: “Every place in nature [is filled with] some specially adapted form.” As well as being the codiscoverer of natural selection, Wallace founded what is now called biogeography. This discipline combines elements of ecology, paleontology, evolutionary biology, and geology to investigate how species arise, disappear, come together, and move apart under the influence of processes such as continental drift, volcanic eruptions, and climate change. Famously, Wallace noticed that the Indonesian islands of Lombok and Bali, although separated by only 25 kilometers of sea, have very different flora and fauna. This divide became known as the Wallace Line, and it marks the boundary between the Asian and Australasian tectonic plates. The islands are only recent neighbors. Most of their species arrived when the two were far apart, and there has been little interchange since they drifted together.

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In the Beat of a Heart: Life, Energy, and the Unity of Nature In a similarly biogeographical vein, Wallace also argued that geological history could account for the greater diversity of tropical animals. Successive ice ages have smothered temperate species under glaciers, “destroying most of the larger and more specialised forms … [and] necessitating the commencement of the work of development in certain lines over and over again.” In the tropics, “evolution has had a fair chance” and, unchecked by periodic catastrophes, was able to accumulate the full, thrilling range of forms, sizes, colors, and behaviors. The tropics are more diverse because they are older, and so provide both a laboratory for producing evolutionary novelties and a preserve where the results of these experiments can survive. Where disaster strikes, we do see large, and long-lasting, reductions in species’ diversity. In 1883 the eruption of Krakatoa sterilized several islands in its vicinity. These islands are green once more, but it takes time to recover from such a catastrophe, and they still have fewer species than similar islands not caught in Krakatoa’s blast. And there is evidence that the most recent ice age, which ended about 12,000 years ago, still casts its shadow over the spread of species. In North America the areas most recently covered by ice harbor fewer bird species, and the comparative youth of the Great Lakes of America may be one reason why they contain vastly fewer fish species than African lakes such as Victoria and Malawi. Pollen analysis shows that tree species took millennia to recolonize American forests in the wake of the glaciers’ retreat. History has been invoked to explain other differences in species diversity besides that between the tropics and the temperate zone. North America has more tree species than Europe because, it has been suggested, Europe’s mountains—the Alps—run east to west. As European trees retreated south before the advancing glaciers, they would have been squeezed between the ice and the mountains, and many would have gone extinct with their backs up against the Alps. American trees faced no such barrier, as the Rockies run north–south, and so could have gone and returned more easily. Past history, then, is an important factor in current biodiversity. Time is needed both for existing species to colonize places depopulated by a catastrophe and for new species to evolve to exploit new habitats. The longer since a place was wiped bare, the more species one

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In the Beat of a Heart: Life, Energy, and the Unity of Nature would expect to see there. But although this reasoning helps explain local differences in species diversity, it is less clear whether history—like all the other explanations encountered so far—can explain the global equator-to-pole gradient. Glaciers do not inevitably wipe out all species in their path. Rather than being eliminated, the temperate zones and their species might simply have shifted to follow the climate. This should be particularly true in the sea, where life is more mobile. And the climatic changes wrought by ice ages also affect tropical forests. Fossil pollen shows that the extent of tropical forests has varied a great deal. It appears that drier and warmer climates lead to large patches of grassland appearing amid the trees. It’s also been argued that isolating clumps of forest from one another promoted diversity, because new species would be more likely to evolve in forest fragments, just as they do on islands. But you can’t have it both ways—either stability promotes greater diversity or it doesn’t. Current conditions seem a better guide to current diversity than past history—the number of species found in a place is correlated more closely to its present climate than to the length of time since that place was last covered in glaciers. There seems to be some balance that, given a few thousand years, restores itself. In the Cretaceous period the world’s climate was much milder. Yet the diversity gradient was still in place. And mass extinctions have hit the tropics just as hard as the rest of the world. It is hard to argue that any one environment is older than any other. Big Places Have More Species But if the tropics are not biodiversity’s museum, they might be its cradle. There is evidence that, even if they are just as likely to die out near the equator, species are more likely to be born there. Two reasons have been suggested for this phenomenon: the tropics are larger, and they are warmer. First, larger. It doesn’t always look it on maps, because the Mercator projection typically used to transform the spherical Earth into a two-dimensional map squashes land at the equator and stretches it at the poles, making Greenland look nearly as big as Africa. But looking at a globe, it is clear that the tropics are by far the largest climatic zone on

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In the Beat of a Heart: Life, Energy, and the Unity of Nature Earth. They cover about 55 million square kilometers, three times more than the next largest zone, the tundra. Likewise in the sea, tropical oceans cover more of the earth’s area than any other land or water mass. What’s more, all the world’s tropical habitat is in one continuous swath lying on either side of the equator. Other zones, such as temperate or subtropical, are found in separate bands in the north and south. A grass seed or a ground squirrel trying to migrate from the North American prairie to the South American pampas has got to cross thousands of miles of hostile tropical habitat. But a monkey could, in theory, travel from Mexico to Argentina without entering alien territory. And within the tropics, between about 25° latitude north and south, the climate is relatively constant. The annual average temperature is about 28°C throughout this zone. As you leave the tropics, the temperature drops by about 8°C for each 10°N you travel. So species adapted to tropical climates have much more land or water to play with compared with their nontropical counterparts. It’s obvious that larger areas will contain more species. They can harbor more individuals and a greater diversity of habitats. The relationship between a place’s area and the number of species found there follows a power law. The exponent in the species-area power law—the bit that is 3/4 in Kleiber’s rule—varies from place to place but is usually between 0.1 and 0.25. So the rate at which you find new species slows down as the area you search expands. Measuring the form of this power law also allows conservation biologists to make predictions about the effects of habitat destruction. For example, the species-area curve for the forests of Eastern America suggests that removing half of the forest will drive about one in every seven bird species to extinction. Sure enough, half of the forest has been lost since 1870, and in that time four of the 30 bird species found there and nowhere else have disappeared. Less obvious, from this line of reasoning, is why the tropics should also contain, as they do, proportionately more species than the temperate regions, a greater number for each square kilometer of area. John Terborgh and Michael Rosenzweig argue that the tropics’ large area creates a high diversity because tropical species, given more land, have larger ranges—and that this makes them both less likely to go extinct and more likely to give rise to new species. Tropical species are less

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In the Beat of a Heart: Life, Energy, and the Unity of Nature likely to go extinct because a species spread over a large area is unlikely to succumb to any single fire, flood, hurricane, or epidemic. Widely spread species are also more likely to bud off new species, because small pockets of their populations will become isolated. Such pockets will stop interbreeding with the rest of the species and so become genetically different from their neighbors. If the isolation lasts long enough, they will no longer be able to breed with the parent population, and a new species will be born. A large range is more likely to be fragmented by inhospitable habitat, be it rivers, mountains, grassland, forest, desert, or sea, creating the isolated populations that give birth to new species. Rosenzweig argues forcefully that any study of what controls biodiversity must take account of area. He has collected evidence to support the idea, comparing environments with similar climates but different areas, and showing that the larger provinces contain proportionately more species. The Amazon River basin, for example, is about 1.5 times the area of the Congo basin but contains well over twice as many fish species, 1,300, compared to the Congo’s 500. But other ecologists have raised objections, and pointed out exceptions, that make it unlikely that the tropics’ great area is a general explanation for their great diversity. Not all habitats are more widespread in the tropics. There is just as much fresh water elsewhere, yet most freshwater fish are still tropical species. Coastal shelves, the shallow seas that border the continents, have no greater extent in the tropics than elsewhere. Yet they harbor a far greater diversity of molluscs, to name one group investigated by biologists. It is also not clear whether tropical species do have especially large ranges. Some ecologists have argued that species living in seasonal temperate climates should roam farther, because the adaptations that let animals and plants cope with wintry weather should also equip them to live in a wide range of different places. And how far a species can spread will depend in part on how many other species it must compete with, so, in addition to range size being one thing that might influence diversity, diversity will also influence range size. It is fearsomely difficult to disentangle the causes and consequences of diversity. Nor is it certain that species with big ranges will give birth to more new species. A species with a really large range will be less likely to become divided.

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In the Beat of a Heart: Life, Energy, and the Unity of Nature Instead of cutting a population in two, a mountain range or a forest could just become an island of inhospitable habitat in the sea of that species’ range. Evolution’s Workshop That’s how being larger might make the tropics more diverse. Now for warmer. As we have seen, the amount of energy pouring into a place is a good guide to the number of species living there. But it seems that the amount of fuel alone cannot account for the difference in diversity. Instead, extra energy might create species by speeding up life’s tempo. This is where metabolic rate re-enters our story. By influencing metabolism, temperature affects evolution. Organisms whose body temperature matches that of their environments—meaning plants and all animals except for the warm-blooded birds and mammals—have faster metabolic rates in warmer climates and so grow more quickly. They will reach maturity quicker, which will make each generation shorter, and so natural selection will have more to work with. The malaria-carrying Anopheles mosquito, which can crank through 10 generations in a year, is likely to evolve more quickly than a temperate species that can only manage one. In general, species with fast generation times can adapt quickly. That is why pesticide resistance in insects, and antibiotic resistance in bacteria, is such a problem. Jim Brown’s team has found that the warmer and smaller a species is, the more quickly its DNA changes. Like cellular metabolic rate, mutation rate over time falls as the −1/4 power of body mass. And besides speeding up the life cycle, hot temperatures cause mutations. Male mammals carry their testicles outside their bodies to keep them cool, in an attempt to reduce the mutation rate in their sperm. The effects of temperature on the pace of life, via metabolism, therefore provide a mechanism by which climate can influence the rate at which new species form. About the same amount of energy is needed to spark a mutation in all cells and species. The effect of temperature on mutation does not translate directly into evolutionary change. How much a species alters through time depends on the selective forces it experiences. But it does show that, other things being equal, the DNA

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In the Beat of a Heart: Life, Energy, and the Unity of Nature of small, hot, fast-burning plants and animals will change more quickly than that of large and cool living things. Metabolism, via its affect on the rate of life, provides a mechanism by which climate can influence the rate at which new species form. Brown’s team used temperature and metabolic rate to build a model of diversity variation with temperature, based on temperature’s affect on metabolism (note that this says nothing directly about the rate of mutation or the appearance of new species). Patterns in the diversity of trees, amphibians, freshwater fish, marine molluscs, and the parasites of marine fish all seem to match the model’s predictions, with a place’s diversity rising as it gets warmer. Fossil evidence supports the idea that the tropics are evolution’s workshop. David Jablonski has shown that most groups of marine animals first appear in equatorial rocks. Many temperate species—including us—are tropical migrants, not creatures born and bred in the cold. Tropical rocks also contain more young species than temperate rocks, suggesting that evolution works faster close to the equator. And from looking at fossil foraminifera, Drew Allen, a former student of Brown, has found that tropical species, which have metabolic rates 15 to 20 times those of their polar cousins, evolve new species more rapidly than their cold-water counterparts. Tropical forams also go extinct more quickly, perhaps because if one species splits into two, each new species has a smaller population. By working out at the molecular level how much energy is needed to cause a mutation and the amount of time it takes for a new species to evolve, Allen has made a back-of-the-envelope calculation that it takes 1022 joules to evolve a new species of foraminifera. About the same amount of energy shines down as sunlight on Earth each day. The link between temperature, metabolism, and evolution is a good candidate for an explanation of why there are more species in the tropics. It is one of the few relationships between environment and biology that ought to hold for every species, wherever it lives; many of the other explanations were derived from observing land plants and animals, and they founder when applied to marine life. It seems to fit with fossil evidence. It predicts roughly how many species we should expect to find in a place of a given temperature—rather than just offering a reason for the shape of the trend—which allows its ideas to

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In the Beat of a Heart: Life, Energy, and the Unity of Nature be tested. Our understanding of how temperature, via metabolic rate, influences development and mutation provides a mechanism for more rapid tropical evolution. And in some senses it looks good by default. Most other reasons for the diversity gradient have been around for a while, giving people time to point out limits to their generality. Ideas about evolutionary rates are relatively new and have yet to be ground down by the mill of academic scrutiny. No Easy Answers After two centuries of research, a toolkit of ideas (or a bunch of hedged bets) is starting to emerge that might enable us to explain why there are more species in one place than another and why there are more species in the tropics than anywhere else. Climate supplies energy, which influences evolution. Climate also influences the harshness and productivity of different environments, which controls what species can live there. History, in the time needed for diversity to recover from past shocks, is surely a factor, as is the world’s geometry, because the tropics are larger than anywhere else and because they are in the middle of their domain. How climate, history, and geometry might interact to control biodiversity is still poorly understood and controversial. Their combined effect is bound to be complex and will vary from place to place. It may be that the simple, top-down approach favored by macroecologists can only get us so far. One possible way forward is through computer simulations. Satellites provide the raw material for such models, in the shape of data on temperature, rainfall, solar radiation, and vegetation cover, which can then be matched to patterns in diversity, particularly the diversity of well-studied groups such as birds and mammals. These models will not be simple, like the 3/4 power law. They will be more like the models that climate scientists use to predict the movements of air and water around the atmosphere. These simulations require vast computer power and give ambiguous results, with sizable errors. Some ecologists are already employing such models to see how their theoretical predictions match up with reality and to forecast how species might respond to climate change. Perhaps the Alexander von Humboldt of the twenty-first century will set sail on a computer.

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