o__ri Sl{f- 1 ~S ,CG Ecology and Evolution of Communities Martin L. Cody N ,..... and Jared M Diamond!' 00 o::t Editors 0) o::t 0 0 0 I!) I!),..... (") (") The Belknap Press of Harvard University Cambridge, Massachusetts, and London, Press England ARLIS Alaska Resources Library & Information Services Anchorage . .£\Jaska 17 Variations on a Theme G. Evelyn Hutchinson 17 A 1 by Robert MacArthur that ' same long ( same other. staten Scientists are perennially aware that it is on some aspects of polymorphism, the exclw best not to trust theory until it is confirmed very elementary theory imported from likewi by evidence. It is equally true, as Eddington population genetics has been most useful, occup pointed out, that it is best not to put too but it seems possible that something ecoloJ much faith in facts until they have been deeper may be waiting in the wings. In in a c confirmed by theory. This is why scientists the fourth section, two rather large and are reluctant to believe in ESP in spite of of Bi relatively unexplored themes are consid­ indisputable facts. This is also why group relate selection is in such dispute among evolu­ ered; they are characterized by their ubi­ gethe tionists. Only when a reasonable theory can quity in ecology, yet are exceptional in that concl account for these facts will scientists believe virtually no attention, theoretical or ob­ of la them. Ecology is now in the position where servational and experimental, has been unifc the facts are confirmed by theory and the paid to them. l'esp• theories at least roughly confirmable by The approach throughout is largely his­ paq facts. But both the facts and the theories torical, but some new biological results are clusi' have serious inadequacies providing stum­ introduced. centl bling blocks to present progress. (Mac­ infiu Arthur, 1972, p. 253-254.) "Unless it Doesn't" gave calle The present essay considers four areas The idea that two similar species are not allie' of ecology illustrating in different ways a likely to live together seems to have been imm little of what Robert MacArthur was writ­ held by several nineteenth- and early teen ing about in the last work that he com­ twentieth-century naturalists. As Hardin olog pleted as the sole author. In the first sec­ (1960) has pointed out, the idea is implicit clusi tion of this chapter some aspects of in Darwin's writings, though never explic­ sort competitive exclusion are considered his­ itly stated. Dr. Martin Cody has kindly den1 torically, because here we have a para­ called my attention to an expression of the to e digmatic case of theoretical and empirical idea in a paper by Hansmann (1857) on It studies progressing dialectically in a thor­ the warblers of Sardinia, where it was in ~ oughly satisfactory manner. In the second noticed by· Dr. Hartmut Walter. Steere's section, on cyclical changes in popula­ (1894) empirical conclusion, that among 1 Altl tions, the relation between the empirical the land birds of the Philippine Islands dane and the theoretical has been less happy, congeneric species did not occur together, the< . suggests the same sort of conclusion. signi and has probably hindered the develop­ case ment of the subject. In the third section, Grinnell (1904; cf. Udvardy, 1959) wrote as n 492 17 A Theme by Robert MacArthur 493 that "two species of approximately the - terra (1926) gave a clear mathematical same food habits are not likely to remain demonstration that under certain reason­ long evenly balanced in numbers in the ably natural conditions competitive exclu­ same region. One will crowd out the sion would be expected, and Gause (1934, other." This is an adequate qualitative 1935) showed how in the laboratory actual statement of the principle of competitive systems in which competing organisms did the exclusion. Ortmann (1906) concluded exclude one another could be constructed, om likewise that two allied species do not that people began to see not merely that ful, occupy the same range under identical such a generalization might be true but ing ecological conditions. Hiltzlieimer (1909), that, more importantly, it might be inter­ In in a discussion of fossil European species esting. md of Bison, used the idea that two closely Schrodinger,2 I believe, said somewhere >id­ related species are not likely to live to­ that Newton's First Law of Motion may tbi­ gether. Monard (1920; cf. Macan, 1963) be recast as: "A body perseveres at rest hat concluded from his study of the benthos or uniform motion in a right line unless ob- of lake Neuchatel that "dans un milieu it doesn't." The last three qualifying words een uniform, restreint dans le temps et conceal the idea of force, which becomes l'espace, ne tend a subsister q'une espece explicit in the second law. Naturalists in his­ par genre," which is the same as the con­ the first decade or so of the twentieth are clusion reached by Steere, a quarter of a century doubtless believed that one of two century earlier. Cabrera (1932), evidently species of the same general biological influenced by Grinnell and by Ortmann, characteristics, brought into a common gave an excellent account of what he habitat, would exclude the other unless it called the ecological incompatibility of didn't. Except to people who had been led not allied coexisting species. As more of the by their experience with birds, bison, or 1een immense body ofwritings of the late nine­ benthos, to believe in something like com­ arly teenth- and early twentieth-century bi­ petitive exclusion, if perhaps in a rather rdin ologists is examined with competitive ex­ naive way, the existence of counter­ ~licit clusion in mind, more examples of this examples underlying the qualifying "un­ Jlic­ sort are likely to be discovered. It is evi­ less it didn't" would make the generaliza­ ldly dent, however, that such concepts failed tion both tautological and insignificant. 'the to elicit much deep interest. Only when there is a strong theoretical ) on It was only when Haldane (1924)1 and, reason for the idea's being correct and a was in an even more important paper, Vol- strong empirical reason for thinking that !re's .ong 1Although I was present in the audience when Hal­ 2 Neither Professor Martin Kline, Professor Saunders mds dane gave the verbal presentation of this paper to MacLane, nor I can find the source of the quotation, .her, the Cambridge Philosophical Society, the ecological which may belong to folklore. Professor Gerald ;ion. significance of the result, obtained as an unimportant Holton and some of his colleagues suggest it is best, case in his genetic argument, struck neither me nor, if somewhat frivolously, attributed to Newton him­ TOte as far as I know, anyone else present. self. G. Evelyn Hutchinson 494 in some cases it is incorrect, does the rameter, keeps the populations of prey problem suddenly become important. species at a low level, in such a way that At the famous symposium of the British what would be the dominant and ulti­ Ecological Society on The Ecology of mately the only species in the community Closely Allied Species held on 21 March without the predator suffers a propor­ 1944 "a distinct cleavage of opinion re­ tionally greater reduction by predation vealed itself on the validity of Gause's than do the other prey species. The most concept" (Harvey, 1945). The Oxford dramatic recently studied cases concern group-Elton, Lack, and Varley-were marine benthic communities (Paine, 1966; obviously stating something of importance Paine and Vadas, 1969; Porter, 1972) or in their implied insistence that what pasture plants grazed by ungulates Gause did in the laboratory had great (Harper, 1969). This type of exception is significance for what they were studying discussed in detail by Connell in Chapter in nature. That extraordinary naturalist 16, and a new theoretical basis for it is Captain Cyril Diver, then Clerk to the provided by Levins through loop analysis Select Committee on National Expendi­ in Chapter I (Figure 6b). ture of the House of Commons, "who gave There may be a few other types of case examples of many congeneric species of that can be subsumed under the classical both plants and animals apparently living Volterra theory by choosing the correct, if and feeding together," was also right in rather unconventional, niche parameters. emphasizing his store of apparent coun­ This may be the case locally with two ter-examples. MacArthur's (1958) war­ species of towhee (Marshall, 1960; also blers, which have played so great a part later discussions by Hutchinson, 1965, and in the later development of the cluster of Cody, 1974). t concepts that we are considering, at the 2. The various situations involving vag­ I time would have seemed to support Diver, ile or fugitive species and more or less s though now we know that they conform rapid or even random environmental ~ to the principle of competitive exclusion changes, causing continuous alterations of in ways that would hardly have appeared the direction of competition between rap­ r possible in 1944. idly reproducing species, provide a second I The work of the past decade suggests type of apparent exception to competitive f that the known exceptions, apparent or exclusion. Such situations in stream com­ c real, to the principle of competitive exclu­ munities are discussed elsewhere in this a sion, may be of four kinds. volume by Patrick (Chapter 15). In these v I. Some exceptions are only apparent cases there is ideally no persistent uni­ 1: and can be rationalized in terms of the directional movement towards a particu­ e theory if the appropriate niche dimensions lar equilibrium position, owing to con­ t are considered.