Life in equilibrium

Life in equilibrium – Edward O. Wilson and Robert H. MacArthur’s 1967 book ‘The Theory of Island Biogeography’ became a cornerstone of new science of population biology

David Quammen


TO SOME PEOPLE, FEW BUT VOCAL, EDWARD O. WILSON IS A BUGBEAR, infamous as the leading enunciator of a branch of science called sociobiology. To others, he is famous and much admired for the same reason. Yes, it’s the same Edward O. Wilson who published Sociobiology: The New Synthesis in 1975, daring to suggest that human social behavior might be partly shaped by genetics. That phase of his life and career is a story in itself (how he was accused of inventing pseudoscientific justification for a racist and sexist status quo; how he was denounced in print by leftist intellectuals and on the placards of angry young demonstrators; how he achieved the unwelcome distinction of having a pitcher of water dumped on his head, by one of those demonstrators, while addressing the American Association for the Advancement of Science). To still other people, more recently, Edward O. Wilson is a brilliant polymath and an elegant writer, a statesmanlike voice of warning about global losses of biological diversity, a wise elder to the conservation movement. And among his entomological colleagues, he is a towering authority on the taxonomy and behavior of ants.

Aware of all those Wilson personae, I’m interested in still another. To me he’s the sole surviving member of the MacArthur-and-Wilson partnership, and therefore a maker of history.

In 1967 Princeton University Press published a small, dense, eye-crossingly mathematical book, The Theory of Island Biogeography, by Robert H. MacArthur and Edward O. Wilson. It was the first in a series that Princeton intended to bring out under the collective rubric Monographs in Population Biology, and its premier position in that series hinted that this drab little volume concerned itself not just with islands and not just with biogeography.

Population biology, at that time, was still a new label for a new combination of scientific concerns and perspectives. It had subsumed a number of disciplines that, throughout earlier decades, were generally treated as distinct: evolutionary biology, taxonomy, biogeography, ecology, genetics. During the 1930s and 1940s, evolutionary biology (as descended from Charles Darwin and Alfred Russel Wallace) had at last become informed by and integrated with genetics (as descended from Gregor Mendel, of whose work Darwin and Wallace were ignorant). Also during that era, taxonomy had been added to the synthesis. By the 1950s, a great ecologist and teacher named G. Evelyn Hutchinson, born British but latterly fetched up at Yale, had started to bring a modestly mathematized style of ecology into conjunction with those other elements. Hutchinson’s own research specialty was limnology, the study islands. It was a canny choice for an ecologist of his bent, in that lakes are neatly bounded ecosystems, relatively small and relatively impoverished, and therefore lend themselves well to relatively thorough investigation. They share that with islands–not surprisingly, since a lake is the reverse image of an island. But the similarity between lacustrine and insular ecosystems isn’t what puts G. Evelyn Hutchinson into this story. Among the graduate students who would go on to add luster to Hutchinson’s reputation as a teacher–foremost among them, in fact–was a young man named Robert MacArthur, born in 1930.

After finishing a master’s degree in mathematics, MacArthur had turned to ecology, arriving at Yale in 19 5 3 for a doctoral program under Hutchinson’s guidance. By 1955 he had published a paper in Ecology. By 1957 he had produced a provocative theoretical contribution that appeared in the Proceedings of the National Academy of Sciences. His dissertation work, a study of community structure and niche partitioning among different species of warbler, also yielded paper for Ecology, which appeared in 1958 and became recognized as a minor classic. Young MacArthur was a hot prospect. He possessed the right combination of talents and ambitions to make a big impact at that particular time on that particular field. He was formidably bright, restless, almighty curios, innovative, and mathematically proficient. He loved the natural world, especially birds, but he had no interest in spending his life as a descriptive field naturalist. He cared about ideas and deep mechanisms, about order and explanation, not just about creatures and landscape. He was eager to change the very character of ecology.

MacArthur felt that the science of ecosystems should venture beyond description. It shouldn’t limit itself to collecting and indexing facts. It should find broader patterns in the natural world, and from those patterns it should extract general. principles. It should measure and count and perform calculational abstractions that would illuminate the essential amid the contingent. It should construct mathematical models that would function as usefully as a slide rule. It should be vigorous enough and bold enough to make predictions. It should offer theory. But ecology as generally practiced was still a loose-jointed, descriptive, non-qualified, non-theoretical enterprise.

MacArthur began to teach. He continued to publish interesting papers. His reputation blossomed. In 1965, after several years at the University of Pennsylvania, he accepted a professorship at Princeton, his role there to include the overall editorship of the Monographs in Population Biology, which offered promise as a forum for the newfangled ecology he favored. By that time he had met and begun sharing ideas with another young scientist, Edward O. Wilson.

WILSON WAS A Myrmecologist, a specialist on the biology of ants. He had grown up in the South, a nature-loving boy who collected insects and kept snakes, spending much of his time alone in the woods and swamps of Alabama and northern Florida. He had become interested in ants by the time he was 9, and soon after that he began doing precociously serious myrmecological fieldwork. At age 13 Wilson began a rigorous study of certain ant groups in the Mobile area and made his first publishable observation. Several years later, with adolescent earnestness, he decided that he should pick an entomological specialty. Based on the challenge and opportunity they offered, he chose flies in preference to ants. But it was 1945 then, and World War II had interrupted the supply of insect pins, which came mainly from Czechoslovakia. Since ants are best preserved in small vials of alcohol rather than mounted on pins, Wilson switched back–and has never regretted it.

He had a first-class scientific brain, as it turned out, which carried him from the University of Alabama to the University of Tennessee and then to Harvard. The particular attraction of Harvard, as put to him, was that its museum held the world’s largest collection of ants. “Get a global view,” an older colleague had told him; “don’t sell yourself short with entirely local studies.” When he met MacArthur, in late 1959, Wilson had recently returned from a long stint of fieldwork in the tropics.

His skills and his interests were complementary to MacArthur’s. Whereas MacArthur had shifted from mathematics to mathematical ecology, Wilson was a taxonomist and a zoogeographer. He had made extensive collections in New Guinea and Australia, on the island of New Caledonia, in the New Hebrides, on Fiji. He knew more about the ants in Melanesia than any man jack on Earth–what they looked like, how they made their livings, which species lived on which islands. Like McArthur, though, he was interested in more than the description of natural history phenomena. As his head and his field notebooks filled with ant data, Wilson had begun to see patterns. For example: The number of ant species on an island tended to correlate closely with island size.

Wilson’s brainload of purely descriptive data seems to have helped bring on an existential headache. In 1961, feeling mildly depressed and unsure about his professional direction, he took a sabbatical from Harvard and went to South America. He loved fieldwork as much as ever, and the rain forests of Suriname, to his great pleasure, were rife with previously unstudied ants. He also visited the islands of Trinidad and Tobago, just off-shore from Venezuela. Trinidad is large, Tobago is small. On Trinidad he found more species of ant than on Tobago.

Pondering the descriptive sort of work he had been doing, Wilson yearned for something more. “It just didn’t seem enough to continue enlarging the natural history and biogeography of ants,” he confides in his short memoir “In the Queendom of the Ants,” published quietly in 1985. “The challenges were not commensurate to the forces then moving and shaking the biological sciences.” His recent acquaintance with Robert MacArthur and some other young mathematical ecologists, and his awareness of Hutchinson’s work, had persuaded Wilson that “much of the whole range of population biology was ripe for synthesis and rapid advance in experimental research; but this could only be accomplished with the aid of imaginative logical reasoning strengthened by mathematical models.” He was a mediocre mathematician, by his account, with no training beyond algebra and statistics. So during his brooding summer in Trinidad and Tobago, besides collecting ants, he taught himself some calculus and probability theory out of textbooks. Returning to Harvard, he enrolled in a mathematics class and sat conscientiously, as an associate professor, in a small desk among a crowd of undergrads. Eventually, after further remedial classes, he had enough math to feel comfortable. Meanwhile he began a collaboration that would take him beyond the descriptive.

“In 1962 Robert H. MacArthur and 1, both in our early thirties, decided to try something new in biogeography,” Wilson relates in his book Biophilia. “The discipline, which studies the distribution of plants and animals around the world, was ideal for theoretical research. Biogeography was intellectually important, replete with poorly organized information, underpopulated, and almost devoid of quantitative models. Its borders with ecology and genetics, specialties in which we also felt well prepared, were blank swaths across the map.” Wilson told MacArthur that he thought biogeography could be made into a rigorous, analytical science.

There were striking regularities in the welter of data, Wilson says, that no one had explained. The species-area relationship, for instance. More particularly, the recurrent ratio to which beetle taxonomist Philip Darlington had called attention: With each tenfold decrease in area came roughly a twofold decrease in species diversity. Among the ant species of Asia and the Pacific Islands, Wilson had noticed another pattern. Newly evolved species seemed to originate on the large landmasses of Asia and Australia and to disperse adventurously from there out to the far-flung islands. As those dispersing species colonized small and remote places such as Fiji, they seemed to supplant the older, native species that had gotten there before them. New species were continually arriving, old species were continually going extinct, and the net effect was … no gain or loss in number of ant species. It looked to Wilson like some sort of natural balance.

Yes, said MacArthur, an equilibrium.

“HE WAS MEDIUM TALL AND thin, with a handsomely angular face,” as Wilson remembers MacArthur. This too is from Biophilia, written a decade after MacArthur’s early death. “He met you with a level gaze supported by an ironic smile and widening of eyes. He spoke with a thin baritone voice in complete sentences and paragraphs, signaling his more important utterances by tilting his face slightly upward and swallowing.” Although it may not have been a complete sentence when MacArthur first spoke the word equilibrium to Wilson, presumably he tilted up his face.

“He had a calm, understated manner,” Wilson continues, “which in intellectuals suggests tightly reined power. Because very few professional academics can keep their mouths shut long enough to be sure about anything, MacArthur’s restraint gave his conversation an edge of finality he did not intend. In fact he was basically shy and reticent.”

To that portrait, subjective and affectionate, Wilson adds a statement that stands as dead accurate: “By general agreement MacArthur was the most important ecologist of his generation.”

The two of them brainstormed during 1961 and 1962 over this notion of a biogeographical balance. They scrutinized Wilson’s ant data from Melanesia. They looked at patterns of distribution among bird species in the Philippines, Indonesia, and New Guinea. They referred to Darlington’s enumeration of beetle and reptile species on the various Antillean islands. They grew convinced that the species lost from an island, during a given span of time under ordinary circumstances, are roughly equal in number to the species gained by the same island over the same span of time. Unless the island itself is very recent in origin or has undergone a sudden disruption, the rates of losses and gains tend to cancel each other out. The result is a dynamic stability. The number of resident species remains steady while, with one species replacing another, the roster of identities changes continually.

Just how are species lost from an island? By local extinction. And how are they gained? Two ways: By speciation, when a single old species splits into a pair of new species; and by immigration, when a species arrives and becomes established. MacArthur and Wilson suspected that the second of those two, immigration, is vastly more frequent than the first. Speciation could be disregarded, then, and the equilibrium they envisioned could be expressed as a balance between immigration and extinction. It’s worth repeating that extinction in this context refers usually to the local extinction of a population, not to the global extinction of a species. Mainlands or neighboring islands, having supplied the flow of transoceanic immigrants, supply still more as extinctions occur.

MacArthur and Wilson drew a simple, vivid graph that showed two sloping fines forming an X: the immigration line coming down, from left to right, and the extinction line going up. The downward slope indicated that immigration events tend to decrease in frequency as an island becomes crowded with species. The upward slope indicated the converse–that extinction events tend to increase in frequency with increasing crowdedness. Each of the fines was curved slightly, into a gentle concave sag, indicating subtle changes in the rate at which immigration decreases and extinction increases on an ever fuller island. In midgraph, the lines crossed; their crossing point marked equilibrium. The number of species corresponding to that graph point is the island’s normal complement of species, remaining roughly constant through time. At least that’s what MacArthur and Wilson hypothesized.

They also created an intricate mathematical model–a long sequence of differential equations capable of accepting numerical data at one end and, like the canning line in a pickle factory, emitting a conveniently transmogrified product at the other end. The product of this canning line was predictions. The model foretold particulars of equilibration (how much elapsed time? how many species?) on a given island. It also, in a broader sense, offered ecologists a radically new way of understanding ecosystems all over the planet in the late twentieth century–as those ecosystems become increasingly fragmented, by human activities, into islandlike pieces.

Wilson is the one living human best qualified to explain how such an informal and seemingly peripheral field as island biogeography became so formalized and so central to the science of ecology. He’s also, as I discover, a generous, mild, and unassuming man. He offers me three hours of intellectual hospitality from the middle of his busy workday as calmly as a good-hearted Methodist minister visiting the sick. “Please call me Ed,” he says. “And I’ll call you David, if I may.”

His office, on an upper floor of Harvard’s Museum of Comparative Zoology, is a large, cheerful room decorated in memorabilia and ants. Unlike the ants that come and go through my own office, his are in cages. The cages are neat plastic units arrayed on long lab tables. Some of them, no doubt the ones holding tropical species, are warmed gently by red electric bulbs. Far up on the left wall hangs a row of imposing black-framed photographs: five venerable men. Are they Ed Wilson’s distinguished predecessors here at the Museum of Comparative Zoology? Five dignified elders, they glower down on everything. Has the Harvard administration issued these photos as mandatory office decor, like the unctuous shot of the president in every post office? On the opposite wall, ironically contrapuntal in identical black frames, five ants glower back.

Far across the room, inconspicuous beside Wilson’s small desk, hangs another photo: the young Robert MacArthur.

Before we get serious about islands, Wilson hosts me to lunch. From his office refrigerator he pulls out turkey sandwiches, bottles of lingonberry juice, and paper-wrapped pieces of baklava, all of which he seems to have shopped for at some local deli himself. He apologizes, this eminent man, this wonderful lunatic of politeness, for the food’s not being fancy. But the turkey is fine and the lingonberry juice has come all the way from Finland.

After the baklava, we carry coffee cups to a seminar room just up the hall from his office. Anticipating the focus of my questions, he has offered to show me some slides. What he means, I discover, is that he’ll give me a private lecture on the origins and development of the MacArthur-and-Wilson theory. A projector sits ready. Wilson clicks up the first slide and then, triggered by that image into an outpouring of scientific and personal reminiscence, talks for almost an hour before clicking on to the next.

The slide shows two young men, dressed for tropical fieldwork, on a sunscorched cay of white sand. One of the two, thin and angular, wears chinos and deck shoes and a red-and-white baseball cap with the brim pulled low, barely revealing a three-day beard.

“MacArthur always hated to be photographed,” Wilson tells me.

In the late 1940s and the 1950s, Wilson says, G. Evelyn Hutchinson gathered around himself at Yale a small circle of extraordinary graduate students. Hutchinson and his students were exploring how evolutionary adaptation played a role in the dynamics of complex communities. One of the Hutchinsonians was a young fellow named Larry Slobodkin, who had just written a little ecology textbook expounding the quantitative model-building approach to evolutionary problems.

Wilson himself, up at Harvard, wasn’t part of Hutchinson’s circle. He had not yet met MacArthur or taken his South American sabbatical. But he knew Larry Slobodkin, and they got along well. Since their interests meshed, he and Slobodkin started planning a joint project.

Slobodkin had some of the mathematical skills that Wilson lacked, as well as a firm grounding in ecological theory. Wilson for his part was strong on myrmecology, on biogeography, and on speciation theory. “Slobodkin and I saw that we could put these two bodies of knowledge and these approaches together, and maybe do a book on population biology,” Wilson recalls. They had even discussed it with a publisher. “Then Slobodkin hesitated. He said, ‘We really need a third author'”–a specific fellow, a certain mathematician whom Slobodkin had in mind–“‘because he’s just so damn good. He’s so brilliant and he’s the coming thing in ecology and full of ideas. I want you to meet Robert MacArthur.'” During a conference in December 1959, they did meet. “And he and I hit it off immediately,” Wilson says.

I gaze at the young man in the red-and-white cap, now famous, now departed, whose image remains frozen on Wilson’s screen.

MacArthur was just back from a postdoctoral year at Oxford. He struck Wilson as “a somewhat ethereal, delicate, slightly Anglicized American who clearly was thinking deeply about these subjects.” Also, just as clearly, he was ambitious to do something exciting and drastic in population biology. Fine; so was Wilson. They quickly became friends, started a dialogue through the mail, and traded ideas toward that proposed three-author book. Wilson even drafted a few chapters. But then for some reason the friendship between Slobodkin and MacArthur went sour–so the book project fell apart. Wilson himself went off on that sabbatical and began his remedial studies in math. When he returned north, he resumed the contact with MacArthur. They met when they could, at conferences, at the University of Pennsylvania when Wilson was invited down there to speak, at MacArthur’s summer home in the small town of Marlboro, Vermont. They had “runaway conversations about biology–about the future of population biology and so on.” They were young and energetic, dreaming the sort of wild plans that energetic young men often dream. But these two weren’t just young and energetic. Though Wilson doesn’t say so, they were also disciplined, scientifically sophisticated, and extraordinarily smart.

The old photo of MacArthur still shines before us. Without a good fan in the projector, this precious slide would have long since suffered meltdown.

“Okay,” says Wilson, “now I’m coming up to the crunch.” He has told the story before, in some of his classes, but never to a smaller or more interested audience. “Slobodkin drops out,” he says. “MacArthur and I become thick as thieves. We’re now enchanted by the idea that something really important might be done with biogeography. So I keep saying, ‘Biogeography, that’s the field of the future.’ Balance of nature. Equilibrium and so on. We’re looking at those curves. Then in ’62, the summer of ’62, after talks in Pennsylvania and Marlboro, at his summer house, I get through the mail a little two- or three-page letter with the equilibrium model–the crossed lines–from MacArthur. And he says, ‘I think that’s the way the whole thing might be approached.’ And I look at this thing. And I say, ‘Yes, that’s it.'”

The equilibrium theory of island biogeography is not a piece of conceptual art. It’s a tool. MacArthur and Wilson developed it for two reasons: to explain and to predict.

Although its mathematical details are egregiously complicated, its essence is simple. Two patterns of real-world data served as the starting points for the theory, which was devised to account for them jointly. First pattern: The species-area relationship. The ants that Wilson knew so well, on islands of the western Pacific, showed a nicely regular version of that relationship. There were more ant species on the bigger islands, fewer ant species on the smaller islands. The beetles and the reptiles and amphibians of the Antilles showed other neat versions of the species-area relationship. The land birds of certain Indonesian islands showed still another. In each case, larger islands contained more species than smaller islands, and when the numbers of species were graphed against the sizes of the respective islands, the graph points arrayed themselves (with a little logarithmic jiggering) as a straight line. Despite its straightness, the line was what scientists call a curve. The slope of each species-area curve could be expressed as a decimal number, which varied from one group of islands to another. It took one value for Antillean beetles, another value for Indonesian birds, still another for ants of the western Pacific.

The second pattern, like the first, had long been familiar to biogeographers: remote islands support fewer species than do less remote islands. This pattern shows itself in several different ways. An island of some given size, if located near a mainland, generally supports more species than a similar-size island far offshore. Also, a small island near a large island (for instance, one of the satellite islands around New Guinea) generally supports more species than a small island with no big neighbor.

MacArthur and Wilson’s predecessors had commonly explained that pattern in historical terms. Remoteness was an impediment that only eons could overcome. Species impoverishment together with remoteness suggested that an island’s history had been relatively brief. Colonization of any new oceanic island took time–vast sweeps of time, if the island was remote–and remote islands were generally not ancient enough to have acquired great richness of species. So said the historical hypothesis.

But MacArthur and Wilson suspected that history wasn’t the answer. Time was the limiting factor only during the earliest period on a new oceanic island, they believed, and most of the world’s island ecosystems had long since come to maturity, to a state of balance, to equilibrium, with the number of species on each a reflection of ongoing processes, not historical circumstances. The ongoing processes that most shaped the balance, they argued, were immigration and extinction.

The sagging-X graph illustrates this ahistorical theory. The immigration curve slopes downward from the left. The extinction curve slopes upward to the right. The decrease in immigration rate and the increase in extinction rate are graphed not against elapsed time but against the number of species present on a given island. As an island fills up with species, immigration declines and extinction increases, until they offset each other at an equilibrium level. At that level, the rate of continuing immigration is just canceled by the rate of continuing extinction, and there is no net gain or loss of species. The phenomenon of offsetting increase and decrease–the change of identities on the roster of species–is known as turnover. One species of butterfly arrives, another dies out, and in the aftermath the island has the same number of butterfly species as before. Equilibrium with turnover.

MacArthur and Wilson’s conceptual model explains the two patterns of real-world data, and its explanatory power is what makes it forceful. Small islands harbor fewer species than large islands. Why? Because small islands receive fewer immigrants and suffer more extinctions. In MacArthur and Wilson’s schema, that’s known as the area effect. Remote islands harbor fewer species than near islands. Why? Because remote islands receive fewer immigrants and suffer just as many extinctions. That’s the distance effect. Area and distance combine their effects to regulate the balance between immigration and extinction. It all fits together ingeniously.

MacArthur had mailed Wilson his little sketch of the crossed lines and Wilson had said “Eureka”; but the full theory, with its supporting arguments and math, was not yet on paper. Then one day they were sitting together near the fireplace in MacArthur’s living room, as Wilson recollects, with notes and graphs spread out before them on a coffee table. This was late 1962. They felt satisfied that their equilibrium model gave a good explanation for the two real-world patterns. But that wasn’t enough. Both patterns–one reflecting the distance effect, one the area effect–were relatively uncomplicated, and another theory might account for them equally well. MacArthur and Wilson needed further evidence. They needed to show an intricate match between what their theory said and what was.

MacArthur suggested a way. Using the mathematical machinery of their theory, they could calculate how quickly a newborn island should approach equilibrium, and how that rate of approach should correspond to the island’s eventual rate of turnover. The numbers would be different for each different newborn island. They should pick one and try it. Having made the calculations, based on the actual area and remoteness of their chosen island, they could compare their theory-derived answers against empirical data. The theory would be either supported or contradicted. It was a good suggestion with one practical drawback: newborn islands are rare. And not many have ever been scientifically studied during the approach to species equilibrium. So there wasn’t much empirical data available. Lonely nubs of fresh lava, cooling like slag in midocean, don’t often attract ecologists.

At this point Wilson had an idea. Let’s look at Krakatau, he said.

Wilson himself had never been there and didn’t propose to go, but he knew something about Krakatau from the literature. He knew that the great eruption of 1883 had killed everything, leaving only a cauterized mound, and that the ash had barely settled before spiders and insects and fern spores began arriving to fill the biological vacuum. He knew that the cauterized mound was effectively a newborn island. He recognized that this mound (under its new name, Rakata) could offer a well-documented case of an island’s approach to equilibrium. It was as close to a theory-testing experiment as two young biogeographers could get.

So MacArthur and Wilson began calculating. They focused on birds. Extrapolating from a species-area curve for the avifaunas of other Asian islands, they figured the number of species that Rakata should support at equilibrium. They figured how much time should have been necessary, after the sterilizing explosion, for Rakata to reach that equilibrium. They figured what the turnover rate should be at equilibrium. Baldly put, their projections were: equilibrium number, 30 species; time t6 equilibrium, 40 years; turnover, one species per year.

Then they went to the empirical data.

After the earliest biological expedition, led by a Professor Treub back in 1886, Rakata had been visited and surveyed again a number of times, notably in 1908, 192 1, and 1934. The data from those surveys had been summarized in a Dutch journal paper of 1948. Consulting it, MacArthur and Wilson found that Rakata in 1908 had supported just 13 resident species of bird. Recolonization had barely begun. Between 1908 and 1921, they found, the number of bird species rose to 27. Then it leveled off. Between 1921 and 1934, there was no net change, with just 27 species recorded again. That suggested a form of stability, dynamic or otherwise. Among these 27 bird species, though, were 5 new species– of the old species were gone. That was turnover. Equilibrium had arrived within roughly four decades, it seemed, and the equilibrium number was roughly 30, as predicted. True, the turnover rate appeared to be lower than their projection, but not so much lower as to discourage them.

“We were very excited,” Wilson tells me. “We said, Good Lord, here was the first time that you could come up with a bare-ass model like that, of what an equilibrium would look like, and you actually have a case where you can measure equilibrium and turnover, and it seems to fit.”

The Krakatau case contributed vividly to the theory’s success. Wilson’s idea about looking at those data had paid off. The Theory of Island Biogeography was published in book form in 1967. At that point Robert MacArthur had five years to live. He continued teaching at Princeton. He wrote 15 more papers. He helped found the journal Theoretical Population Biology. In 1968 he went down to the Florida Keys and visited Wilson, who had another sabbatical and was spending it there. They talked excitedly about island biogeography as a quantitative paradigm and where it might possibly lead. MacArthur was always a great talker, and conversation with his students and his colleagues was one of the chief ways he exerted his magic influence. Sometimes the conversation involved scribbled equations, groping efforts to put nascent ideas into the crisp language of mathematics. Not many ecologists could match his mathematical sophistication. But mathematics for MacArthur was only a means toward the larger goal: understanding how evolutionary processes and ecological tensions combine to shape biological communities.

One theme underlay most of his work. This theme–it seems almost a truism now, but MacArthur himself considered it worth stating–was the search for patterns. He emphasized patterns and equilibriums and ongoing processes, while de-emphasizing the sort of onetime, contingent events that figure in historical explanations. Where lies the distinction between those two types of explanation, the process-oriented and the historical? A historian pays special attention to the differences between phenomena because they shed light on historical contingency. “He may ask why the New World tropics have toucans and hummingbirds,” MacArthur wrote, “and parts of the Old World have hornbill and sunbirds.” The hornbills of Africa and Asia are large-bodied, omnivorous birds with huge beaks, allowing them to fill roughly the same ecological niches as the toucans of tropical America; likewise the sunbirds of Africa and Asia are small-bodied, bright-colored nectar drinkers, filling roughly the same niches as American hummingbirds. The history-minded biogeographer wonders why hummingbirds, not sunbirds, have occupied the suitable niches on a given continent. MacArthur himself was more interested in the similarities among phenomena because similarities reveal the workings of regular processes. He was more inclined to wonder why hummingbirds and sunbirds, despite different ancestries and independent histories in two different regions of the planet, are so similar.

For MacArthur, the geographical distribution of animal and plant species was full of deeply interesting questions. Why did such and such a community contain these species but not those? Why did such and such species live there but not here? Answers existed, MacArthur knew, and those answers were important, he believed. Of course the best of the earlier biogeographers, Darwin and Wallace, had also searched for repeated patterns and sought answers for the deeply interesting questions those patterns raised; MacArthur was merely continuing their tradition while inventing new methods. For most of the twentieth century, biogeography had been descriptive, not theoretical; it hadn’t maintained the standard of profound provocation that Wallace and Darwin had set. Robert MacArthur, like his distinguished Victorian. predecessors, was a biogeographer with a hungry ambition toward theory.

Sometime in 1972, MacArthur’s illness was diagnosed. He had renal cancer. It progressed quickly.

He went up to the house in Vermont and, working against time, with no access to libraries, produced a short volume tided Geographical Ecology: Patterns in the Distribution of Species, summarizing his own final view of what mattered most in ecological science. He gave special thanks in the preface to Ed Wilson, who “showed me how interesting biogeography could be and wrote the lion’s share of a joint book on islands.”

By late autumn, MacArthur was home again in Princeton. Although he and Wilson had pursued different projects since their collaboration, the friendship remained strong. Wilson tells me of their last conversation. He had heard that MacArthur was low, so he telephoned. “I knew he was dying. I didn’t realize he was within hours of passing on.”

“What did you talk about?” I ask.

It was just the sort of chat he’d had with MacArthur many times. They discussed ecology. The future prospects, the unanswered questions. “We talked about recent developments in the field, and we gossiped about who had his head screwed on right,” Wilson says. MacArthur barely mentioned his illness, as though he had a hundred years to live. The call lasted half an hour. Wilson made notes immediately afterward, thinking not so much about deathbed wisdom from a historic personage as about his own later sentimental interest. He stuffed the notes into a file, he says, and hasn’t looked at the file in two decades. He seems to prefer trusting his memory. Memory knows things that notes could never remember.

About those notes: “I took them down as to what we said, and as to what we thought of certain people, and this and that. And that was it. I mean, there was no . . . We didn’t inject sentiment.” What he seems to mean is that true emotion, of which there was plenty, is often wordless. “Or say farewells. I didn’t really expect that it would be that quick, anyway. That was it.”

The death of MacArthur as perceived by Wilson–that can’t be it, I think. There’s undoubtedly more. There’s surely some little human detail, incidental, so poignant that even memory chooses not to remember it. I want to say: Let’s look at the notes. Lees see what it is you’ve forgotten. But of course I don’t.

By now I’ve cored the heart out of Wilson’s workday, and it’s time to let him escape back into the present. I’ve caused distraction enough, not least by reminding him of the note file. The file is still around somewhere, presumably in the ant-filled office, where the photo of Robert MacArthur hangs like a private icon.

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