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ROBERT HELMER MACARTHUR April 7, 1930-November 1, 1972 BY EDWARD O. WILSON AND EVELYN G. HUTCHINSON IN NOVEMBER ~ g7 ~ 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," wrote Martin Cody anct Jarec! 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 pop- ulation and community ecology within the reach of genetics. By reformulating many of the parameters of ecology, bio- geography, anti genetics into a common framework of fun- damental theory, MacArthur more than any other person who worked cluring the decisive decade of the 1960s set the stage for the unification of population biology. MacArthur was the youngest son of John Wood Mac- Arthur, a professor of genetics at the University of Toronto and Marlboro College in Vermont. After completing his unclergraduate education at the latter institution and taking a master's clegree in mathematics at Brown University, Robert MacArthur took a Ph.D. in 1957 at Yale University, under the direction of G. Evelyn Hutchinson. In order to receive ad- clitional training in field ornithology, he spent the academic year ~ 957-l 958 with David Lack at Oxford University. Hutchinson, Lack, and an older brother, the physicist John 319

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320 BIOGRAPHICAL MEMOIRS W. MacArthur, Jr., were dominant influences in shaping MacArthur's unique blend of mathematical and ecological in- terests. From ~ 958 to ~ 965, MacArthur advanced from as- sistant professor to full professor at the University of Penn- sylvania. He then moved to Princeton University, where he ended! his career as Henry Fairfielc! Osborn Professor of Biol- ogy. In 1952 he marries! Elizabeth Bayles Whittemore, with whom he had four children (Duncan, Alan, Elizabeth, and Donald). MacArthur began his career with three articles that re- vealec! an unusual power and originality of approach. The first (1955) was the proposal of a way to measure community stability taken from information theory, formalizing for the first time a concept that, until then, could only be expressed through verbal description. Soon afterward (1957) came the celebrated "broken-stick" mocle} of the relative abundance of bird species. Although the specific hypothesis of competition embodied in the bro- ken-stick distribution has been ctisputed and the approach was later dismissed as obsolete by MacArthur himself, we should not overlook the real significance of this contribution, which dicl indeed appear to describe what happens in nature in some as yet imperfectly studied circumstances. In three short pages, MacArthur audaciously confronted a central problem of community ecology that previous writers had scarcely formulated in words. 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 imagina- tion so long as such efforts are ctisciplined by the postula- tional-cleductive method. Reviewers sometimes forget that the broken-stick hypoth- esis was only one of three frequency distributions presented in the article, each derived from a clifferent, competing set of biological hypotheses. The method of multiple working

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ROBERT HELMER MACARTHUR 321 hypotheses was thereby introduced to this branch of ecolog- ical theory. The 1957 article set the tone for all of Mac- Arthur's later work. Inevitably, his approach was condemned by some ecologists as oversimplification, butright 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 anal- ysis of niche division in warblers (19581. For this somewhat more conventional study, he received the Mercer Award of the Ecological Society of America. The warbler stucly re- vealecl the real secret of MacArthur's success, his almost unique status as a mathematician-naturalist. He was a math- ematician of professional gra(le, having been trained in the discipline before commencing the formal study of ecology. He shared the conviction of pure mathematician G. H. Harcly, whom he resemblec! very much in temperament anct philosophy, "that a mathematician was a maker of patterns of ideas, and that beauty ant! seriousness were the criteria by which his patterns shouIcl be judged." In conversation, MacArthur would say that the best sci- ence comes, to a great extent, from the creation of de novo ant! heuristic classification of natural phenomena. "Art," he enjoyed quoting Picasso, "is the lie that helps us to see the truth." But MacArthur was also a born naturalist. He watcher! . birds with the patience and skill of a professional ornitholo- gist, visited the tropics as often as he could, and delightecl in the encIless 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 decade of the 1960s was a period of intense activity for Robert MacArthur. While serving on the faculty of the

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322 BIOGRAPHICAL MEMOIRS University of Pennsylvania and then at Princeton, he began a parallel series of investigations, many in collaboration with colleagues anct students, that touched on a wicle range of topics around the central problem of species cliversity. Part of his special genius was an ability to work closely with per- sons of widely varying talents and interests, to turn them into fast friends, anct to bring out the best in their scientific labors. One of them, E. O. Wilson, who coauthored this memoir, recorder! the following impression of him: He was medium tall and thin, with a handsomely angular face. He met you with a level gaze supported by an ironic 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 face slightly upward and swallowing. He had a calm understated manner, which in intellectuals suggests tightly reined power. Because very few intellectuals can keep their mouths shut long enough to be sure about anything, MacArthur's restraint gave his conversation an edge of finality he did not intend. In fact he was basically shy and reticent. He was not a mathematician of the first class- very few scientists are, otherwise they would become pure mathemati- cians but he joined superior talent in that field with an extraordinary creative drive, decent ambition, and a love of the natural world, birds, and science, in that order. MacArthur and his coworkers analyzed the evolution of the demographic parameters, established the environmental correlates of bird (diversity, and formulated and partly solvecl the species packing problem. One of his most influential works, The Theory of Island Biogeography (1967), written with Wilson, create(1 species equilibrium theory. This theory ex- plored the many ramifications of a balance of species number on islands and on "habitat islands," any sharply demarcated habitatsuch as a lake, or even, for insects at least, a tree in ~ E. O. Wilson, Biophilia (Cambridge, Massachusetts: Harvard University Press, 1984), p. 68. A further account of MacArthur's collaborative work and its impact on ecology has been given by Sharon E. Kingsland in Modeling Nature: Episodes in the History of Population Ecology (Chicago: University of Chicago Press, 1985).

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ROBERT HELMER MACARTHUR 323 the micicIle of a field. During colonization, the extinction rate in species/unit time rises as the number of species on the island rises ant! the immigration rate falls. When the two converge, a dynamic equilibrium is attained in which the con- tinuing turnover varies according to the speed with which the colonization took place. The models predict an increase in species numbers with larger island! area and greater prox- imity to the mainIanct. Other investigators have addect many refinements to this basic theory. Experimental tests have also been performed on isolated habitats, from bottles of nutri- ents to full-scale islancts and islanct habitats in the Florida Keys ant} the Brazilian Amazon. The current theory of islancl biogeography, while still very inadequate for the largest ant! most complex systems, has worked well enough to become an important part of both ecology ant] biogeography. It is also a cornerstone of the new field! of conservation biology because of its relevance to the study of the extinction process and the planning of natural reserves. As time passed MacArthur spoke of himself increasingly as a biogeographer ant! made the subject the focus of his teaching at Princeton. In 1971, when he learner! he hac! only a year or two left to live, he quickly brought the many threads of his work together in the single book, Geographical Ecology: Patterns in the Distribution of Species. The clarity and incisive- ness of this synthesis show him at the height of his power. 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.

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324 BIOGRAPHICAL MEMOIRS SELECTED BIBLIOGRAPHY 1955 Fluctuations of animal populations, and a measure of community stability. Ecology, 36:533-36. 1957 On the relative abundance of bird species. Proc. Natl. Acad. Sci. USA, 43:293-95. 1958 With P. Klopfer. North American birds staying on board ship dur- ing Atlantic crossing. Br. Birds, 51 :358. A note on stationary age distributions in single species populations and stationary species populations in a community. Ecology, 39: 146-47. Population ecology of some warblers of northeastern coniferous forests. Ecology, 39:599-619. 1959 With G. E. Hutchinson. A theoretical ecological model of size dis- tributions among species of animals. Am. Nat., 93:117-25. With G. E. Hutchinson.- On the theoretical significance of aggres- sive neglect in interspecific competition. Am. Nat., 93:133-34. On the breeding distribution pattern of North American migrant birds. Auk, 76:318-25. 1960 On the relative abundance of species. Am. Nat., 94:25-36. With P. Klopfer. Niche size and faunal diversity. Am. Nat.,94:293- 300. On Dr. Birch's article on population ecology. Am. Nat., 94:313. On the relation between reproductive value and optimal predation. Proc. Natl. Acad. Sci. USA, 46:144-45. Population studies: Animal ecology and demography. Q. Rev. Biol., 35:82-83. Review of Cold Spring Harbor Symposium, vol. 22.

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ROBERT HELMER MACARTHUR 325 1961 Population effects of natural selection. Am. Nat., 95: 195-99. With P. Klopfer. On the causes of tropical species diversity: Niche overlap. Am. Nat., 95:223-26. With J. W. MacArthur. On bird species diversity. Ecology, 42:594- 98. 1962 With I. W. MacArthur and }. Freer. On bird species diversity. II. Prediction of bird censuses from habitat measurements. Am. Nat., 96:167-74. Some generalized theorems of natural selection. Proc. Natl. Acad. Sci. USA, 48: 1893 -97. 1963 With M. L. Rosenzweig. Graphical representation and stability con- ditions of predator-prey interactions. Am. Nat., 97:209-23. With E. O. Wilson. An equilibrium theory of insular zoogeography. Evolution, 17:373 -87. 1964 Environmental factors affecting bird species diversity. Am. Nat., 98:387-97. With D. Garfinkel and R. Sack. Computer simulation and analysis of simple ecological systems. Ann. N. Y. Acad. Sci., 115:943- 51. With V. G. Dethier. A field's capacity to support a butterfly popu- lation. Nature, 201:728-29. With R. Levins. Competition, habitat selection, and character dis- placement in a patchy environment. Proc. Natl. Acad. Sci. USA, 51: 1207-10. Ecology. In: New Dictionary of Birds, ed. A. L. Thompson, pp. 230- 33. New York: McGraw-Hill. 1965 Patterns of species diversity. Biol. Rev., 40:510-33. Ecological consequences of natural selection. In: Theoretical and

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326 BIOGRAPHICAL MEMOIRS Mathematical Biology, ed. T. H. Waterman and H. Morowitz, pp. 388-97. New York: Blaisdell. 1966 With H. Recher and M. Cody. On the relation between habitat selection and species diversity. Am. Nat., 100:319-32. With R. Levins. The maintenance of genetic polymorphism in a spatially heterogeneous environment: Variations on a theme by Howard Levine. Am. Nat., 100:585-89. With E. R. Pianka. On optimal use of a patchy environment. Am. Nat., 100:603-9. A review of The Pattern of Animal Communities by C. S. Elton. Am. Sci., 54:497A. With }. Vandermeer. A reformulation of alternative b of the broken stick model of species abundance. Ecology, 47: 139-40. Note on Mrs. Pielou's comments. Ecology, 47:1074. With I. H. Connell. The Biology of Populations. New York: John Wiley & Sons, Inc., Publishers. 1967 With R. Levins. The limiting similarity, convergence, and diver- gence of coexisting species. Am. Nat., 101:37~-85. With E. O. Wilson. The Theory of Island Biogeography. Princeton: Princeton University Press. 1968 Selection for life tables in periodic environments. Am. Nat., 102:381-83. The theory of the niche. In: Population Biology and Evolution, ed. R. C. Lewontin, pp. 159-76. Syracuse: Syracuse University Press. 1969 Patterns of communities in the tropics. Biol. J. Linn. Soc., 1:19- 30. The ecologist's telescope. Ecology, 50:353. With H. S. Horn. Foliage profile by vertical measurements. Ecol- ogy, 50:802-4.

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ROBERT HELMER MACARTHUR 327 With R. Levins. An hypothesis to explain the incidence of mono- phagy. Ecology, 50:910-11. Species packing and what competition minimizes. Proc. Natl. Acad. Sci. USA, 64: 1369-71. 1970 Graphical analysis of ecological systems. In: Some Mathematical Questions in Biology, ed. J. D. Cowan, pp. 61-73. Providence, R.I.: American Mathematical Society. Species packing and competitive equilibrium for many species. Theor. Popul. Biol., 1: 1 - 11. 1971 Patterns of terrestrial bird communities. In: Avian Biology, ed. D. S. Earner and J. R. King, vol. 1, pp. 189-221. New York: Aca- demic Press. 1972 With J. M. Diamond and I. R. Karr. Density compensation in island faunas. Ecology, 53:330-42. With H. S. Horn. Competition among fugitive species in a Harle- quin environment. Ecology, 53:749-52. With D. MacArthur. Efficiency and preference at a bird feeder. I. Ariz. Acad. Sci., 7:3-5. With R. M. May. Niche overlap as a function of environmental var- iability. Proc. Natl. Acad. Sci. USA, 69:1109-13. Strong, or weak, interactions. Trans. Conn. Acad. Arts Sci., 44: 177-88. Coexistence of species. In: Challenging Biological Problems, ed. I. Behnke, pp. 253-59. New York: Oxford University Press. Geographical Ecology. New York: Harper & Row. 1973 With l. MacArthur, D. MacArthur, and A. MacArthur. The effect of island area on population densities. Ecology, 54:657-58. 1974 With A. T. MacArthur. On the use of mist nets for population stud- ies of birds. Proc. Natl. Acad. Sci. USA, 71:3230-33.