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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.