Exultation and Explanation

G. Evelyn Hutchinson, Sterling Professor of Zoology Emeritus at Yale, is unquestionably the world’s greatest living ecologist. In his most famous article—entitled “Homage to Santa Rosalia, or why are there so many kinds of animals?”—Hutchinson emphasizes the fundamental theme of ecology by citing an anecdote of J.B.S. Haldane. The great British biologist “found himself in the company of a group of theologians. On being asked what one could conclude as to the nature of the Creator from a study of his creation, Haldane is said to have answered, ‘An inordinate fondness for beetles.”‘¹

Since Linnaeus set the modern style of formal naming in 1758, more than a million species of plants and animals have received Latin binomials. More than 80 percent of these names apply to animals; of the animals, nearly 75 percent are insects; of the insects, about 60 percent are beetles. It is stunning that a single mode of organic architecture should engender such diversity—indeed, it continues to amaze me every time I think about it, and I have thought about it often. From the tiny trichopterygids less than 1/100 inch in length, to the all-female Micromalthus reproducing as a larva in rotting wood, to the quarter pound Goliath beetle and nearly foot long Batocera of New Guinea, beetles embody nature’s finest display of her principal theme—multifarious diversity. The science of ecology probes this richness for regularities. As Hutchinson asked in his subtitle to the article graced by Haldane’s anecdote: “Why are there so many kinds of animals?”

Ecologists must live in tension between two approaches to the diversity of life. On the one hand, they are tempted to bask in the irreducibility and glory of it all—exult and record. But, on the other, they acknowledge that science is a search for repeated pattern. Laws and regularities underlie the display. Why are there more species in tropical than in temperate zones? Why so many more small animals than large ones? Why do food chains tend to be longer in the sea than on land? Why are reefs so diversely and sea shores so sparsely populated with species? As an explanatory science, ecology traffics in differential equations, complex statistics, mathematical modeling, and computer simulation. I haven’t seen a picture of an animal in the leading journal of evolutionary ecology for years.

Many ecologists have escaped this tension by focusing their work on a single approach—exultation or explanation—and by treating the other side with territorial suspicion and derogation. Hutchinson has practiced and loved both all his life. He had all the advantages of an upper-class, English, intellectual background. The Kindly Fruits of the Earth, an autobiographical account of his early life up to his appointment at Yale in the late 1920s, reads like an extended idyll. More than half the book traces his education through the English public schools to Emmanuel College, Cambridge (his father had been a mineralogist and don at Pembroke College). The last three chapters recount his early professional experiences in Italy, South Africa, and the United States. Hutchinson apparently enjoyed all of it, from botanizing in the woods to tea with a variety of notables.

Perhaps one has to be a bit more dyspeptic or analytic than Hutchinson is to communicate the meaning of such advantages to those of us who did not grow up enjoying them. Moreover, since Hutchinson is as legendary for his kindness as for his brilliant mind, this memoir only records the good deeds of dead people—for if he follows the old motto, de mortuis nihil nisi bonum (say only good of the dead), of the living he chooses to say absolutely nothing at all! (Hence the book must exclude Hutchinson’s last half century—the period of his greatest achievements.) This benign style may not win many readers beyond the large group of professionals who admire Hutchinson and who wish to learn more about the illustrious scientists associated with him, but it is somehow comforting to discover that such elemental decency actually exists.

During his early years at Yale, Hutchinson worked primarily in biogeochemistry, insect classification, and limnology—the science of fresh waters, especially lakes and ponds.² Indeed, now in his mid seventies, he is completing the fourth and final volume of his monumental Treatise on Limnology. When Hutchinson entered ecology, it was largely a profession of empirical recorders. It boasted few organizing concepts beyond “succession theory”—a somewhat mystical idea that treated communities of species as “superorganisms,” and tried to identify an orderly succession of communities in a history of colonization within a geographic region. The foundations of modern population ecology had been established in equations of demography and population growth by such mathematicians as A.J. Lotka, V. Volterra, and G.F. Gause, but most ecologists either didn’t know the work or questioned its relevance to real field data.

Hutchinson had a major part in fostering the two major directions of modern theoretical ecology. One approach, production or systems ecology, emphasizes the flow of energy among the components of ecosystems, for example the efficiency of “conversion” between levels in a food chain—how many calories of green plants are needed to sustain herbivores on land, what weight of consumed herbivores maintains a population of carnivores, etc. Raymond Lindeman, the Pergolesi of ecology, wrote the founding document of this discipline, but died in 1942 at age twenty-six while it was still in press.³ He developed many of his ideas as a post-doctoral student with Hutchinson. Moreover, after the journal Ecology had rejected Lindeman’s paper on the advice of two traditional ecologists, Hutchinson’s intercession proved crucial to a favorable reconsideration. Hutchinson wrote a private letter to the editor of Ecology⁴ that remains a stirring defense of theoretical work as a guide for empirical research, not a denial of its importance. He ends with Sir Thomas Browne’s defense of fruitful error—“the certainty hereof let the arithmetick of the last day determine.”

Hutchinson played a more direct role in establishing the second approach, population ecology. This begins with the central Darwinian postulate that nature manifests no higher principle than the struggle of individual organisms to maximize their own reproductive success. Notions of community and natural harmony, however illuminating as metaphors, do not reflect nature’s primary unit, the population of individuals within a species. Modern ecology begins with equations of population growth, including differing rates of fertility and mortality. It attempts to build toward a theory that will explain the size of populations, their rates of change in size, and the co-existence of many populations in a single area through competition and avoidance. An Introduction to Population Ecology is Hutchinson’s recent survey of this central subdiscipline in modern evolutionary theory. It is a textbook, and lay readers (and some professionals as well) may have to review their algebra before tackling it. But, unlike most examples of the genre, it is accessible, intelligent, and enjoyable to read as literature.

As a word, ecology has been so debased by recent political usage that many people employ it to identify anything good that happens far from cities and without human interference. But it is, among professionals, the science that tries to understand why there are so many kinds of organisms and why some habitats and some parts of the world harbor more kinds than others. It attempts, in other words, to explain the spatial and temporal structure of organic diversity. Its fundamental concept is the niche—an expression of the location and function of a species in a habitat.

Hutchinson made his most important contribution to theoretical ecology by establishing a workable, quantifiable concept of niche.⁵ Before him, ecologists debated whether a niche represented an organism’s address or its profession—in other words, are niches environmental spaces that exist whether or not organisms live in them, or are they created by the range of activities (feeding, nesting, etc.) performed by each species in its unique way? If niches are addresses, then the problem of organic diversity might be reduced to a study of physical habitats and their change through space and time. If they are professions, then diversity in an area will be set, in large part, by the kinds of organisms that settle there. We must then wonder whether a nearly infinite subdivision of niches might be possible and whether the notion of a limit to diversity makes any sense at all.

Not surprisingly, Hutchinson’s definition embodied fruitful aspects of both views—organisms do create ecospace through their activities, but the nature of physical space and resources sets important limits. Hutchinson measured the niche of a species by defining graded components in the environment that must be important for survival—temperature, particle size of food, or nesting height in trees among birds, for example. He then depicted these environmental components as mathematical axes at right angles to each other. (Axes at right angles are mathematically independent and, although we cannot visualize it, an axis may stand at right angles to as many other axes as we like in appropriate spaces higher than three dimensions.) For each axis, we can plot the total range used by a species; the volume of space defined by the intersection of these ranges for each axis is the niche. (See illustration.) Since useful quantification is so often the key to fruitful science, this concept of niche as a definable segment of resource space has become the basis for studies of diversity and its limits.

After developing this definition, Hutchinson noticed⁶ that closely related species inhabiting the same area often space themselves out quite regularly along individual resource axes. This led to what I regard as the most arresting and important concept in our attempt to understand the structure of diversity—limiting similarity. Is there a maximum similarity (or niche overlap) beyond which two related species cannot coexist in the same area? If such limiting similarity can be defined, then we will have a theory of maximum organic diversity. Ecologists have not yet developed a general theory of limiting similarity, though it is a field of active research today. In the absence of a general theory, we might probe empirically for any regularities in the similarity of related species living together in a single area and feeding at the same level of a food web. Again, Hutchinson was the pioneer. He noticed that such related species differ by a length ratio averaging 1.28—that is, the larger species is 1.28 longer than the smaller for critical dimensions related to feeding. This corresponds almost exactly to a doubling of weight—that is, the larger species weighs twice as much as the smaller. (Length varies as the cube root of weight, and the cube root of two—corresponding to a doubling of weight—is 1.26.)

Indeed, Hutchinson may have discovered something fundamental about the structure of similarity in general systems. In a recent article, Princeton ecologists Henry Horn and Robert May provide a compendium of further biological cases for the 1.26 ratio in biological systems.⁷ But they also note that length ratios for consorts or ensembles of similar musical instruments meant to be played together tend to obey the rule—the sequence of recorders, sopranino, descant, treble, tenor, bass, and great bass, runs 1.2, 1.5, 1.3, 1.6, and 1.4. The four stringed instruments of the modern orchestra are way off; they increase too rapidly in length (1.8 for cello to viola and 1.7 for bass fiddle to cello). But they were not designed to play as a group. Carleen Hutchins has built a new “violin family” of “acoustically matched stringed instruments” analogous to the old consorts and meant as an ensemble. The eight instruments have successive ratios of 1.2, 1.2, 1.3, 1.3, 1.3, and 1.3. Horn and May also measured a sequence of 1.3, 1.2, 1.3, and 1.2 in wheels of a tricycle-bicycle sequence and 1.2, 1.2, 1.2, and 1.3 for a single manufacturer’s set of iron skillets. Who knows what it all means; I do not think it is an accident. Horn and May conclude that Hutchinson’s rule may well derive from generalities about assembling sets of tools, rather than from any biological peculiarities.”

All this is only half of Hutchinson. It helps to explain why he is a great scientist, but it does not capture the special character that makes him a great man as well, an object of that rarest of all attitudes among scientists—reverence. For Hutchinson exults as much in the pure diversity of all knowledge as in the hunger to explain nature’s patterns. He is, to use an expressive English word virtually unknown in America, a polymath. He cherishes every detail of every subtle difference among objects and casts his web of knowledge over more disciplines than many colleges teach. His favorite themes, to put the matter biologically, are diversity and ornamentation—seeing the great and general in the small and the detailed. He wrote an entire book on ornament in Tibetan culture.⁸ For more than ten years he wrote a column for the American Scientist entitled “Marginalia,” an extended and delightful set of prose footnotes and general wanderings around the periphery of biology and human life. Now he works on literal marginalia—the illuminations of plants and animals that adorn so many medieval manuscripts.

The love of detail for its own sake pervades both books. The textbook on population ecology is adorned with historical footnotes and tangential wanderings on etymology and minutiae of natural history. Some pages even attain the scholarly ideal—all footnote with no text at all! They give to the genre that rarest of traits—a human face—and make this textbook a delight rather than a chore to read.

Hutchinson’s emphasis on unevaluated detail is not so successful in his autobiographical memoir—for it is the content here, rather than a commentary upon it. Hutchinson may view all bits of knowledge as equal in interest and impact. More ordinary mortals tend to be choosy. Even I, a New Yorker who rarely missed the San Gennaro festival, and who even mastered the nickel-through-water-into-cup game, flagged a bit after ten pages on whether and, if so, how and why the relic of the blood of St. Januarius liquefies during holy times in Naples. And much as I love the Onycophora, that curious group of potential intermediates between worms and insects, I wished the several pages on their detailed taxonomy were shorter and the paragraph on race relations in South Africa much longer. Still, such excursions are more than amply compensated by the insights liberally scattered throughout this memoir.

Many scientists undoubtedly view this discursive side of Hutchinson as marginal to his “real” accomplishments in constructing theories and explanations. Henry Ford’s equation of history and bunk ranks as good mathematics for many professionals holding the false view that science progresses by gathering objective information and manipulating it with a timeless device known as the scientific method. They might regard Hutchinson’s remarkable brand of scholarship as commendable but surely irrelevant to professional activity.

Such an attitude compromises science, as much as its disheartening Philistinism holds the ideals of learning in contempt. An experimentalist, using technological machinery under a strict and repeated protocol, might get by in ignorance of the history and implications of his field. Those who work directly with nature’s multifarious complexity cannot afford such narrowness. We are tied to historical habits of thought (doctrines of progress, gradual change, and linear causality, for example) and methods of procedure (reduction to component parts as a mode of explanation, rather than direct study of interaction). When we fail to recognize that these are habits of inertia rather than nature’s truths, new paths are closed off. Scientists ignorant of history are not so much condemned to repeat it, as to be confused and unenterprising. Hutchinson’s work has been richer for its combination of explanation and exultation.

In the coda to his textbook, Hutchinson tells of the solitaire, a bird extinguished by European settlers on the island of Rodriguez in the Indian Ocean. According to an accurate early eighteenth-century chronicler, solitaires mated for life and held a territory 200 yards or so in radius. Each pair incubated a single egg and reared its helpless offspring for several months after hatching. Hutchinson continues:

We now reach the extraordinary part of the story. Leguat [the chronicler] says that when the chick had left the nest for some days, a company of 30 or 40 adults brought another chick and, being joined by the parents, the two young birds were escorted to some unoccupied territory. This extraordinary ritual he describes as the marriage of the solitaire. Leguat seems so reliable on most other aspects of the bird’s biology, and the marriage, if it occurred, must have been so conspicuous, that it is hard to doubt its reality.

Shall we regret the solitaire’s passing because its unique behavior might have suggested new generalities in the currently “hot” field of adaptive strategies in mating behavior? Or shall we simply mourn the lost opportunity for watching something so fascinating and so different? We had best cherish exultation and explanation with equal tenacity, though I myself would trade several good generalities for the chance to witness such a spectacle.