MACARTHUR, ROBERT HELMER

(b. Toronto, Ontario, Canada, 7 April 1930; d. Princeton, New Jersey, 1 November 1972)

ecology.

“In November 1972 a brief but remarkable era in the development of ecology came to a tragic, premature close with the death of Robert MacArthur at the age of 42.” So wrote Martin Cody and Jared Diamond in the 1975 memorial volume Ecology and Evolution of Communities.

MacArthur will be remembered as one of the founders of evolutionary ecology. It is his distinction to have brought population and community ecology within the reach of genetics. By reformulating many of the parameters of ecology, biogeography, and genetic into a common framework of fundamental theory, MacArthur, more than any other person who worked during the decisive decade of the 1960’s, set the stage for the unification of population biology.

MacArthur was the youngest son of John Wood MacArthur, a professor of genetic at the University of Toronto and Marlboro College in Vermont. After completing his undergraduate education at the latter institution and the requirements for a master’s degree in mathematics at Brown University. Robert MacArthur took a Ph.D. in 1957 at Yale University under the direction of G. Evelyn Hutchinson. He spent the academic year 1957-1958 with David Lack at Oxford University in order to receive additional training in field ornithology. Hutchinson. Lack and an older brother of Robert’s, the physicist John W. MacArthur, Jr., were dominant influences in shaping MacArthur’s unique blend of mathematical and ecological interests. From 1958 to 1965 MacArthur advanced from assistant professor to full professor at the University of Pennsylvania: he then moved to Princeton University, where he ended his career as Henry Fairfield Osborn Professor of Biology. In 1952 he married Elizabeth Bayles Whittemore; they had four children (Duncan, Alan, Elizabeth, and Donald). He died of renal cancer.

MacArthur began his career with three articles that revealed an unusual power and originality of approach. The first (1955) was the proposal of a measure of community stability taken from information theory, formalizing for the first time a concept that previously had been the subject of simple verbal description. MacArthur employed entropy, the sum of the product of the frequencies of occurrence of species and the logarithms of the frequencies, a very general and intuitively satisfying way of condensing a large amount of complex data. Soon af terward (1957) came the celebrated “broken-stick” model of the relative abundance of bird species.

MacArthur proposed that the relative abundance of mutiple related species living in the same community (such as the songbirds of an island) can often be generally approximated by comparison with a stick broken into segments of randomly selected lengths, each segment representing the abundance of a particular species. The rationale was that when species compete, they divide the environment randomly and in a mutually exclusive fashion, like a stick divided into segments.

Although the specific hypothesis of competition embodied in the broken-stick distribution has been disputed, and the approach was later dismissed as obsolete, by MacArthur himself, we should not overlook the real significance of this contribution. In three pages MacArthur audaciously confronted a central problem of community ecology that had scarcely been put into words by previous writers. He characterized the issue in such a way as to suggest that the deepest remaining mysteries of natural history can be reached by leaps of the imagination, so long as such efforts are disciplined by the postulational-deductive method.

Reviewers sometimes forget that the broken-stick hypothesis was only one of three frequency distributions presented in the article, each derived from a different, competing set of biological hypotheses. The method of multiple working hypotheses was thereby introduced to this branch of ecological theory. The 1957 article set the tone for all of MacArthur’s later work. Inevitably, his approach was condemned by some ecologists as oversimplification, but right or wrong in particular applications, it energized a generation of young population biologists and transformed a large part of ecology.

MacArthur’s third early contribution was an elegant analysis of niche division in warbler (1958). For this somewhat more conventional study, he received the Mercer Award of the Ecological Society of America. In a sense the warbler study revealed the real secret of MacArthur’s success, his almost unique status as a mathematician-naturalist. He was a mathematician of professional grade, having been trained in the discipline before commencing the formal study of ecology. He had the convictions of a pure mathematician, which, according to G. H. Hardy (whom he resembled very much in temperament and philosophy), are simply “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.” MacArthur would say in conversation that the best science comes to a great extent from the creation of de novo and heuristic classification of natural phenom ena. “Art,” he enjoyed quoting Picasso. “is the lie that helps us to see the truth.”

But MacArthur was also a born naturalist. He watched birds with the patience and skill of a professional ornithologist, visited the tropics as often as he could, and delighted in the endless facts of natural history, which were temporarily exempted from his Cartesian scalpel. The store of random information thus accumulated and the shadowy play of its many patterns were the real inspiration of his theoretical work.

The 1960’s was a period of intense activity for MacArthur. While serving on the faculty of the University of Pennsylvania and then at Princeton, he began a parallel series of investigations, many in collaboration with colleagues and students, that touched on a wide range of topics around the central problem of species diversity. Part of his special genius was an ability to work closely with persons of widely varying talents and interests, turn them into fast friends, and bring out the best in their scientific labors. MacArthur and his coworkers analyzed the evolution of the demographic parameters, established the environmental correlates of bird diversity, and formulated and partly solved the species packing problem, in which the relation of niche breadth is correlated with the number of species crowding into the same area. One of his most influential works was The Theory of Island Biogeography (1967), with E. O. Wilson, which created species equilibrium theory. In this approach, consideration is given to the numbers of spcies belonging to a particular group of organisms (such as birds or ants) found on a give island or isolated patch of habitat. The numbers are seen as the result of the balance between the immigration rates of new species and the extinction rates of species already present. The rates in turn vary with the size of the islands and their distance from other islands.

As time passed. MacArthur spoke of himself increasingly as a biogeographer, and he made the subject the focus of his teaching at Princeton. In 1971, when he learned he had only a year or two left to live, he quickly brought the many threads of his work together in the book Geographical Ecology; Patterns in the Distribution of Species. The clarity and incisiveness of this synthesis show him at the height of his powers. Geographical Ecology is both the reflective memoir of a senior scientist and the prospectus of a young man whose creative effort ended at the point of its steepest trajectory.

BIBLIOGRAPHY

I. Original Works. A complete bibliography of MacArthur’s writings is provided by Martin L. Cody and Jared M. Diamond in the volume they edited Ecology and Evolution of Communities (Cambridge, Mass., 1975) 13-14. MacArthur presented many of his successful ideas, either originally or in review, in The Theory of Island Biogeography (Princeton, 1967), written with Edward O. Wilson; and in Geographical Ecology; Patterns in the Distribution of Species (New York, 1972; repr. Princeton, 1984)

II. Secondary Literture. An evaluation of much of MacArthur’s work is provide by the multiple authors of Ecology and Evolution of Communities (see above), and by many textbooks of the 1970’s and 1980’s, among which Mark H. Williamson, Island Population (Oxford and New York, 1981); and James H. Brown and Arthur C. Gibson, Biogeography (St. Louis, 1983), are perhaps the most thorough. A brief personal portrait of MacArthur, with a firsthand account of the development of the theory of island biogeography, is provided by Edward O. Wilson in Biophilia, (Cambridge, Mass., 1984), 68-71, 73-74.

Edward O. Wilson

MacArthur, Robert Helmer

Canadian-Born American Biologist 1930-1972

Robert Helmer MacArthur was born in Toronto, Canada. MacArthur was an important scientist in the field of ecology, the study of the relation between living creatures and their natural environment. When MacArthur started his studies in ecology, it was a merely descriptive science. Under his influence, ecology developed into a science based on quantitative, or measurable, data.

MacArthur moved to the United States at the age of seventeen to study at Marlboro College in Vermont, where his father was a professor of genetics . In 1951 he earned a bachelor's degree, in the field of mathematics, from Marlboro. Two years later, in 1953, he achieved his master's degree in mathematics from Brown University. While pursuing a Ph.D. at Yale, MacArthur switched from mathematics to zoology, with a concentration on ecology. After receiving his Ph.D. in 1957, MacArthur spent a year in England studying birds. In 1958 he was appointed as an assistant professor of biology at the University of Pennsylvania. In 1965 he became a professor of biology at Princeton. He held this position until his death from cancer at the age of forty-two.

MacArthur's first studies were on five similar species of birds called warblers that were living together in a spruce forest in New England. Some scientists believed that these birds might be an exception to the generally accepted competitive exclusion principle , which states that in stable environments, no two species occupy the same niche (the specialized role of an animal in its environment). However, MacArthur's studies showed that the birds occupied different parts of the trees, and thus did indeed follow the principle. This work earned for MacArthur the Mercer Award of the Ecological Society of America (1959).

MacArthur used his background in mathematics to focus on population biology. He studied how the population sizes of bird species varied with the size of their habitats . MacArthur and biologist Edward O. Wilson studied populations of birds living on islands. Their findings were published in 1967 in the book The Theory of Island Biogeography.

MacArthur and Wilson also developed the idea of life history strategies. They noted that some species have short lives characterized by very fast growth and high reproductive rates, then a sudden and drastic decline in numbers. An example of this type of species—called r -selected species — is the lemming. MacArthur and Wilson compared these animals to species that have slow growth and stable populations. An example of this type of species—called a k -selected species —is the elephant.

In 1971, when MacArthur learned that he had cancer and might live only a few more years, he decided to compile his many ideas into a single book. This book, Geographic Ecology: Patterns in the Distributions of Species, was published in 1972 shortly before his death.

see also Life History Strategies; Wilson, E. O.

Denise Prendergast

Bibliography

McMurray, Emily J., ed. Notable Twentieth-Century Scientists. Detroit: Gale Research Inc., 1995.

Millar, David, Ian Millar, John Millar, and Margaret Millar. Chambers Concise Dictionary of Scientists. Cambridge, U.K.: The Press Syndicate of the University of Cambridge, 1989.

Muir, Hazel, ed. Larousse Dictionary of Scientists. New York: Larousse, 1994.