Robert Helmer MacArthur (1930 – 1972)
Canadian biologist and ecologist

Few scientists have combined the skills of mathematics and biology to open new fields of knowledge the way Robert H. MacArthur did in his pioneering work in evolutionary ecology . Guided by a wide-ranging curiosity for all things natural, MacArthur had a special interest in birds and much of his work dealt primarily with bird populations. His conclusions, however, were not specific to ornithology but transformed both population biology and biogeography in general.

Robert Helmer MacArthur was born in Toronto, Ontario, Canada, on April 7, 1930, the youngest son of John Wood and Olive (Turner) MacArthur. While Robert spent his first seventeen years attending public schools in Toronto, his father shuttled between the University of Toronto and Marlboro College in Marlboro, Vermont, as a professor of genetics. Robert MacArthur graduated from high school in 1947 and immediately immigrated to the United States to attend Marlboro College. He received his undergraduate degree from Marlboro in 1951 and a master's degree in mathematics from Brown University in 1953. Upon receiving his doctorate in 1957 from Yale University under the direction of G. Evelyn Hutchinson, MacArthur headed for England to spend the following year studying ornithology with David Lack at Oxford University. When he returned to the United States in 1958, he was appointed Assistant Professor of Biology at the University of Pennsylvania.

As a doctoral student at Yale, MacArthur had already proposed an ecological theory that encompassed both his background as a mathematician and his growing knowledge as a naturalist. While at Pennsylvania, MacArthur developed a new approach to the frequency distribution of species . One of the problems confronting ecologists is measuring the numbers of a specific species within a geographic area—one cannot just assume that three crows in a 10-acre corn field means that in a 1000-acre field there will be 300 crows. Much depends on the number of species occupying a habitat , species competition within the habitat, food supply, and other factors. MacArthur developed several ideas relating to the measurement of species within a known habitat, showing how large masses of empirical data relating to numbers of species could be processed in a single model by employing the principles of information theory. By taking the sum of the product of the frequencies of occurrences of a species and the logarithms of the frequencies, complex data could be addressed more easily.

The most well-known theory of frequency distribution MacArthur proposed in the late 1950s is the so-called broken stick model. This model had been suggested by MacArthur as one of three competing models of frequency distribution. He proposed that competing species divide up available habitat in a random fashion and without overlap, like the segments of a broken stick. In the 1960s, MacArthur noted that the theory was obsolete. The procedure of using competing explanations and theories simultaneously and comparing results, rather than relying on a single hypothesis, was also characteristic of MacArthur's later work.

In 1958, MacArthur initiated a detailed study of warblers in which he analyzed their niche division, or the way in which the different species will to be best suited for a narrow ecological role in their common habitat. His work in this field earned him the Mercer Award of the Ecological Society of America . In the 1960s, he studied the so-called "species-packing problem." Different kinds of habitat support widely different numbers of species. A tropical rain forest habitat, for instance, supports a great many species, while arctic tundra supports relatively few. MacArthur proposed that the number of species crowding a given habitat correlates to niche breadth. The book The Theory of Island Biogeography, written with biodiversity expert Edward O. Wilson and published in 1967, applied these and other ideas to isolated habitats such as islands. The authors explained the species-packing problem in an evolutionary light, as an equilibrium between the rates at which new species arrive or develop and the extinction rates of species already present. These rates vary with the size of the habitat and its distance from other habitats.

In 1965 MacArthur left the University of Pennsylvania to accept a position at Princeton University. Three years later, he was named Henry Fairfield Osborn Professor of Biology, a chair he held until his death. In 1971, MacArthur discovered that he suffered from a fatal disease and had only a few years to live. He decided to concentrate his efforts on encapsulating his many ideas in a single work. The result, Geographic Ecology: Patterns in the Distribution of Species, was published shortly before his death the following year. Besides a summation of work already done, Geographic Ecology was a prospectus of work still to be carried out in the field.

MacArthur was a Fellow of the American Academy of Arts and Science. He was also an Associate of the Smithsonian Tropical Research Institute, and a member of both the Ecological Society and the National Academy of Science. He married Elizabeth Bayles Whittemore in 1952; they had four children: Duncan, Alan, Donald, and Elizabeth. Robert MacArthur died of renal cancer in Princeton, New Jersey, on November 1, 1972, at the age of 42.

RESOURCES

BOOKS

Carey, C. W. "MacArthur, Robert Helmer." Vol. 14, American National Biography edited by J. A. Garraty and M. C. Carnes. NY: Oxford University Press, 1999.

Gillispie, Charles Coulson, ed. Dictionary of Scientific Biography. Vol. 17–18: Scribner, 1990.

MacArthur, Robert. Geographic Ecology: Patterns in the Distribution of Species. Harper, 1972.

——. The Biology of Populations. Wiley, 1966.

——. The Theory of Island Biogeography. Princeton University Press, 1967. Notable Scientists: From 1900 to the Present. Farmington Hills, MI: Gale Group, 2002.