He joined Slobodkin and me, a thin, diffident young man who spoke with an American accent but in the British style of cautious understatement, perhaps acquired at Oxford. The book is an attractive idea, he said. We should explore it further. He had a headache. He wanted to go home. We shook hands, and he left.
Nothing more happened for nearly a year. That was my fault. I simply put the book project completely aside in order to return to field work. The tropics had reasserted their pull. A dream stirred deep within me that El Dorado was still there unattained. I had to go. In February 1961 Renee and I traveled to Trinidad, where we stayed as guests of an Icelandic native and widow named Asa Wright. Her property, Spring Hill Estate, was perched near the head of Arima Valley, in the North Range. It had become a popular stopover for naturalists and serious birders from around the world. Broken rain forest ran down the valley to Simla, the research station founded by William Beebe. The great naturalist was in the last year of his life, and I was grateful for the opportunity to meet him. Renee and I dined occasionally with him and his capable assistant Jocelyn Crane at Simla, admiring the silver candlesticks given to Beebe by his friend Rudyard Kipling and talking tropical natural history here in the place where so much of the best research on it had been conceived.
The tropics in those days nourished a strange collection of intellectuals. At Spring Hill we sat on the screened veranda listening to stories by another famous visitor, Colonel Richard Meinertzhagen, who had first served as an officer under Queen Victoria and fought with T. E. Lawrence in the Middle East during the First World War. I looked him up later in Lawrence’s Seven Pillars of Wisdom, and sure enough, there was Meinertzhagen, reported in the same episodes he had described to Renee and me. Meinertzhagen was at Spring Hill to visit oil birds in a nearby cave and to collect the fruits of native forest trees. Given all this, and the aging Asa Wright’s retrograde colonial attitude toward the Trinidadians of color, we felt we had been propelled backward in time fifty years.
There were adventures to savor, this time shared with Renee. Once a pet donkey wandered across the Spring Hill veranda into the open dining room, its hooves clopping loudly on the hardwood floor, and consumed a chocolate cake set out for afternoon tea. The maid quickly chased him back out. Soon afterward, as Renee sat in a corner of the veranda, waiting for my return from the field, she could not help overhearing Asa’s reaction to the news: “Oh my god, Eutrice, do the Wilsons know?”
“No ma’am,” Eutrice lied.
The donkey was tethered to a veranda post at night, and sometime during the evening it was usually visited by vampire bats that flew in from the neighboring forest. In the morning one or two dried rivulets of blood streaked some part or other of its flanks or legs. Such bloodletting was a common problem for livestock in the area, and the bats carried rabies. Seated on the veranda, armed with flashlights, Meinertzhagen and I watched eagerly for the arrival of the vampires late into the night. We never saw one. That is the talent of vampires, evading detection.
Two months after arriving Renee and I departed for Suriname, to add field work on the South American mainland. We proceeded by freighter out of Port of Spain to the bauxite mining town of Moengo, then back to the capital of Paramaribo. We lived in a pension there while I explored the forests south as far as Zanderij. We then returned to Spring Hill for a while before proceeding to Tobago for the final three months, June through most of August, of our tour.
I felt completely at home again in the heat and smell of rotting vegetation, although Renee did not, especially when she learned about the vampire bats. Discoveries came easily, as always for me in tropical forests. I acquired a colony of the giant, primitive dacetine ant Daceton armigerum from a nest high in a tree in Suriname and made the first study of its social organization. I rediscovered the apparently “true” cave ant Spelaeomyrmex urichi in a central Trinidad cavern, and proved that the species also lives in the open forest of Suriname—and thus is not an obligatory cave ant. I puttered here and there, in the opportunistic spirit that had always guided me.
But early on this field trip, while beginning work on Trinidad, I found the tropics less than paradise. To my dismay I slipped into a depression for the first time in my life. I began to worry again about the broader canvases of ecology and evolution, and the need to get on with the agenda of my young evolutionists’ conceptual revolution. I hated the corresponding diminution of my naturalist’s ardor. I was anxious about my own inadequacy in mathematics. I felt certain that the future principles of evolutionary biology would be written in equations, with the deepest insights expressed by quantitative models. I set out to remedy my deficiency by teaching myself calculus, probability theory, and statistics from textbooks I read on verandas and beach cupolas in Trinidad and Tobago. Progress was slow; I was not gifted; I worried even more. Here I was, thirty-two years old, time and the main chance about to slip away—or so it seemed. Would I miss out on the real action coming?
Soon after our return home in late August, Renee and I bought our first house, a small two-story cape in the suburban town of Lexington, ten miles west of Cambridge. It cost $19,000, about twice my gross annual salary at the time. By scrimping on expenses during our sabbatical trip, we had just managed to save the minimum $3,000 down payment for a first mortgage. Now five years into our marriage, we at last felt rooted and secure. I felt more confident in my work and in the knowledge that I would probably stay at Harvard for the remainder of my career. My math anxiety faded.
Soon afterward MacArthur and Slobodkin joined me at Harvard for a one-day meeting to resume planning our book on population biology. We drew up an outline, divided chapter assignments, and went our separate ways. As much as I admired Slobodkin, I felt a stronger personal attraction to MacArthur. In subsequent correspondence and visits we discovered a surprising range of common interests, among which was a passion for biogeography—the geographic distribution of plants and animals. The traditional discipline, in which I had been steeped throughout my career, was in chaos. Grand chaos, in fact, since the subject matter is the largest in physical scale of all biology, and it spans the entire history of life.
In 1961, when MacArthur and I focused on it, biogeography was still largely descriptive. Its most interesting theory was the Matthew-Darlington cycle of dominance and replacement. Otherwise its main substance comprised such topics as the origin of the fauna and flora of the West Indies—whether by immigration across dry land bridges that once connected the islands to the mainland or by the chance arrival of organisms borne on water and in the air. Biogeography seemed ripe for the new thinking that was emerging in population biology. I showed MacArthur some of the curves in my files linking the area of individual islands to the numbers of resident species of ants and other organisms. I told him about my conception of the taxon cycle and the balance of species.
MacArthur’s interest in these and related subjects grew rapidly. As our discussions deepened, and spread to include gossip and personal anecdotes, we became close friends. Our backgrounds proved similar in several respects that matter most in scientific collaboration. Although he had majored in mathematics at Marlboro College, and had a conspicuous talent for it, his heart was in the study of birds. He was a naturalist by calling, and seemed happiest when searching for patterns discovered directly in Nature with the aid of binoculars and field guides. It was his calling to scan the tangled bank of Nature and skeletonize it in his own and others’ minds to its essential abstract features. As a mathematician-naturalist he was unique, approached only by his mentor, Evelyn Hutchinson. He was not as expansive in his interests as Hutchinson, but quicker and more deeply penetrating at strategic points. He shared the conviction of the great mathematician G. H. Hardy, whom he resembled in temperament and philosophy, “that a mathematician was a maker of patterns of ideas, and that beauty and seriousness were the criteria by which his patterns should be judged.” He wished above all to discover beautiful true-life patterns.
In conversation, MacArthur wo
uld say that the best science comes to a great extent from the invention of new classifications of natural phenomena, the ones that suggest hypotheses and new rounds of data gathering. “Art”—he enjoyed quoting Picasso—“is the lie that helps us see the truth.” His methodology bore testimony of the strength of an inherent naturalist: he knew what he was talking about, and he was concerned more with the tapestry of Nature and his power to see it independently than with what others thought of it, or of him.
MacArthur watched birds with the patience and skill of a professional ornithologist. He visited the tropics as often as he could, and delighted in relating endless facts of natural history. The store of random information thus accumulated and the play of its intersecting patterns were the inspiration of his theoretical work, by which he described the process of the origin of biological diversity.
When I first met him he was an assistant professor at the University of Pennsylvania, soon to be promoted to associate and then to full professor. He later moved to Princeton, where in a short time he was named Henry Fairfield Osborn Professor of Biology. His demeanor was subdued and pleasing. Of medium height, with a handsomely rectangular face, he met you with a disarming 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 head slightly upward and swallowing. He had a calm, understated manner, which in intellectuals suggests tightly reined power. In contrast to the excessive loquacity of most professional academics, MacArthur’s restraint gave his words an authority rarely intended. In fact he was basically shy and loathed being caught in a careless error. He was nevertheless conscious of his status among colleagues and felt secure about it. Although he was generous by instinct and capable of lavish, almost Hutchinsonian praise during private conversations for work he thought important, he did not hesitate to describe the foibles and weaknesses of others with pitiless accuracy. But he harbored no malice I could detect, only a taxonomic interest in other scientists and a frequent disappointment that tempered his enthusiasms.
He joined superior talent with an unusual creative drive and decent ambition. He placed his family, Betsy and the four children, above all else. After that came the natural world, birds, and science, in that order. One day as we strolled along a road in the Florida Keys, I told him of the effort I was making with several others to conserve Lignumvitae Key, one of the last islands of Florida with a relatively undisturbed Caribbean forest. He reacted with a warmth that surprised me—I had not even thought of mentioning it earlier. He declared that he would rather save an endangered habitat than create an important scientific theory.
MacArthur launched his scientific career with two articles that revealed his unusual powers. The first, in 1955, suggested a way to predict stability in a community of plants and animals by the use of information theory. It formalized a concept that until then could be expressed only through verbal description. Soon afterward, in 1957, came the famous “broken stick” model of relative abundance of bird species. To capture the essence of his approach, imagine that the combined numbers of a certain guild of birds, say warblers, found in a particular forest is represented by the length of a stick. Make the stick one meter long, representing 100,000 warblers, so that each bird is represented by a fraction of a millimeter somewhere on the stick. Have the guild consist of ten warbler species. Break the stick into ten pieces at random, with their lengths randomly distributed. Let the length of each piece represent the number of individuals of a particular species. One species, let us say, gets 200 millimeters, or 20 percent of the stick; it is therefore assigned 20,000 individual birds. Another species gets 5 millimeters, or 5,000 birds. And so on for all ten pieces and the individual birds their lengths represent. Because the pieces and thence the species are not allowed to overlap, the array of numbers for the whole ensemble of ten warbler species will be the same as if real warblers divided up the resources of the forest among themselves competitively, so there is no sharing of resources, and the fraction each species has acquired is a random variable. The niche of each species is also unique. If real warblers were found to fit a numbers array like this array (more technically put, if its “species abundance distribution” fits the broken-stick model), we would be justified in supposing that the warblers are really segregated by competition for resources. At least we must keep that possibility open, subject to confirming studies of other kinds. What would be the alternative to the exclusion model? One proposed by MacArthur was that the species receive pieces of the stick with lengths being determined randomly, but that the pieces can be overlapping; in other words, the bird species do not exclude one another by competition. Because the exclusion model turned out to fit one set of bird data and MacArthur’s first disposal more closely than did the alternatives he conceived, he concluded that competition is likely to be important in determining the abundance of birds.
The specific hypothesis of competition captured by the broken-stick distribution was later disputed by others, and MacArthur himself eventually dismissed his methodological approach—prematurely, I thought—as obsolete. Even while fading, however, the conception represented a breakthrough in ecological theory. In three pages, MacArthur confronted a central problem of community ecology with competing hypotheses expressed as numbers, in contrast to previous theorists, who had formulated the same general idea more vaguely by words. He characterized the issue in such a way as to allow logically possible alternatives to be tested and a choice made. By working out this example, he showed that the deepest remaining mysteries of natural history might be solved by leaps of the imagination, so long as such efforts are disciplined by clear postulation tested by data taken from the field.
The method of multiple working hypotheses was thereby introduced to the branch of ecology concerned with whole communities of species. MacArthur’s 1957 article set the tone of all his later work. Inevitably, his entire approach, not just the broken-stick model, was correctly criticized by some ecologists as oversimplification. That defect matters little in the long haul of history. It was a step in the right direction. Right or wrong in particular applications, it energized a generation of young population biologists and transformed a large part of ecology. It helped us to think clearly.
As MacArthur and I extended our conversations, I expressed three convictions of my own. First, that islands are the key to rapid progress in biogeography. The communities they contain are discrete units that are isolated by the sea and can be studied in multiples. Second, that all biogeography, including even the histories of faunas and floras, can be made a branch of population biology. And finally, that species on islands are somehow in balance in a way that can be modeled quantitatively. MacArthur quickly agreed, and began to apply his powers of abstraction to the data sets I showed him. In the following exchange I have telescoped our conversations and letters on the subject in order to convey the crucial steps in the origin of species-equilibrium theory.*
Wilson: I think biogeography can be made into a science. There are striking regularities no one has explained. For example, the larger the island, the more the species of birds or ants that live on it. Look at what happens when you go from little islands, such as Bali and Lombok, to big ones like Borneo and Sumatra. With every tenfold increase in area, there is roughly a doubling of the number of species found on the island. That appears to be true for most other kinds of animals and plants for which we have good data. Here’s another piece in the puzzle. I’ve found that as new ant species spread out from Asia and Australia onto the islands between them, such as New Guinea and Fiji, they eliminate other ones that settled there earlier. At the level of the species this pattern fits in pretty well with the views of Philip Darlington and George Simpson. They proved that in the past major groups of mammals, such as all the deer or all the pigs taken together, have tended to replace other major groups in South America and Asia, filling the same niches. So there seems to be a balance of Nature down to the level of the species, with waves of replace
ment spreading around the world.
MacArthur: Yes, a species equilibrium. It looks as though each island can hold just so many species, so if one species colonizes the island, an older resident has to go extinct. Let’s treat the whole thing as if it were a physical process. Think of the island as filling up with species from an empty state up to the limit. That’s just a metaphor, but it might get us somewhere. As more species establish themselves, the rate at which they go extinct will rise. Let me put it another way: The probability that any given species will go extinct increases as more species crowd onto the island. Now look at the species arriving. A few colonists of each are making it each year on the wind or on floating logs or, like birds, flying in on their own power. The more species that settle on the island, the fewer new ones that will be arriving each year, simply because there are fewer that aren’t already there. Here’s how a physicist or economist would represent the situation. As the island fills up, the rate of extinction goes up and the rate of immigration goes down, until the two processes reach the same level. So by definition you have a dynamic equilibrium. When extinction equals immigration, the number of species stays the same, even though there may be a steady change in the particular species making up the fauna.
Look what happens when you play around a little with the rising and falling curves. Let the islands get smaller. The extinction rates have to go up, since the populations are smaller and more liable to extinction. If there are only ten birds of a kind sitting in the trees, they are more likely to go to zero in a given year than if there are a hundred. But the rate at which new species are arriving won’t be affected very much, because islands well away from the mainland can vary a lot in size without changing much in the amount of horizon they present to organisms traveling toward them. As a result, smaller islands will reach equilibrium sooner and end up with a smaller number of species at equilibrium. Now look at pure distance as a factor. The farther the island is from the source areas, say the way Hawaii is farther from Asia than New Guinea, the fewer new species that will be arriving each year. But the rate of extinction stays the same because, once a species of plant or animal is settled on an island, it doesn’t matter whether the island is close or far. So you expect the number of species found on distant islands to be fewer. The whole thing is just a matter of geometry.
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