IN THE SPRING OF 1958, G. Evelyn Hutchinson, a Yale University zoologist, was on a field trip in Sicily. A specialist in the biology of lakes, he was looking for a type of water bug known to live on the island. Most of the land was farmed, however, and there were hardly any suitable pools. Hutchinson’s luck changed on a sightseeing trip up Monte Pellegrino, a hill outside Palermo. There, in a small pond, he found two species of the bugs he was seeking. The pond was fed by water trickling out of cracks in the mountain, just below a church dedicated to a twelfth-century hermit, Santa Rosalia. The saint was already the patron of Palermo; in his gratitude, Hutchinson added evolutionary biology to her duties.

Yet Hutchinson saw a riddle in his good fortune. Why were there two species of bug in the pond and not one—or 20? He decided to use his forthcoming address to the American Society of Naturalists, of which he had just been elected president, to pose the question. His presidential address, titled “Homage to Santa Rosalia, or Why Are There so Many Kinds of Animals?” is probably the most influential lecture in the history of ecology.

Evelyn Hutchinson made no great discoveries about nature. Nor was he a particularly brilliant fieldworker or experimentalist; in fact, he could be rather butterfingered. Early in his career, Hutchinson pioneered the use of radioactive isotopes to track the movement of chemical elements through living things. Rowing out onto Linsley Pond, in Connecticut, with a vial of radioactive phosphorus, he spilled the contents into the boat. The only way for him to get the phosphorus into the lake was to scupper his craft.

Hutchinson’s genius lay rather in his combination of intellectual gifts. Like many biologists, he took a sensual delight in the natural world. In his case it was part of a broader aesthetic, a general fascination with beautiful things. He loved ornament and variety, and late in life wrote a book on decoration in Tibetan and Indian culture. But along with an eye for detail came a gift for spotting patterns—things, he came to believe, were connected. Ecologists often focus their thinking on either the organisms they are studying—counting their numbers, measuring their attributes—or the connections between them, such as the flow of nutrients through a food chain. Hutchinson could see nature’s objects and processes simultaneously and so see ecological problems in new ways. This talent, combined with his belief in the power of mathematics to describe natural systems, made him one of the key figures in the transition from natural history to ecology, from cataloguing the living world to explaining it. He was also as much the midwife as the father of modern ecology. Born in Cambridge to a well-to-do academic family, at Yale he was the archetypal Englishman abroad. His exotic demeanor coupled with an exotic erudition attracted a group of remarkable students, many of whom had never thought of becoming ecologists before they met him.

One of the reasons Hutchinson’s question has been so stimulating is that it is rather vague. One of the things he meant was: Why is there the number of species that there is and not some other number? He was particularly interested in the idea that, in the absence of environmental upheavals such as floods, droughts, fires, ice ages, or meteor strikes, the number of species in a place arrives at an equilibrium and in whether there is a theory that can predict and explain this number. Fifty years after Hutchinson’s lecture, there is no one theory that can

do this; or rather, there are lots of them, but all have their caveats and exceptions, and all inspire argument, which at times has been bitter. None of the ecologists following in Hutchinson’s wake have quite been able to draw the teeth of his question. To quote one of them, we still do not know why there are 700 species of birds in North America and not seven.

In the Homage, Hutchinson suggested that one explanation for the number of different species lay in the range of possible biological professions and addresses. Belonging to a species is like having a job: It’s a specialized way of making a living, and an evolutionary choice that closes off other employment options. By becoming excellent at one way of life, through adaptation, animals and plants become inept in others. You would no more set a sheep to catch a rabbit than you would employ a plumber to cut your hair. Tropical orchids would struggle on the tundra. Cows are good at digesting grass, bad at ant eating; pangolins, vice versa. Species divide up resources, and each species can exploit some so well that it can monopolize them. But due to life’s ubiquitous trade-offs, the ability to hog some resources comes at the cost of being able to use all of them, leaving other jobs vacant. If all organisms use energy in the same way, maybe diversity reflects the number of different ways to get it.

The most fundamental difference in biological jobs is between those organisms that get energy from nonliving sources, called autotrophs, or self-feeders, and those that consume other organisms, called heterotrophs. Plants are the most obvious autotrophs—they use sunlight to build carbon dioxide into sugar—but not the only ones. Some microbes can fuel themselves from the chemical bonds in compounds such as ammonia and methane, using geothermal heat to break them apart. The first autotrophs, born in the ocean more than 3 billion years ago, probably did something similar.

Heterotrophs feed on sunlight by proxy, consuming the cells around them. This is how a food chain works, with autotrophs such as plants feeding herbivores and predators eating the herbivores and each

other. The most obvious heterotrophs are impressive beasts such as bison, giraffes, lions, and us, but the vast majority are unobtrusive organisms such as fungi and microbes that get their energy from decomposing dead material. The worms that eat us when we die are above us in the food chain. In fact, ecologists now speak of food webs rather than chains, because in any place most species eat several different kinds of plants, animals, or both and in turn have to avoid several different predators or parasites. Food chains can become loops—the worms might get you in the end, but in the meantime if you eat a chicken that feeds on worms, you can reestablish your trophic superiority. A complex network of links is needed to describe who eats whom.

One way to create diversity is to put another link in the food chain. But as a biological job creation scheme, such a ploy is not very effective. Most food chains are only four or five species long. The ecologist who worked out why they are not longer was a protégé of Hutchinson—Ray Lindemann. Hutchinson and Lindemann were among the first scientists to think about how the patterns shown by a group of different species living in the same place, called an ecological community, could be the result of the way that energy flowed through them—“If the community is an organism, it should be possible to study its metabolism,” Hutchinson once wrote. And before he moved to Yale to work with Hutchinson, Lindemann spent five years studying a Minnesota lake called Cedar Creek Bog, plotting the links between plants taking up energy and materials, consumers eating plants and one another, and decomposers feeding on corpses and recycling their nutrients to plants. In the process, Lindemann realized that, in energetic terms, living things are leaky vessels and that the loss of energy as living matter passes up the food chain can explain the pattern of declining numbers and diversity.

For starters, Lindemann reasoned, the laws of thermodynamics prevent energy from being perfectly converted from one form to another. But there are other leaks besides this one. Much of the energy a plant or animal obtains will be used up before it is eaten, and much of it will be converted into tissues that are indigestible to its predator, although not to microbes in the predator’s gut or the soil. A large portion of the energy entering one level of a food chain is lost. All flesh is

grass, but only about 10 percent of the energy in grass makes it into herbivore flesh. Predators eating herbivores are similarly inefficient. (The efficiency of consumers varies between about 5 percent and 50 percent in different food webs; researchers have sought regularities in the amount of energy passing between levels in a food web, but in this instance the variability of nature has so far confounded them.) So the first predator gets only about 1 percent of the energy in the plants. This is why you can feed more people with a tonne of wheat by baking bread than by feeding the grain to burger-bound beef cattle. The longest food chain found so far has nine members, counting the autotroph as number one. The calories in the top predator have been through eight other bodies, and about 99.999999 percent of the sun’s energy has seeped away en route.

In the Homage, Hutchinson pointed out that several other things, besides leaking energy, limit the length of food chains. For example, each predator tends to be larger than the one below it. If you want to kill something, it helps to be bigger than it, although this is not true for herbivorous insects and parasites, such as caterpillars, ticks, and mosquitoes, which are important though poorly understood players in food webs. So, in addition to there being less energy at the top of the food chain, the largeness of top predators forces them to spread out. Eventually, each animal needs so much land or water that its lifestyle becomes unsustainable—it will be impossible to find mates, for one thing. This restriction puts a limit on how big and fierce animals can become. Cannibalism further shrinks the potential length of a food chain by reducing the amount of energy available to the next level up. Predators that take their prey from several levels in the food web—omnivores, in other words—have the same effect. A super-efficient predator might eliminate the species between it and those lower down the chain. This is what humans are currently doing in the sea and in the bushmeat trade. As big species such as cod or apes disappear, smaller ones such as herring and rats come onto the menu.

After moving to Yale, Ray Lindemann wrote up these ideas in a paper called “The Trophic-Dynamic Aspect of Ecology.” The editors at Ecology rejected it as too speculative, but Hutchinson was able to convince them of its worth. It is now considered one of the field’s seminal

works and helped found the field of ecosystem ecology. But Lindemann never saw it published: While it was in review, he died of liver disease, at age 26.

So biodiversity is not a product of long food chains. Most diversity is found within the levels of food webs rather than between them. The 1,500 orchid species in Costa Rica all occupy the same level of the web, as do the thousands of herbivorous insects that feed on them, or the birds and lizards that eat the insects, and the birds of prey and cats that catch the birds. This struck Hutchinson as paradoxical. By the late 1950s, lab and field experiments, on microbes in a jar or grasses in a plot, had already shown ecologists that, if two species were made to fight over the same suite of resources, one would always eliminate the other. Yet diversity is ubiquitous. Seemingly featureless places, such as prairies, invariably contain many species of grasses and herbs. Plankton puzzled Hutchinson most of all. The open water of a lake seems a homogenous environment, like a giant lab flask, yet somehow hundreds of species are able to share it. It takes a lot of effort to minimize the number of species in a place, as gardeners and farmers know.

Besides posing a puzzle, the lab experiments gave a clue as to what might be going on. The outcome of contests depended on the conditions in the arena—tweaking the temperature, or the amount of nitrogen in the soil, changed which species won. This variability seemed to open up more job opportunities for species, more potential specializations. Species can also vary by when they breed, the temperature they prefer, whether they grow best in shady conditions or sunny, whether they are active at night or day, and in many other ways. A few years before his encounter with Santa Rosalia, Hutchinson devised a way of thinking about a species’ place in nature—its niche. He saw the niche as a set of coordinates specifying each species’ lifestyle choices and how it fit into biological and environmental space. One axis might represent the temperature range a species could tolerate. Another might be the range of food items that species would eat. Tree-dwelling birds could be defined by the height at which they built their nests.

Plants would have a range of light or moisture levels within which they thrived and beyond which they withered—and so on, until all the species’ habits were accounted for.

Natural environments vary in space and time. They contain wet and dry spots, leaves and fruit, air and soil. Their temperature, rainfall, and altitude all vary. This variety creates a niche space too broad for any one species to monopolize, and so evolution will favor some degree of specialization. Hutchinson argued that the spread of niches in environments allowed species to coexist, as long as their needs did not overlap too much with those of any another species.

A species’ niche is a force field around it, from which it must exclude others or perish. The strength of the field varies, however. Organisms do best in environments close to the center of their niches and worst at the edges. A plant will usually grow quickest at a temperature just cooler than the hottest it will tolerate and will struggle as it approaches its limits; a hawk that prefers to catch mice and voles might be capable of tackling a beetle or a sparrow but will only attempt it if nothing else is on offer. At the edge of a species’ niche, other species better adapted to exploit those conditions can muscle in. For example, in Scotland the barnacle Chthamalus lives on the higher, drier parts of rocky shores. It has no trouble surviving in the wetter environment lower down, but normally other, faster-growing species crowd it out. But to achieve their fast growth rate, these competitors have sacrificed their ability to withstand drying out at low tide, leaving Chthamalus a piece of the beach to itself.

Put another way, evolution favors animals and plants that can avoid competition by moving into less crowded regions of ecological space. As Darwin saw, the struggle for existence brings about natural selection and results in life forms that fit their particular environment. Bats are less agile in the air than swifts and swallows, but they are better at navigating in the dark. By dividing up the day, both groups can feed on flying insects. Giraffes’ long necks allow them to reach foliage that other browsers cannot, dividing up food in space. So one reason for there being lots of species is that there are lots of potential niches. This is the multiple Goldilockses view of biodiversity: The three bears’ breakfasts can feed more girls if some like either cold or scalding porridge.

Another scientist inspired by Hutchinson, Robert MacArthur, developed these ideas about niches. MacArthur was another who thought that the job of ecologists was to spot patterns—even if this meant ignoring some of the detail—to find their causes and to express them in elegant mathematical language. In this way, he wrote, the study of nature could avoid “degenerat[ing] into a tedious set of case histories.” He believed that ecologists should be more like physicists and less like historians. They should seek mechanisms to explain what they saw, not just tell a story of how things might have gotten to be that way. More than any other ecologist, he championed the approach to modeling taken by West, Brown, and Enquist—of building theories that make predictions, based on a set of assumptions about underlying causes, and then testing them against data, rather than starting with the data and ending up with a model that describes it. His models were not ways to make detailed predictions. Often their conclusions were very broad, giving answers such as “larger” or “more” rather than “3/4”. Instead, they aimed to reveal common mechanisms underpinning biology’s many facts.

MacArthur had taken a master’s degree in mathematics before coming to work with Hutchinson, but his program was not purely mathematical. He always sought to test his theories against data. This approach would lead to modified theories, which would suggest new experiments, and so on. He was also an expert bird watcher. In his Ph.D. work he looked at five species of warbler, living in coniferous forests, which were thought to be so similar in their feeding habits and behavior that they violated the idea that species could not share identical niches. Through painstaking observation, MacArthur found that each species preferred to feed in different parts of trees. One spent its time around the tree base, another at the treetops. One used outer branches, another inner—evidence that apparently identical species have evolved to coexist by finding many subtle ways to divide up resources. MacArthur also showed that the number of bird species in an area rises steadily with the number of forms of vegetation there, such as grassland, scrub, and tree canopies—in other words, more niches means more species.

MacArthur’s influence on ecology was equal to Hutchinson’s. Besides being a brilliant and original thinker, he had an aura that made him a prophet for his view of life. He radiated enthusiasm for nature and science and, at least toward the like-minded, was generous with his time, attention, and encouragement. His belief that ecology could be explained and generalized was seductive, and those who spoke with him, particularly young scientists, often remember coming away transformed, feeling that things were suddenly much clearer and that they themselves were suddenly much cleverer.

For a decade beginning in the late 1950s, MacArthur, together with Hutchinson and other colleagues, produced theories seeking to explain many of the key problems of ecology. Many of these theories addressed the ways that species might coexist by dividing up resources (there’s a story that the problem of sharing resources had occupied MacArthur since childhood, when he had tried to work out the best way to divide a cake). One of the problems MacArthur pursued using this resources-based approach was why some species are common and some rare. Broadly speaking, a few species are common, but most are rare. Ecologists had sought mathematical descriptions and explanations of the numbers of rare and common species in each place for decades, and they still are. MacArthur built a model that envisaged the environment as a stick (a cake would have been just as good) and then broke the stick into pieces of random length. The length of each piece represented both the numbers of each species and the size of its niche: More common species got longer bits of stick. He then looked to see whether the patterns in the lengths of random stick fragments matched the abundances of real species. The image of the environment as a broken stick does not capture the possibility that species’ niches can overlap, that two or more can share the same part of the stick. So MacArthur refined it by trying to work out how similar species could be—how much they overlapped on the environmental stick—without one forcing the other out. Working this out, and combining it with a measure of the full range of resources available, might lead to a prediction of the number of species that could share a place. MacArthur also tried to unite competition and evolution, by working out how the pressure to avoid competition might cause competing species thrown

together to evolve differences through time. All in all, MacArthur’s ideas became a route map for ecology.

The Homage came at the beginning of a golden age in ecology. In the 1960s and 1970s there was optimism that a unified theory of ecology and evolution—that would explain how natural selection acting through ecological laws produced the structure of nature—was close at hand. Hutchinson and MacArthur made competition between species the focus of this research program. The strength of competition between species, and the ways in which it could be avoided, came to be seen as the forces that controlled the number and type of species that could coexist and the evolutionary changes that would occur in similar species sharing a habitat. The hunt was on for a set of rules that governed the way animals and plants divided up resources and environmental conditions—niche space—between themselves.

Body size is crucial here. The extent of an animal’s niche, and so the number of species that can pack into any part of ecological space, depends on its body size. Besides trying to work out how an organism’s size affects its biology, scientists have spent decades looking at the question the other way around—trying to work out what determines body size. Whatever the determining factors are, they seem to favor smallness. There are mammals weighing everything from 2 grams to 100 million grams, but three-quarters of species weigh less than 1 kilogram. As for birds there are more species of warbler than eagle. There are more small tree species than large; the same is true for fish. This seems to be true however you slice up life: There are more species of insects than mammals, for example.

Hutchinson and MacArthur thought that niches had something to say about this too. To a browsing giraffe, a leaf is less than a snack. But the same leaf might feed one insect species nibbling at its edge and another that burrows in and eats it from within. Other insects could live on the tree’s bark, bore into its trunk or roots, or trick the plant into growing around them, creating a gall. Small animals use up less ecological space, and so more of them should pack into that space. The analogy that I used earlier, of packing balls into a crate to describe how trees fit into a forest, could apply here also, except that each ball represents a species, not an individual. More recent ecologists have analyzed

this packing using fractal geometry. If you imagine the environment’s resources as a fractal, rather than a stick, small species will see that fractal at a higher magnification than large ones. Small species will see, and exploit, all the twists, turns, and branches in resource space invisible to larger species. Large animals perceive the world at a coarser scale and need more space—from feeding ground, to waterhole, to breeding ground, to roost—to get life’s jobs done.

On Monte Pellegrino, Hutchinson noticed that the two bug species in the pool were of different sizes. If body size determines how much ecological space a species occupies, differences in body size seem an obvious way to occupy different niches and thus avoid competition. Different-sized organisms need different amounts of space and food, they can get and use different types of food, and they can tolerate different environments. Hutchinson sought a rule to explain size differences in coexisting species. In groups of animal species exploiting the same resource, he noticed, each species is often about twice the weight, or 1.3 (the cube root of 2) times the length, of its nearest neighbor. Hutchinson thought that the critical size difference was between the body parts used to obtain food. If you line up the Galapagos finches studied by Darwin in order of their size, each species has a bill about 1.3 times longer than the next smallest finch. Insects go through several different larval stages before adulthood, and each stage is roughly 1.3 times longer than the next. The idea caught on, and many more examples of what became known as Hutchinson ratios were spotted. In 1977 the ecologists Henry Horn and Robert May noted that in consorts of viols and recorders each instrument, as one moves from treble to tenor or tenor to bass, is about 1.3 times longer than its neighbor. They seem to divide up musical space so that each has a separate job and does not compete with the other group members. And in some sets of iron skillets sold together, they pointed out, each is 1.3 times wider than the next. No one would buy a set of five identical frying pans, but like animals the coexisting skillets have been selected to specialize in handling food items of different sizes. Horn and May suggested that Hutchinson’s ratios might apply generally to sets of complementary tools.

Jared Diamond, a friend and colleague of MacArthur, offered another example of how competition between species might control

diversity. Diamond studied the distribution of 141 bird species on the 50 islands of the Bismarck Archipelago off the coast of Papua New Guinea. Diamond sought the rules—he called them “assembly rules”—that could explain which species lived on which island. He looked at factors such as body size, the altitude a species lived at, where in the trees it fed, what it ate—such as insects, fruit, or nectar—and how far it would travel (many tropical forest birds will not cross even small stretches of water). Some combinations of species, Diamond discovered, were never found together. For example, two closely related fly-catchers that fed by picking insects off vegetation were never found on the same island. The same went for two similar-sized species of nectar-eating birds. Sometimes, this created a checkerboard pattern. Each island was like a square on a game board—if it was already occupied by one species, it was no longer open to others. Other rules were more complex. Which combinations of species were able to live together could also depend on the size of the island and sometimes on the presence, or absence, of other species. Some species were “supertramps” that could tolerate many different conditions and travel long distances. They did well on small isolated islands, but could not gain a beachhead on islands already containing many more specialist species. Diamond’s assembly rules may not have provided a quick and easy route to a general understanding of diversity, but they did—along with many other similar studies—suggest a procedure that, applied in specific places, could explain why a certain number of certain species lived there.

Not everyone agreed with Diamond’s assembly rules. In particular, a group at Florida State University led by Daniel Simberloff, another mathematics major turned ecologist, argued that ecologists had been too eager to invoke competition as the cause of patterns in nature. The true way to do science, said Simberloff and his colleagues, was to try and falsify a hypothesis, not find data that supported it. Just because a pattern matched one interpretation didn’t mean that a true underlying cause had been discovered. There might be several other mechanisms capable of producing the same pattern—just as a Ptolemaic model of the solar system can predict the planets’ movements almost as well as a Copernican one. Ecologists should be testing these alternative explanations against one another.

Similarities and differences are easy to spot but difficult to pin down. If you measure enough things for any species you are bound to find lots of differences, but working out which, if any, are the reasons that they can or cannot live together is another matter. Likewise, measure enough traits and you are almost bound to find some that vary in a ratio similar to 1.3:1. The pebbles on a beach come in many different sizes, but that does not mean that the existence of small pebbles forces others to be big, or vice versa—they are just spread out randomly across a certain range. In 1981, Daniel Simberloff and William Boecklen showed that most examples of Hutchinson ratios were more like pebbles than skillets: a random spread, not a matching set. Similarly, Simberloff’s team claimed that many of the patterns Diamond had observed would be reproduced if species had strewn themselves around the Bismarck Archipelago at random. They also pointed out that the bones found on Monte Pellegrino, thought to be those of Santa Rosalia and to have miraculous properties, had turned out to belong to a goat. The MacArthurians, in turn, found flaws in the work from Simberloff’s team (or the Tallahassee Mafia, as they were nicknamed) and claimed that Simberloff’s models were much less random than they at first appeared.

Now, both sides of the debate sound reasonable. Considering alternative hypotheses is important, as is statistical rigor. Devising meaningful models in the face of messy ecological data is difficult. Some studies of competition wilted under tough scrutiny; others stood firm: Several of Diamond’s pairs of bird species are far less likely to be found together than chance alone would predict. Scientists spend much of their time picking holes in what other scientists claim to have found out, but it only occasionally gets personal. So why did the debate between the Tallahassee team and the MacArthurians become, in the words of one journalist at the time, “as acerbic and acrimonious as any that has stirred the combative instincts of academia”? One reason was that MacArthur’s approach had already polarized ecologists. His ecology and philosophy inspired devotion and admiration in those on his side, but they could apppear arrogant to those who took a different approach, and MacArthur could be dismissive of his less conceptually ambitious colleagues. He hated being wrong but was not afraid to try

and push things along by publishing half-formed ideas or suggestive but inconclusive data. Sometimes, mistakes were later found in these studies, which confirmed him as a charlatan to those who disagreed with his science or resented his fame and influence.

MacArthur saw being interesting and provocative as part of his job; tidying up could come later. A few years after he published it, for example, he decided that his broken-stick model was wrong. Theory would naturally run on ahead of data. It takes days to build a mathematical model but months or years to do a field study. But for MacArthur there was no later: He was diagnosed with kidney cancer in 1971 and died the next year, at age 42. His untimely death left many of his theories only partly formed and tested. It also created a power vacuum. Ecology was a young and growing science, and the generation of researchers following MacArthur was hungry for prestige. Continuing his work was one obvious way to do this; knocking it down was another. The theory of competition, said Simberloff, “[had] caused a generation of ecologists to waste a monumental amount of time.” Diamond and his colleagues called the criticisms “silly” and “lacking common sense.” All the arguing dissuaded the next generation of ecologists from working on these problems; the discipline shifted back toward experimental studies, and researchers wanting to study the large-scale patterns in nature found it difficult to get funding.

By the early 1980s the preceding decades’ optimism had dissolved. Many ecologists despaired that studying competition would bear fruit. The criteria that might explain diversity—the heterogeneity of the environment and the competitive interactions between the many species that share it—are each hugely complicated. In models a small tweak to any one variable can change dramatically the number and type of species that can coexist. Measuring them in the wild is also dauntingly difficult. There were also theoretical reversals. It turned out that any differences between species could allow them to coexist, so theorizing about or looking for a minimum difference seems meaningless. This made it seem as if the problem was why there were so few species, not so many. Even in a jar, one species can’t eliminate another solely through competition. There must be some other factor causing extinction. This factor could simply be chance, a random fluctuation

in the rate of reproduction that tips a population over the point from which it cannot recover. It was at this time that Jim Brown set about looking for some other way to explain diversity besides competition. Other ecologists spoke of their discipline as being “in crisis,” “repugnantly complicated,” and “in a quagmire.”

French wine makers have a concept called terroir, meaning each vineyard’s unique combination of soil, climate, and geography. The idea is heavy with mystique, and the link between terroir and a wine’s flavor is held to be inexplicable and impossible to unpick. Perhaps biodiversity comes about as the result of some sort of ecological terroir, with the number and type of species in a place being due to idiosyncratic local conditions interacting in unfathomably complex ways. Perhaps, although species obviously occupy different niches, the search for generalities is misguided, and the niche is a sterile mathematical concept that cannot be measured in the real world. But many ecologists think not, and for them competition and the niche are alive and well.

Some ideas still focus on how competing species apportion resources. In 1976, David Tilman, then a Ph.D. student at the University of Michigan, found that two species of algae could share Lake Michigan because they differed in their ability to use phosphorus and silicon. One species was especially good at getting phosphorus out of the water and so could thrive in a low-phosphorous environment. But it needed lots of silicon, which the other species was better at obtaining. Each had traded off its competitive abilities in one arena against its competence in another, and, as long as silicon and phosphorus were spread unevenly, both could coexist. Since then, Tilman and his colleagues have developed this idea into a more general theory, particularly applicable to plants, which argues that each species is a specialist for a certain set of conditions. Every point in niche space becomes a potential niche—again, the potential number of coexisting species is limitless. Species thrive in some places and wither in others, but because environments are variable many species can share a place.

Another, more recent, idea of how species divide up resources, devised by Mark Ritchie and Han Olff, resuscitates the MacArthurian idea that biodiversity is a question of how many species you can pack into ecological space and combines it with our old friends scaling and fractals.

Imagine a group of species all fighting over a pot of resources, such as grasses for light, water, and nutrients, or grazers competing for grasses, big cats for grazers, and parasites for hosts. What each species needs is not spread evenly across the world. It comes in clumps, such as a moist, well-lit piece of bare ground, or some juicy young shoots, or a carcass. To survive, organisms must find these patches amid the barren areas that cannot feed them—grass seeds must disperse over areas of woodland; wildebeest must follow the rains; lions must find the wilde-beest. Frogs must find ponds; blackbirds, worms; fleas, dogs.

Just as not all places are equally hospitable, neither are all patches. They will vary in size and quality. A grass seed could find itself on a prairie or in a crack between some paving stones; a flea could find itself on a glossy young pup or a mangy old mutt. Whether a patch of habitat can keep an animal or a plant going depends on its size and quality. It also depends on the properties of the organism seeking to exploit that patch—principally its body size, because this property controls how much food an animal needs and how long it can go without starving. Big animals need more food, but they can go for longer periods between meals, and so can use large but thinly spread patches. Smaller ones need less food but in higher concentrations. Such a multiplicity of factors makes working out how the environment might control the diversity of organisms seem horribly complicated. A glance at any map shows that each landscape is a unique mishmash of wet, dry, rocky, fertile, high, low, windy, and sheltered places. How can we hope to predict what sort of patches it contains and what sort of life these patches can support? We can look for regularity hidden amid the chaos.

Take a map and scan it into a computer. Get some software to look for all the patches of one sort of habitat, such as woodland, and work out how big each patch is. Then get the computer to divide the patches into size classes, such as 0.1 to 1 hectares, 1.1 to 10 hectares, and 10.1 to 100 hectares, and plot how many patches fall into each size class. The

spread of patches sizes turns out to follow a power law distribution. Woods—or grasslands, or ponds—are like earthquakes. There are a lot of small ones and a few big ones, and their commonness declines as their size increases in a regular, logarithmic way. This is why maps and landscape photos need scale bars: Their features look more or less the same at all magnifications. The same goes for patch quality—there are lots of places that offer slim pickings and a few gold mines. So although we can’t predict where habitat or resources will be in a landscape, we can predict that they will be divided up like a fractal. In a way this is unsurprising, because the physical processes that create landscapes, such as earthquakes, erosion, and landslides, are also fractal.

Fractal thinking also offers a worm’s- (blackbird’s-, wildebeest’s-, grass seed’s- …) eye view of natural resources. One feature of fractals is that the finer the scale you use to measure them, the longer they appear. Small species have short measuring devices and will perceive more resources. A forest pool is a drinking fountain to a passing bear, a home to a beaver, and a universe to a single-celled alga. The same scale-dependent view applies to food within patches of resources. The browsing giraffe bites off the whole leaf, getting lots of tough and indigestible plant tissue in the process. The herbivorous insect that tunnels between the leaf’s veins eats only the nutritious green flesh, leaving the skeleton behind. How much we see in nature depends on how closely we look at it, which depends on our size.

So small species will find a high concentration of resources, but they need to, because they can only exploit small patches. Larger species will find food more thinly spread but will be able to cover more ground to get it. Goldilocks redux—for a species of any size, some patches will be too small, others will be big enough but too sparsely provisioned, and a few will be just right. The landscape is a template that is filled in with the species fit to each patch of resources.

So if you know the amount of resources in a place, you can predict the fractal way that these resources will be divided, how many species that place will be able to support, and also how different those species should be, because their sizes ought to match the holes in the environmental template. What’s most impressive about Ritchie and Olff’s idea is that it predicts the number of species that a place should

be able to support. The duo tested their model by trying to predict how many herbivore species would be found in each of 28 different East African wildlife reserves (where the quantity of resources, in the form of plant food, can be predicted from the amount of rainfall) and how many grass species should be found in experimental plots on the Minnesota prairie (where the amount of nitrogen in the soil is a good measure of total resources). The results were impressive: The number of species at each site, and their size differences, matched the model’s predictions well. But Ritchie and Olff’s idea is not all-embracing. It works well only on smallish scales, areas up to a few thousand times larger than the individual home ranges of the organisms under study. For soil bacteria this would be a few cubic centimeters; for a plant it’s a field; for a grazing herbivore it’s the area of a game park. Over greater areas other factors, such as whether organisms can get between distant patches, come into play and so its predictions start to break down—it can’t tell you why there are 700 species of birds in North America and not 7.

Both Tilman’s and Ritchie and Olff’s theories are based on the Hutchinsonian idea that biodiversity is the result of some sort of balance in how species divide up the environment. But other ecologists think that nature is not in balance and that it is the very disturbances that Hutchinson ignored—the droughts, floods, and fires—that allow species to live together. Even if two species share identical niches, one does not instantly eliminate the other. The speed with which this happens depends on what else is going on in the environment. In a stable environment, competition will run quickly. Nothing else is going on. But in the real world, lots of things can send the competitors back to the starting line, shift the balance between them, or so preoccupy them that they never get around to competing at all. A tree cannot crowd out its neighbor if it is struck by lightning or blown over. But the next lightning bolt will strike another species of tree, allowing a third species to survive. On a human timescale, it looks as if the trees in a rain forest have struck a happy balance. In fact, they might be strangling one another, but so slowly and haphazardly that we never see a killer blow. Taking competitors out of the game reduces the strength of competition, perhaps to a level where competition is not strong enough to

curb diversity. Maybe the harshness of the world prevents nature’s bus from ever becoming so full that species must fight over seats.

Another thing, besides environmental hazards, that opens up seats on the bus is species’ habit of eating one another. This could be a particularly effective way of maintaining biodiversity because, whereas storms strike all the trees in a forest more or less equally, predators and herbivores tend to focus disproportionately on the most common species in their environment. By reducing the numbers of species they feed on, these consumers prevent their prey from overwhelming their neighbors. If starfish are taken off a beach, the mussels they feed on take over, and the total number of species goes down. Likewise, herbivores tend to choose plants that are juicy and fast growing but not well defended. Without the browsers and grazers, these plants would take over. But with them other plants that are slower growing, because they have invested in poisons, thorns, or other defenses, can survive. One idea argues that every tree in the forest inadvertently attracts herbivores that specialize in eating it. Mature trees can cope with this, but it’s no place to raise a child—any young tree of the same species trying to establish itself in the same neighborhood is in trouble. These herbivores, however, would ignore nearby trees of other species, allowing a patchwork of diversity to build up. There is some evidence that tropical trees do indeed find it harder to germinate and survive close to members of their own species. The crucial common aspect of these predator effects is that they weigh heaviest on common species. This creates a benefit to being rare that maintains diversity by reducing strong competitors’ advantage.

These are just a few of the many ideas, none of them mutually exclusive, that different ecologists currently favor to explain why species do and don’t live together. There are variants of these, but they are all twists on the basic concepts of competition and how species avoid it, either by dividing up space, time, and resources, or because of forces beyond their control.

But maybe competition between species is not strong enough to control biodiversity. Maybe species are not adapted to slot into a niche,

dividing and ruling ecological space. Maybe they just drift around, with chance and history controlling the combination found at any one place and time. Maybe ponds, or forests, or coral reefs are in flux, not in balance. Maybe there had been three species of water bugs living on Monte Pellegrino the week before Hutchinson showed up, but one had died out. Or maybe there had been one, and a pregnant female of another species had only just flown in, and another species arrived the week after he left.

Nature certainly changes. Deposits of tree pollen in Europe and America show that over the past few thousand years species have come and gone and that a place can contain a different set of plants from one millennium to the next. And for the past few millennia, humans have been inadvertently challenging the idea that species are packed tightly into ecological space. In many places the dominant wild plants and animals are recent colonists, aliens that travel along with people. In the past two centuries the native prairies of California have been almost entirely replaced by immigrant European grasses—the state is now home to more than 500 introduced plant species. If the natives were wedged firmly into their niches, we would expect invaders to have difficulty establishing themselves. Perhaps immigrants could only succeed by being bigger or meaner than the residents, like the carnivorous snail Euglandia. This species was introduced onto the Pacific island of Moorea in an attempt to control another exotic, an African land snail, but instead ate its way through the native tree snails to the extent that several species went extinct and others now survive only in captivity. If all invaders were like Euglandia, this would challenge the idea that diversity has reached some stable point, but it would at least preserve the notion that environments were full and that niches must be fought over. But, while invasive species are rightly thought to be a serious conservation problem, many, perhaps most, exotics slot into their new homes without causing a fuss and with no discernible influence on the organisms already there. One survey of alien species in estuaries estimated that more than 90 percent of immigrants had no discernable effect on the residents. If environments were running at full capacity, we would not expect this. And we would expect places with more species to be harder to invade. This too seems not to be the case.

So ecological communities are fluid, unstable things. But just because there might be no balance of nature does not mean there can be no laws of nature. A whole class of theories, a school with a history just as long as that to which Hutchinson and MacArthur belonged, offers alternative explanations for biodiversity based on the idea that nature is unstable. The difference between these and niche-based models is that they rely on probabilities, rather than causes, and that instead of competition they see migration as the most important ecological process.

One of the boldest and most controversial current ideas about biodiversity was born in the same place that Enquist now works, the San Emilio plot in Guanacaste, Costa Rica. This plot was established in 1976 by Steve Hubbell. Hubbell came to Costa Rica to study the ants and bees, to track their movements and work out how they fed. One of his main interests was in different species’ niches. He had already discovered how different bee species drank flowers’ nectar at different times, some arriving en masse when the blooms were fresh, others coming along later in ones and twos to mop up the leftovers.

To track the ants’ movements, Hubbell began marking the trees. Once he had marked all the trees in his study plot, he was struck by how patchily the different species were arranged. Many species lived closer to others of their own kind than would be predicted if the trees were driving their offspring away by attracting specialist herbivores. When Hubbell looked at the species in San Emilio, the individuals of most were either spread at random or found in groups, not evenly spaced.

Hubbell began to suspect that the patchwork of tree species in his forest was too irregular to be explained by patterns of herbivores or the range of niches found there. In a mature forest, vacancies appear unpredictably. When a tree falls, it creates a gap that the seeds and saplings that have wound up in that spot race to fill. It seemed to Hubbell that who would win the race was as unpredictable as where the gaps would be, and that being in the right place at the right time was more important than being specialized to a particular set of conditions. Hubbell lost interest in ants and bees and began thinking about how the forest could have come to be.

Forests are big. Trees are long lived. So to understand forests, you

need to study a large area over a long time. Al Gentry’s rapid-fire censuses had shown that it was possible to collect detailed yet large-scale information on forests. Hubbell took things further. In 1980 he and his colleagues set about mapping and identifying every tree in a 50-hectare block of forest—equivalent to 500 gentrasos, nearly double the combined area of all of Gentry’s original plots—on Barro Colorado Island in the Panama Canal, where the Smithsonian Institution has a research station. Unlike Gentry’s surveys, this one would be permanent. You could return year after year and see who had died, who had arrived, and who had grown and by how much. This method, called the Forest Dynamics Plot, has now become the standard for long-term studies of forests. There are plots in forests from Puerto Rico to Cameroon to Thailand; in total 3 million trees belonging to 6,000 species are being monitored.

Over the years, with each census, Hubbell became more convinced that the spread of niches was not controlling the diversity and abundance of the trees on Barro Colorado Island. Most species didn’t seem to care whether they grew in a small or large gap or in a shady or well-lit spot. Each individual was surrounded by a diverse and unpredictable set of neighbors. This unpredictability ought to have prevented trees from evolving into specialized competitors, because they could not predict what they would be competing with. Species also came and went. Four shrubs went extinct in the dry El Niño year of 1983. Others, confined to just one corner of the island, seemed to be recent arrivals.

When not counting and measuring trees, Hubbell spent his time playing with theoretical models, trying to find the levels of birth, death, and migration that would reproduce the number of species, and their population sizes, that he saw on Barro Colorado. Fortunately, there was a ready-made theory at hand, and, perhaps paradoxically, it too was the work of Robert MacArthur.

In 1963, MacArthur, working with Edward (E. O.) Wilson came up with an idea to explain the number of species on an island. MacArthur and Wilson’s model could hardly be simpler. The number of species on an island, they reckoned, is a balance between the rate at which new ones turn up and the rate at which the inhabitants go extinct. In its most basic form this model doesn’t need niches, or competition, or

any sort of interaction between species. It’s just a question of probabilities. On an island with few species, almost every immigrant is likely to represent a new species. But if an island has lots of species already, the chances that an arrival will be a novelty are small. So the immigration rate of new species falls as the number of species on an island rises. Each species also has a certain chance of going extinct. The more species you have, the more will go extinct in any time period. The extinction rate rises with the number of species. The island’s total number of species will be where these two trends balance out. The number of species will be stable, but which species they are will change.

Of course, the real world is unlikely to be that basic, and many ecologists have customized MacArthur and Wilson’s model to try and make it more realistic. They have looked at how competition might reduce the chances of immigrants establishing on a crowded island or how the distance from the mainland affects the immigration rate. But to get started Hubbell needed just the basic model.

So, Hubbell thought, perhaps the tree species on Barro Colorado were just a random subset of the species in the wider region, the whole of Central America. New species could arrive blown on the wind, or stuck to a bird’s foot, or fall to the ground in monkey droppings. Sometimes a storm would fell the last tree of its kind on the island, making that species locally extinct. But no species is better suited to life on Barro Colorado than any other. In the 1980s, Hubbell began tinkering with the chassis of island biogeography—keeping a low profile because of the field’s recently bloody history—to see if he could get it to reproduce the pattern he saw in Panama.

For starters, Hubbell stopped looking at the rate that species arrived in and left the forest and switched the emphasis to the birth, immigration, and death of individuals. Then, instead of starting with an empty island, he assumed the opposite—that the forest was saturated. There were no gaps in the canopy, and for a new tree to grow, an old one must go. His next tweak was to add new species by evolution, as well as migration, in the form of a number giving the probability that any individual will give birth to a new species. Although this scenario is unlikely—species are far more likely to arrive in a forest from outside

than by evolving there—plants can sometimes produce offspring that are genetically very different from themselves. Plus it was the easiest way to add evolution to the model without complicating the mathematics unduly.

What Hubbell never included was specialization. He wanted to see how much he could explain by imagining that forests are filled with an average universal tree. If real trees were close to this average, with broad niches that overlapped with the others in the forest, his theory was potentially powerful. If they were very different, his ideas would probably not explain anything.

In the resulting model the species that fills the gap depends not on how wet, or shady, or rich in nutrients the gap is. It just depends on who gets there first. The probability of surviving or reproducing is also equal for all species. The reason for there being so many different types of life is that the differences don’t matter.

Hubbell’s eureka moment came at 2 a.m. one night. He realized that by multiplying the (very small) number giving the probability that a new species would spring into being with the (very large) number of individuals in the whole region, such as Central America, he got a new number that seemed to control the entire community. If you fed in the right value, it predicted the number of species in an area. It also predicted relative abundance: which species should be common, which rare, and how common or rare each should be. Hubbell seemed to have cracked ecology’s two great problems at once.

He called his magic multiple the universal biodiversity number. When this number is small (when the population is small or the probability of speciation low), you get a forest dominated by a few species, similar to the conifer forests of Canada and Siberia. When the number is large, the forest has many species, and the numbers are more equal, as in a tropical forest. In the universal biodiversity number, Hubbell thought he had discovered the e = mc² for ecology.

In Hubbell’s model the population size of each species fluctuates at random. In effect, this means that all species eventually go extinct in a place. This is so because no population can become infinitely large, so in the long term the only way for any species to go is down, even if it originally dominated the entire forest. Eventually it will have a run of

bad luck that wipes it out. But, particularly for common species, this extinction will take so long that to a human observer—particularly one on a three-year research grant—the forest will seem like a stable set of species. Hubbell called his theoretical framework neutral ecology, a deliberate echo of an older theory called neutral evolution, which says that much of the change in an organism’s DNA confers no selective advantage or hindrance on it.

Neutral ecology is not biodiversity’s theory of everything. It can only predict the species within one level of a food chain; for any one place the plants, herbivores, carnivores, and decomposers would each need their own model. Neutral models are also likely to be more successful at predicting the diversity of plants than animals. Forests are planes tessellated with individuals, a lot like Hubbell’s theoretical environment. Species share similar requirements, new individuals can only establish themselves when a gap opens, and once they have settled they cannot move somewhere more favorable. Animals are much more flexible in both what they consume and where they consume it. To a rain forest bird, the world is not a two-dimensional plane—it’s a complex three-dimensional space. In many ways, animals can carve out their own ecological spaces.

But on its own terms, neutral ecology is remarkably powerful. Get the right population size and the right balance between birth, death, and speciation, and a near-perfect re-creation of a forest pops out of the model. With just three numbers, for example, Hubbell was able to predict both that a Malaysian rain forest should contain 800 species (which it did) and the individual abundances of each species.

Neutral evolution was originally controversial but is now well established. Neutral ecology is still in its controversial phase. About half a dozen journals rejected Hubbell’s first paper describing the universal biodiversity number. It eventually sneaked out in 1997 in an obscure journal called Coral Reefs. And Hubbell’s model still makes many ecologists deeply uncomfortable. At one conference, someone called him “the Antichrist of ecology.” This is partly philosophical. Biologists have traditionally studied what makes species different from one another, and they are impressed by how many aspects of the species they study seem to fit a particular environmental job. Neutral ecology

seems to start from assumptions that are clearly wrong. It seems like throwing ecology away.

There have also been challenges to the theory’s science. In particular, several teams have claimed that there is more structure in nature than neutral ecology can account for, that who lives with who is predictable, suggesting that the species in a place are evolved to match the conditions there and are not a random subset of the whole. Neutral ecology also seems best at predicting patterns in diversity at intermediate scales, from a few hundred square meters to tens of square kilometers. On larger scales, environmental differences become important—just as a plant from the tundra would not survive in a desert. On smaller scales, interactions between individuals do seem to have an effect on who can live with whom, and ecologically similar species are less likely to be found very close together.

Experiments can test whether plants live where they do through chance or because they have found their niche. You just move the plant from its wild home to somewhere else. In a world of niches, residents will be better off and migrants will struggle. In a neutral world, travel will be easier. A team led by Graham Bell, an ecologist who developed models of neutral ecology independently of Hubbell, found that plants were quite happy to be moved around Canadian forests. There was no sign that they had a strong preference for a particular environment or set of neighbors. And neutral ecology makes predictions about time as well as space: Widespread and abundant species are more likely to be old, because it takes a long time to become common relying on dumb luck alone. Rare species are more likely to have evolved recently. Comparing DNA, which gives clues about how long ago a species came into being, might answer this question. A bit like Kleiber’s rule for body size and metabolic rate, the real world is bound to deviate from neutral ecology’s predictions: how much it deviates, and how, will show what needs to be explained by other means, such as competition, and what doesn’t.

There is also the issue of whether a mathematical model that reproduces a natural pattern, however uncanny its accuracy, really explains

what is causing that pattern. Tweak the universal biodiversity number just right and it will give a good fit to patterns of biodiversity—but does it really capture what, if anything, is controlling which species live where? Some ecologists have claimed that other mathematical models of commonness and rarity give a better description of nature than Hubbell’s neutral scheme. And models based around niches and competition, if they are detailed enough, can do just as good a job of producing patterns in diversity as neutral models.

Hubbell has an interesting take on this issue. He draws parallels between neutral ecology and physics. Physicists, he points out, tend not to worry about the underlying mechanism of their theories as long as they can describe nature. No physicist thinks Newton’s laws wrong because they do not describe how gravity works. In the same way, the laws of thermodynamics or electrical resistance were discovered by observation and correlation, not through studying the properties of individual atoms or electrons. Like Jim Brown, Hubbell evokes the physics that uses statistics to describe the behavior of groups. The laws that describe the properties of gases were not discovered from the bottom up, by considering what every molecule was doing. They are exercises in emergence, describing how the group as a whole behaves, based on an assumption that all molecules are identical and respond to their environment identically. Ecological problems are harder than physical ones because their particles—living things—are so much more variable and unpredictable. But Hubbell and Brown believe that ecology can adopt some aspects of physics’ worldview, by looking for statistical regularities in the behavior of nature as a whole, rather than trying to assemble it from its parts. To look for a deeper level of explanation is to step onto a slippery slope: There is always something more fundamental waiting to be explained, another term to be bolted on to the equation. The best criteria for a good theory, Hubbell says, are that it should be simple and make accurate predictions—“that it should fail in interesting ways”—not that it should be an exact replica of the thing it is trying to explain.

There are certainly aesthetic criteria to what makes a good theory, the main one being elegance through simplicity. Making a model more complex will always make it reproduce patterns more accurately, but it

does not necessarily make it a better tool for understanding nature. The more parameters you have, the more things you must measure to test them. The mathematics is also wont to become opaque—it can be as hard to work out why a very complex model gives the predictions it does as it is to understand the system you are trying to model. On the other hand, neutral ecology is not like the physical theories Hubbell invokes, because it says definite things about how the patterns it reproduces come about. It’s true that the equations relating the pressure of a gas to its temperature cover up a mass of details, but these equations are only about phenomena, not mechanisms. They are formal descriptions of the system that go from data to theory and back again, like allometry. Neutral ecology, on the other hand, does contain ideas about mechanisms, even if they are very hard to get a handle on. Like a good MacArthurian model, it goes from theory to data. This makes the model potentially disprovable, which is a good thing. The rate at which new species appear, and the total number of trees in Southeast Asia, either is what Hubbell’s model predicts or it isn’t.

The exciting thing about neutral ecology is that it is challenging and counterintuitive, and its implications for our view of nature are a powerful incentive to carry out the studies that could test it. Herculean efforts such as FLUXNET and the Forest Dynamics Plots show that such tests, although daunting, are possible. This will all take time. Neutral evolution took more than a decade to become accepted, and nobody needed to measure 3 million trees. It will take at least as long to put neutral ecology through the wringer.

A theory of biodiversity ought to be able to predict the consequences of changing biodiversity. Achieving this goal would be more than a matter of intellectual satisfaction. Plants, animals, microbes, and fungi keep soil fertile and air and water clean; they pollinate crops and control their pests and provide food, fuel, building materials, and drugs. The services provided by wildlife are estimated to be worth $33 trillion to humanity each year. Lately, humans have been unpicking this life-sustaining fabric at a rate seen rarely in Earth’s history. The fossil record

shows that about 10 species go extinct each year, but currently about 30,000 species are lost each year (both numbers are very approximate, and the real value could be different by a factor of 10). We are in the midst of the sixth mass extinction of the past 600 million years. At the current rate, between a quarter and a third of all species will have disappeared within 50 years. We have also been inserting species into new places, taking plants and animals around the world deliberately, as crops, livestock, pets, and ornaments and, inadvertently, as stowaways in cargo and ballast water. What will result from all this messing with biodiversity?

It might seem that the lesson of neutral ecology, and of the Enquist group’s work on the structure of forests, is that, if many of the patterns in nature are not caused by the differences between species, then losing or gaining a few isn’t going to matter much. On the broadest scale, this might be true (although we are losing more than a few species). Most invaders have little effect, and most species are probably loosely packed into their surrounding environments. Pulling them out, or cramming a new species in, does not have much impact on the species around them. But on a local scale the details of species matter. Ecosystems are complex and unpredictable and do not equal the sum of their parts. There are many examples of the removal or addition of a single species having serious consequences. And until we make the change, we can hardly predict what these influential species will be.

Often, the most dramatic effects of changing biodiversity come from chain reactions acting through food webs. For example, as D’Arcy Thompson predicted, by the end of the nineteenth century Russian hunters seeking fur had nearly driven Alaskan sea otters extinct. What he couldn’t have foreseen is that, because otters eat sea urchins, the urchins boomed in their absence. Urchins eat kelp, so the crash in otters led to a crash in kelp. Kelp helps stabilize shorelines, so storms and erosion became more damaging. Otters recovered, but twentieth-century overfishing may have triggered the same chain reaction, by removing the food of killer whales and forcing them to eat more otters.

Species like the sea otter, which directly or indirectly have a large effect on many of the other species in their environment, are called keystone species. They need not be predators: They could equally be

plants or herbivores. It is hard, if not impossible, to tell from looking at a food web which, if any, species are keystones—and for most places we have no idea of the structure of the food web. Ecosystems are a bit like the game of Kerplunk!, that staple of 1970s childhood, in which the aim is to gradually dismantle a lattice of plastic sticks without letting the marbles the lattice supports fall through. It is seldom obvious which sticks it is safe to remove and which will trigger an avalanche of marbles. We are playing Kerplunk! with the planet.

In addition to each species’ ecological influence, the sheer number of species in a place is important. One rarely, if ever, sees a tropical tree stripped bare by caterpillars. On the other hand, places where there are fewer species, such as a field of cabbages or a conifer forest, are much more prone to plagues of pests, such as the spruce bark moth, which has killed tens of millions of Alaskan trees in the past 20 years. Experiments likewise show that places with more species tend to have a greater biomass and are less prone to booms and busts in their populations.

There are several reasons this might be so. Diverse ecosystems could have some redundancy—a more diverse portfolio—so if one species disappears, there is another to do the same job. And a diverse group of species could spread life’s load more evenly, with each responding differently to environmental conditions and coming with its own set of predators and diseases. Everyone suffers a bit—on inspection, many of the leaves in the Costa Rican cloud forest are riddled with holes chewed by insects—but disasters become rare.

Biodiversity also makes itself felt through its influence on the structure of food webs. If a predator is too effective and too specialized, it will cause a crash in its prey’s numbers, followed by a crash in its own numbers as it starves. But adding a competing predator that feeds on the same prey weakens this interaction, because some of the energy in the prey species takes another route through the food web. In computer models of food webs of this sort, the dynamics of the community as a whole become more stable as more species are added—the total number of organisms, the biomass, and the chemical processes all become less variable and quicker to recover from perturbation. Each individual species, however, becomes more likely to go extinct because it has a smaller population.

It has been a long time since I mentioned metabolic rate. Because metabolism controls processes in individuals such as growth and life span, some ecological properties, such as a population’s density and growth rate, seem closely linked to metabolic rate. But how metabolic rate might be linked to the quality of life—biodiversity—as well as its quantity is still unclear. But food webs, which describe the way that energy is split between species, suggest a link between the way that individuals use energy and biodiversity.

Like blood vessels, food webs are networks through which energy flows. This is one of the things Hutchinson meant when he commented on studying the ecosystem’s metabolism. But we cannot carry the analogy too far. The two types of networks have rather different geometries, and food webs are dynamic—their links grow weaker and stronger, switch on and off, and rearrange themselves in a way that blood vessels do not. But the way that energy flows in and out of the environment, via living things, depends fundamentally on the flow through individuals. And, as we have seen, the flow of energy through individuals—their metabolism—depends on their size, temperature, and chemical supplies and demands. So it is reasonable to expect that size and metabolic rate will have something to say about food webs and ultimately diversity.

There are also hints that links are beginning to form between niches and metabolic ecology. In a 2003 book, Ecological Niches: Linking Classical and Contemporary Approaches, designed partly as a response to the challenge of neutral ecology, Jonathan Chase and Matthew Leibold describe how ideas about niches are changing: “Rather than use vague concepts such as ‘niche overlap’ and ‘niche breadth’,” they say, “we focus on measurable aspects of the biology of organisms such as growth rates, consumption rates, and death rates.” These, of course, are the very things that an understanding of metabolic rate can help to explain.

The energy in sunlight, refracted through the prism of the environment, produces the spectrum of biodiversity. The structure of that prism is fearsomely complicated. It consists of the physical world—the

lay of the land, the spread of water, the chemistry of the soil, the changing of the weather, and the other organisms that all living things meet with as food, predators, cooperators, or competitors. Such complexity has fostered intellectual diversity. The tightly focused worldview of ecologists in the 1960s and 1970s has dissolved into a babel of ideas. It can seem as if there are as many theories as there are theorists.

This problem may well reflect the real world: The different things that biologists have suggested might control biodiversity are not mutually exclusive, and it seems fanciful to imagine that they will collapse into some theory of everything. Plants, animals, and microbes each have very different needs and different ways of fulfilling those needs. The reason there were two species of water bug in the pool on Monte Pellegrino in 1958 is surely different than the reason that there were 60-odd species of tree in the Savegre gentraso in 2005, or 59 species of butterfly in Britain, or about 4,300 mammal species on Earth. Diversity looks different depending on the scale on which you consider it, and each scale will probably need its own explanation. But it is possible to imagine a set of ideas, including the processes of metabolism, the ubiquity of power laws and self-similarity, the structures of niches and food webs, and the processes described by neutral ecology that together will answer Hutchinson’s teasing question: Why is there biodiversity?

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