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AMER. ZOOL., 21:889-901 (1981) Interspecific Competition and Species' Distributions: The Ghosts of Theories and Data Past1 JEREMY B. C. JACKSON Department of Earth and Planetary Sciences, The Johns Hopkins University, Baltimore, Maryland 21218 SYNOPSIS. The idea that biological interactions between species restrict their distributions beyond limits set by the inorganic environment was established by plant ecologists well before the work of Volterra, Gause, and the development of modern niche theory. Mechanisms of competition between plant species, and the influence of predation and environmental factors on the outcome of interspecific competition, were demonstrated by extensive field experiments. These early botanical achievements were ignored by zoologists whose subsequent, independently derived ideas about competition paralleled those of plant ecologists to a remarkable degree. Application of niche theory to the real world was closely tied to the advent of the "modern synthesis." The primary innovation of niche theory was not simply a new appreciation of the possible importance of competition between animals, but the incorporation of an evolutionary perspective to distributional problems largely absent among ecologists since Darwin. INTRODUCTION The role of competition between species in affecting their distributions is today hotly debated in many quarters (e.g., Connell, 1975, 1978, 1979, 1980). Regardless of position, these discussions almost invariably focus on the Volterra-Gause niche theory of competition and the principle of competitive exclusion, as if the importance of interspecific competition rises or falls on the strengths or weaknesses of these points of view (e.g., Hutchinson, 1975, 1978; Strong, 1979; Heck, 1980). In 1978, for example, Jared Diamond published a paper titled "Niche shifts and the rediscovery of interspecific competition, why did field biologists so long overlook the widespread evidence for interspecific competition that had already impressed Darwin?" According to Diamond, ecologists before 1959 missed the boat because, among other reasons: 1. They did not know what to look for. 2. They did not understand the niche concept. 3. They were misguided reductionists searching solely for physical explanations. 4. They did not have a theoretical framework for the information they did not know how to look for. In this paper I will demonstrate that Diamond's story simply is not true. Rather, ecologists long before 1959 thought a great deal about competition, they did know what to look for, they were not all "reductionists," they did not need a mathematical formulation of the niche to have the same idea, and they had a well developed theoretical framework, namely succession. The fact that their theory was wrong by current views is immaterial; it stimulated a great deal of important work. At this point it is probably worth saying that I was reared on Hutchinsonian niche theory and, unlike many ecologists today, still find it enormously stimulating. My argument here is simply against a misguided sense of the originality of ideas characteristic of the revisionist history written by successful, creative scientists. I will proceed as follows: 1. Outline what I take to be the generally accepted genealogy of niche theory pre-1960. 2. Summarize what was actually being published in the principal English-language ecological journals of the time. 1 From the Symposium on Theoretical Ecology preIdeas about fundamental versus resented at the Annual Meeting of the American Soalized niches, and the roles of compeciety of Zoologists, 27-30 December 1980, at Seattle, tition and other processes in shaping Washington. 889 890 JEREMY B. C. JACKSON Population Genetics ^ Mathematical Pooutation I92O'S Ecology (PEARL, VOLTERRA, LOTKA) (FISHER, WRIGHT, HALOANE) i 1 "Modern Synthesis" (HUXLEY, MAYR. 006ZHANSKY) I930'S-(940'S Laboratory Experimental (GAUSE, PARK, CROM8IE) Distributions of Natural Populations (HUTCHINSON and LACK) I940'S-I950'S 1 1 1960's Modern Niche Theory FIG. 1. Partial genealogy of modern niche theory pre-1960. Details of right side of figure in Hutchinson (1975, 1978). species distributions, have a considerable pedigree. 3. Point out what was apparently new with modern niche theory, namely an evolutionary perspective regarding distributional problems. Lack's first great contributions on this topic are his 1944 paper "Ecological aspects of species formation in passerine birds" and its subsequent expanded version, his 1947 book Darwin's finches. In these works Lack focused on the role of competition in shaping distributions of closely related bird species and developed his now famous axiom that, because of competition, such species are either allopatric, or if sympatric, live in different microenvironments and/or eat different foods. His debt to the modern synthesis is clear. Mayr and Huxley seem to have influenced him more than Gause, and he says of Huxley (1942) that he ". . . is apparently the only previous worker to suggest that these differences in size are primarily correlated with differences in food." (Lack, 1944, p. 279) Lack (1944, p. 284) acknowledged Huxley for his "extensive criticism of both the facts According to Diamond (1978), Hutch- and views here expressed, which were ininson (1975, 1978), and others, the path to valuable in completing the final draft." Beunderstanding of the ecological impor- fore 1944 Lack had seemingly no interest tance of interspecific competition looks in competition and believed that bird something like the right hand side of Fig- species distributions were explainable by ure 1. This story, that mathematical habitat selection (Lack, 1933). How such models stimulated laboratory experiments habitat selection might have evolved apwhich in turn stimulated understanding of parently did not interest him until he the real world, is the way interspecific com- abandoned the hypothesis as evolutionarpetition is commonly introduced in basic ily unsound in 1940. texts (e.g., Krebs, 1972). The left hand side Hutchinson's (1941, 1944) seminal paof Figure 1 is usually ignored, yet, as we pers dealt with the role of competition in shall see, the "modern synthesis" was es- plankton succession in lakes. In these pasential to what Lack and Hutchinson had pers he cited Volterra and Gause but igto say. I will not discuss the history before nored the botanical literature despite the Hutchinson and Lack as it is widely known obsession of plant ecologists with succes(see especially Hutchinson, 1975, 1978). sion at that time. In his famous 1951 paper One point however, deserves special men- "Copepodology for the ornithologist" tion here. Gause himself did only labora- Hutchinson cited Lack and Huxley. There tory experiments, but in his influential and in his later paper on the niche (1958) 1934 book The strugglefor existence, he care- and "Homage to Santa Rosalia" (1959) fully reviewed much of what was already there is no mention of terrestrial plant known of competition in the real world. ecology. The same is true of all Lack's This chapter is largely one of plant-envy, works on interspecific competition. for Gause knew what most ecologists have Through these papers of Hutchinson since forgotten, namely that a great deal and Lack, niche theory became firmly eswas known of competition among plants. tablished among ecologists. Consequently Hutchinson and Lack were the first to views of competition became strongly successfully apply ideas of competitive ex- biased in at least two ways. First, competiclusion to the real world in a big way. tion between species became equated with GENEALOGY OF MODERN NICHE THEORY 891 INTERSPECIFIC COMPETITION THE PUBLISHED RECORD OF COMPETITION STUDIES BEFORE 1959 i/> 4 o ER 100 the niche theory of Volterra and Gause. Second, all questions about competition that came to be considered interesting were denned for the equilibrium case. a. 0) r 128- 4- .\ v\ P/v • \ \_ _ *, A * # ••* Let us now examine the published re- X cord of people who called themselves ecol- 2 ogists who were writing during the time of 1915 1925 1936 1945 1955 the scenario just described. Data are from YEAR the three original English-language ecoFie. 2. Number of published papers mentioning inlogical journals, Journal of Ecology (JE- terspecific competition as a factor influencing organCOL), Journal of Animal Ecology (JAE) and ism distributions per 1,000 journal pages from 1920Ecology (ECOL), plus The American Natu- 1959. Data points for five-year intervals (e.g., 1920— ralist (AMNAT). The period covered is the 1924, 1925-1929, etc.). P = JECOL (Journal of Ecolforty years previous to and including publi- ogy), A = JAE (Journal of Animal Ecology), and E = cation of Hutchinson's "Homage to Santa ECOL (Ecology). Rosalia" (except for JAE which began publication in 1932). I read at least the summary and scanned every article for any ref- These are, in order of numbers of papers: erence to interspecific competition as a 1. Studies of distributions for their own distributional factor. Indices to the joursake: 36% of 332 papers. These include nals are unfortunately incomplete and/or Eltonian animal ecology and much unreliable. For example, Hutchinson's plant ecology not directly concerned (1951) "Copepodology for the ornitholowith succession theory. There is no gist" and MacArthur's (1958) "Population temporal trend in the frequency of this ecology of some warblers of northeastern group of papers in JAE (Fig. 3A); there coniferous forests" are not listed under is an increase in JECOL and perhaps in competition in the Ecology index. ECOL with waning interest in plant succession. There are 336 papers that deal in some way with interspecific competition in 2. Applied research: 31% of 332 papers. These include, for example, papers by 71,610 pages of the four journals from plant ecologists trying to save the 1920-1959. Of these, only fourteen are American prairie and tropical biologists from AMNAT, many of whose authors concerned with parasites and disease. were busy inventing population genetics There is a general peak (Fig. 3B) in this and the "modern synthesis." Excluding group of papers in the interval 1940AMNAT, there are 322 papers in 50,436 1945, apparently reflecting previous journal pages or 6.4 articles on interspenatural disasters rather than the ongocific competition for every thousand pages ing war. in the three major ecological journals of the time. That is quite a lot! So, right away, Neither of these first two groups of pawe can dismiss Diamond's (1978) claim that ecologists were not studying interspe- pers could be said to be directly motivated cific competition before 1959. There is not by theory at all. even any reasonable temporal trend in the 3. Successionists, in the sense of Clements number of papers (Fig. 2). and Tansley: 30% of 332 papers. Authors of papers in this group also wrote What were these authors thinking about many papers in the first two groups. Bewhile studying interspecific competition so lief in succession necessitated belief in enthusiastically? The papers fall readily competition as the driving force of sucinto four slightly overlapping groups (so cession, and many were interested in they sum to slightly more than 100%). HI JEREMY B. C. JACKSON B 1955 IOO-I 1955 P lOO-i \ 80- 80- \ A \ CESSI P 60- \ [f v E _ 40; _ 20— ¥ z \ N 40- \ \ \ \ • i 20- \ \ 1925 N/ \ \ \ • 1915 1N / \ \ I / E . y^ A —f 1935 1945 1955 YEAR 196 1925 1935 1945 1955 YEAR Fie. 3. Intellectual framework of papers mentioning interspecific competition published from 1920-1959. A, studies of distributions for their own sake; B, applied research; C, studies invoking succession theory; D, studies invoking niche theory. A-C for P = JECOL, A = JAE, and E = ECOL; D as for above plus N = AMNAT (American Naturalist). er and editor of the journal since its inception in 1914. A. S. Watt and E. J. Salisbury were also influential contributors. The quality of papers in this period was consistently excellent by modern standards. There followed a lapse in the 1930s of primarily descriptive vegetation studies, apparently motivated by a rather uncritical but abiding faith in community succession. After this, there emerged an important series of papers by A. S. Watt and others on pattern and process in plant communities. When ECOL first appeared in 1920 it The primary theme in JECOL until the early 1930s was succession, and the domi- was, like JECOL, primarily a botanical nant figure was Sir Arthur Tansley, found- journal. Succession was again the major observing or measuring this force. There is a sharp decline in the frequency of such papers (Fig. 3C) with declining interest in plant succession itself. 4. Volterra-Gause niche theory: 13% of 332 papers. This, by far the smallest group, includes authors in the tradition of the right side of Figure 1. There are slow increases in the frequencies of papers in this group in ECOL and JAE, no papers at all in JECOL, and a striking increase in AMNAT (Fig. 4D). INTERSPECIFIC COMPETITION theme, and F. E. Clements the dominant figure, though he published few papers in the journal. In the 1930s management of the prairie became a major topic along with succession. Later still there was no unifying theme or dominant figure evident. JAE was Charles Elton's journal. He founded it to get out from under the thumb of the plant ecologists. The early theme of the journal was causes of variations in animal numbers through time. This came straight from his 1927 book Animal ecology, the introduction to chapter 9 of which reads: "The numbers of animals never remain constant for very long and usually fluctuate considerably and rather regulariy-" Contrast this with the first sentence of Lack's 1954 book The natural regulation of animal numbers. "Most wild animals fluctuate irregularly in numbers between limits that are extremely restricted compared with what their rates of increase would allow." Between these two divergent statements, in 1944, was the famous British Ecological Society symposium on Gause's work and the ecology of closely related species. About this time Elton became more interested in competition than he was in 1927 (Elton, 1946), but not in the same way as Lack, who wrote numerous niche theorycompetition papers after 1944 (e.g., Lack, 1945, 1946). In 1954, Elton and Miller considered Lack's approach, said it had merit, but complained of lack of tests for his ideas and appealed for experiments comparable to those done for plants that Lack and Hutchinson ignored. As an example of what could be done, Elton and Miller praised Brian's (1952) experimental study of interspecific competition in ants. Let us summarize what we have discovered so far relative to Diamond's (1978) scenario. First, there was a great deal of work on interspecific competition before niche theorists (in the current sense) got around to it. Second, much of this work was stimulated by a simple desire to un- 893 derstand distribution patterns for their own sakes, or to solve practical problems, without regard to any theoretical framework. Third, most of the work on interspecific competition that was stimulated by theory had another theoretical perspective, now much out of favor, i.e., plant succession. Evidently dislike (e.g., Hutchinson, 1940) of the superorganism point of view regarding succession and its proponents' pervading fascination with community classification prevented modern niche theorists from recognizing the considerable accomplishments of these plant ecologists. This is interesting because much of what is considered original to modern niche theory of competition, except the mathematics, was well formulated and understood by many plant ecologists, especially in England, as early as 1914. I will demonstrate this in some detail for two subjects of major interest both to early plant ecologists and most ecologists today. • These are (i) the concepts of what are now called fundamental versus realized niches and (ii) the interacting roles of competition, predation, and physical disturbance in determining community structure. EARLY PLANT ECOLOGISTS' CONCEPTS OF THE NICHE Ideas virtually identical to Hutchinson's (1958) concept of the niche were put forth by prominent, well-known, and influential British plant ecologists more than forty years before the Cold Spring Harbor Symposium. The first clear statement is that of Tansley (1914) in his presidential address to the first annual meeting of the British Ecological Society. In his paper Tansley defined the field of ecology as he saw it. In a section titled "Competition and Succession" he says (p. 198): One of the most important factors which will be at once met with in this field is competition, the competition of plants of the same species and of plants of different species growing in the same community. A closed plant community, such as a wood or a meadow or a heath, is generally relatively stable apart from human interference. That is not to say that 894 JEREMY B. C. JACKSON it is not undergoing change, but the change is comparatively slow. The plant population is in a condition of relative equilibrium. The balance between its different members has been worked out, and fresh invaders can only gain an entrance sporadically. In such conditions competition, though it is by no means in abeyance, is largely masked. In order to determine the powers of the different species, we must resort to experiment, i.e. we must give the individuals of the various species a freer field than they have in the restricted conditions of a closed association. The simplest way in which this can be done is by clearing a patch of ground of some or all of the species present and seeing what happens. By suitable modifications of this procedure we should be able to disentangle the factors which have led to the actual distribution of species in—that is, to the structure of—the closed association. We shall find that it depends on several factors. In the first place the general habitat conditions, depending primarily on the nature of the soil, determine what species can [Tansley's italics] exist in a given spot. Secondly, we have to determine what species can actually gain access to the spot by seeds or other propagative organs, and having gained access whether they can germinate and establish themselves. This is a remarkably modern outlook. Besides appealing for experiments, to which we shall return, note that Tansley is thinking in terms of equilibria for most plant communities. He did, however, recognize the importance of communities "in which the habitat is constantly and rapidly changing, so that equilibrium cannot be established" (p. 199). This is "The paradox of the plankton" (Hutchinson, 1961) in 1914. Note also that Tansley made a clear and deliberate distinction between what Hutchinson (1958) termed the fundamental versus the realized niche when he contrasted "what species can exist in a given spot," which he defines largely in terms of soil conditions, and "what species can actually gain access to the spot . . . and . . . can germinate and establish themselves." This distinction has been a major theme in British plant ecology ever since. For example, Tansley (1917) reported results of an experimental study of competition between two species of the bedstraw Galium (see also Hutchinson, 1978). Tansley showed that both species could live on calcareous soils and peat but that the outcome of competition resulted in different dominants in the two soil types, i.e., it was not sufficient to know under what conditions a plant could live alone, but also where it could survive amongst other plants. In 1929, E. J. Salisbury gave his presidential address to the British Ecological Society in which he wrote (p. 201): . . . culture of plants in conditions far removed from those of their natural habitats is possible with a large number of species provided always that the surrounding soil is kept sedulously weeded. Such facts emphasize how delicate is the balance between species in the struggle for supremacy and how an adverse condition even far removed from the lethal point may weight the scale in one direction or the other. It is not my purpose to treat here of the relation of species to the external factors of the habitat, though it is obvious that before we are in a position to assess the contributory causes of dominance and subordination of species their optimal requirements as regards light, water supply, soil reaction, supply of essential nutrients, etc. will have to be determined. The more nearly the habitat conditions simultaneously approach the optimum requirements for a given species in these several respects the less susceptible will it be to suppression by its competitors unless the same conditions bring about a corresponding increase in their vigour also. Thus for Salisbury, as for Gause (1934) and others later on, the more similar two species' requirements, the more intense was competition likely to be between them. Interest in potential versus realized occurrences of plants continued in a big way. The British Ecological Society funded a 895 INTERSPECIFIC COMPETITION major experimental investigation of the range of physical and chemical environmental conditions under which common British plant species could survive. For this purpose they did transplant experiments, bringing together plants from all over Britain to a single place where soils, watering, and nutrients could be varied in a systematic and controlled fashion. In the first report of this work Marsden-Jones and Turrill (1930) point out that the plants were deliberately grown in the absence of competitors to test the importance of what they termed "edaphic factors," i.e., they were determining the boundaries of the species' fundamental niches. This was expensive work done on a massive scale for more than a decade; obviously the members of the Society thought it was important. Early plant ecologists also considered mechanisms of competition between plants (Clements and Weaver, 1924; Clements et al, 1929; Salisbury, 1929). Three questions of particular interest were (i) the relative importance of competition for light versus that for soil, water and nutrients, usually termed shading versus root competition (e.g., Moore, 1929); (ii) why particular soils like the English chalk favored one species or another, e.g., the reasons for the outcome of Tansley's (1917) experiments (Salisbury, 1920; Rayner, 1921); and (iii) how established plants prevented others from becoming established by preventing their seeds from reaching the soil or germinating (Cameron, 1935). To these ends plant ecologists did numerous laboratory and field experiments. By laboratory experiments I mean manipulative plantings in greenhouses or field. In these they varied plant densities, species compositions, and resources such as light, minerals, or water. Field experiments (sensu Connell, 1974; Paine, 1977) comprised manipulations of natural communities, usually with care to change only a single factor. For example, ditches were dug to separate the roots of different plants while still allowing them to potentially shade each other (Fricke, 1904 (not seen, cited in Connell, 1975); Fabricus, 1927 (not seen, cited in Salisbury, 1929); Phillips, 1928; 80-i 60- a x UJ 40- 20- 1915 1925 1935 1945 1955 1935 1945 1955 B 80-! 60- 5 S ato si 40- m 580 20- n A: 3 a 1915 1925 YEAR Fic. 4. Percentage of papers mentioning interspecific competition based on (A) field experiments or (B) field experiments and/or observations of displacement of one species by another. P = JECOL, A = JAE, and E = ECOL. Tourney, 1928; Holch, 1931; Toumey and Kienholz, 1931; Watt and Fraser, 1933; Korstian and Coile, 1938; Lutz, 1945; Shirley, 1945; Robertson, 1947; McVean, 1956; Ellison and Houston, 1958). They also selectively shaded plants (Phillips, 1928; Nedrow, 1937; Harley, 1939; Chapman, 1945; Ellison and Houston, 1958) and examined competition between established and newly germinating plants (Cameron, 1935; Bramble and Goddard, 1942). Many of the first experiments were not properly controlled or replicated, but over time many became quite sophisticated. None other than R. A. Fisher more or less invented Latin squares and parametric statistics for agriculturalists, and many plant 896 JEREMY B. C. JACKSON ecologists followed suit with some truly sophisticated work (e.g., Summerhayes, 1941; Bramble and Goddard, 1942). Because of the great importance currently attached to field experiments (Colwell and Fuentes, 1975; Connell, 1974, 1975, 1978, 1979, 1980; Paine, 1977; Strong, 1979; Heck, 1980), and widespread belief in their novelty, it is worth considering the extent of earlier similar work. Some 78 of the 322 interspecific competition papers published in JECOL, JAE, and ECOL from 1920 to 1959 involved field experiments. Two hundred and nine involved plants, and of these 70 or 34% used or explicitly referred to results of field experiments. In contrast, of the 113 papers about animals, only eight or 7% used field experiments. The patterns over time for the three journals are shown in Figure 4A. Percentages of papers utilizing field experiments were always 20% or more in the plant journals (JECOL and ECOL) and much lower in JAE. Expanding the criterion of reasonable demonstration of competition in nature to include carefully observed displacements of species by others (see Connell, 1975), the percentages are higher overall, but are still lower for animals than for plants. These differences apparently reflect differences in scientific attitudes of plant and animal ecologists of the time as much as greater difficulties of working with highly mobile animals such as birds. Plant ecologists worried about the validity of their techniques and even wrote "methods" papers. For example, in his paper "Exclosure technique in ecology," Daubenmire (1940) comments on problems of what are now called "cage effects" and their interpretation (apparent versus real causes of results). Certainly there is no support here for the view of Connell (1975, 1978, 1979, 1980) and others today that reliable field demonstrations of interspecific competition are lacking. Another recent issue (e.g., Sanders, 1968; Paine, 1974) involves the relative importance of biological versus physical factors in distributions of marine organisms. Early plant ecologists envisioned no such dichotomy. Rather they designed their competition experiments to bring out the subtle relationships between so-called edaphic factors such as soil pH and the outcome of competition. To quote Salisbury (1929, p. 199): " . . . the vigour of a species and its consequent capacity for dominance may depend on apparently quite small and insignificant changes in the environment and provides a salutory warning against neglecting such as unimportant." Thus a large number of plant ecologists studied gradients in environmental conditions not because they were misguided reductionists unaware of competition, but because they knew the complexity of the multitudinous interactions hidden behind that name. Certainly the same must be true of physical gradients on rocky shores. COMPETITION, PREDATION, DISTURBANCE, AND DIVERSITY A second area long of interest to plant ecologists involves the nature of the combined effects of competition, predation, and physical environmental disturbances on plant community structure and diversity. From the beginnings of this century, British and American plant ecologists, farmers, and ranchers have been impressed by the effects of herbivores on the composition of vegetation, especially in heaths, fields, and the American prairie. To quantify the nature and magnitude of these phenomena they did many field experiments, mostly using exclosures (Farrow, 1916, 1917, 1925; Watt, 1923, 1934, 1957; Tansley and Adamson, 1925; Taylor, 1930; Talbot etai, 1939; Daubenmire, 1940; Godwin, 1941; Hope-Simpson, 1941; Summerhayes, 1941; Bramble and Goddard, 1942; Fitch and Bentley, 1949; Gimingham, 1949; Gillham, 1953, 1955; Goodman and Gillham, 1954). The scale of manipulations ranged from modest cages to entire forests and fields. As before, many of the early experiments were lacking in details of design, but many of the later studies were excellent. The nearly universal result of exclosures was a dramatic change in dominant plant 897 INTERSPECIFIC COMPETITION species. Sometimes replacement was merely of one grass or herb by another, but other times, especially for exclosures maintained ten to twenty years, trees or woody shrubs began to replace grasses or ferns (Farrow, 1941; Hope-Simpson, 1941). These studies were not done just to demonstrate the processes of competition or predation in nature. Everyone seems to have accepted the importance of both as obvious. Rather, they were designed to unravel the extent of effects, the mechanisms of their action, and their consequences in shaping the major vegetational features of England and the American west. They were also done to determine how commercially important pasture and range were being ruined by drought and/or overgrazing and what to do about it. Ranchers, for example, worried about the possible worth in cattle forage of consumption of vegetation by rodents (Taylor, 1930; Talbot et al., 1939; Fitch and Bentley, 1949). Two studies merit special mention for their relevance to current research. The first by Tansley and Adamson (1925) was part of a long term observational and experimental study of the vegetation of the English chalk. Exclosure experiments yielded new dominant plants. In their words (pp. 205-206): ". . . the two interlocked factors, viz. absence of grazing and consequent increase of competition leading to the success of the taller growing plants, appear to be far the most potent influences in changing the vegetation." Tansley was one of the leading Clementian successionists. To him competition was the driving force leading to the local climax vegetation. Predation was termed the "biotic factor" and was viewed as a force that deflected or prevented formation of climax vegetation. In this light, Tansley and Adamson's study was theoretically motivated, with the theory being tested by experiments. I have summarized Tansley and Adamson's results with regard to species diversity as they discussed them but failed to present in graphical form (Fig. 5). The curve has a familiar form (e.g., Paine, Tansley and Adamson 1925 J. Ecology pp. 205-6 ; Many : None No Rabbits Many Rabbits Too Many Rabbits FIG. 5. Graphical representation of Tansley and Adamson's (1925) observations on the relation of plant species diversity to grazing by rabbits. 1977; Lubchenco, 1978), and needs no further comment other than to emphasize that the authors' explanation of their results would be well received today. The second paper, titled "Pattern and process in the plant community," was A. S. Watt's (1947) presidential address to the British Ecological Society. This paper is a thoroughly modern statement of the ways physical disturbances like wind erosion, herbivory, and competition between plants interact to affect distribution and abundance patterns of plants. It was cited by Krebs in his influential text (1972) but not by most ecologists working on similar problems today (e.g., Levin and Paine, 1974; Platt, 1975; McNaughton, 1979; but see Connell, 1978; Bormann and Likens, 1979). Watt (1923) first studied regeneration of beech woods, doing experiments and describing vegetational changes. Twentyfour years later he had done similar work in a variety of plant communities and was ready to make general pronouncements. He was much less interested in succession than when he started. Rather he viewed plant communities as a mosaic of various dominant associations pock-marked by disturbance-generated patches in varying states of recovery. Watt understood why the same association does not always become reestablished in the same place after a disturbance, i.e., the effects of history or 898 JEREMY B. C. JACKSON so-called multiple stable points (Sutherland, 1974). He was also a student of morphology and growth patterns, which he used to explain the comparative competitive success of different species. Watt had great influence. Some of the finest derivative work was that of Gillham (1953, 1955) and Goodman and Gillham (1954) on the effects of wind erosion, grazing, and sea bird dung on the vegetation of the Pembrokeshire Islands. These authors did experiments on grazing effects and observed blowouts of the dense vegetation mats exposed to severe winds. Speaking of winds and grazing, Goodman and Gillham (1954, p. 313) state: Ecologists like Tansley, Watt, and Elton were interested in immediate causes of distribution and abundance patterns. Some species of plants propagated vegetatively, others only by seeds. These differences affected their colonization and competitive abilities and thus affected distributions. That was simply the way it was. For all their splendid work there was little interest in the origin of life history patterns by plant or animal ecologists before the 1950s. One striking exception was E. J. Salisbury's (1929) "The biological equipment of species in relation to competition" to which I have referred previously. This is a truly extraordinary paper and yet it has been almost entirely forgotten. Below These two factors have a strong influ- I list only the life history topics discussed ence on the competitive powers of a in this paper: species and hence are to a great extent indirectly responsible for its abundance 1. Sexual versus asexual reproduction in a particular place. Thus the domiand its relation to dispersal and comnance of Armeria maritima over Festuca petitive ability. rubra on the spray-washed cliff-tops is 2. Parent-offspring conflict and sibling largely the result of the inability of the rivalry in plants. latter to compete under the handicap of 3. Dispersal by fragmentation. heavy grazing. 4. Morphological variation in stolon lengths relative to rates of vegetative Note especially that they viewed competidispersal, competitive ability, and lotion and predation to be interrelated faccation of new habitats. This was studtors. Predation acted to change the comied experimentally. petitive dominant, not to eliminate the 5. Tradeoffs in above-ground versus beimportance of competition. The new domlow-ground stolon propagation. inant was simply more resistant to the preAbove-ground growth was faster but vailing conditions of herbivory. Unlike bore greater risks of stolons being eatsome ecologists today (Connell, 1975, en or burned; below-ground growth 1978, 1979, 1980), Gillham, Watt, and was slower but safer. Tansley would not have thought to argue 6. Production of litter, e.g., leaves that do about the relative importance of competinot easily rot, to make the soil envition and predation. Both were important ronment unfavorable to other plants. most of the time, and they had the exper7. Production of far more seeds than iments to prove it. could ever survive to decrease chances of competitors' seeds surviving. LIFE HISTORIES AND EVOLUTION: 8. Length of life and why it varies. T H E NEW PERSPECTIVE 9. Relationship of fecundity to survivorReading all these earlier papers made ship. Salisbury's admittedly qualitative me wonder whether the Volterra-Gause understanding of reproductive value niche theory had contributed anything just predates Fisher's (1930) formal fundamentally new to ecologists' ideas treatment. about interspecific competition besides the 10. Mast years. mathematics. I believe the answer is yes, 11. Simultaneous versus continuous versus but not for reasons usually claimed. Very discontinuous germination. simply, niche theory forced ecologists to deal with evolutionary arguments. Salisbury ends with a Harperesque appeal 899 INTERSPECIFIC COMPETITION for numbers on all the above parameters and good statistics. Salisbury's paper was his presidential address to the British Ecological Society, hardly an obscure offering, yet it has been , lost from even the botanical literature except for citation by Watt in 1947. Why was it lost? I believe the explanation is simple. Poor Salisbury was so far ahead of other ecologists in his understanding of natural selection that they simply were not interested. Ecologists had long had a difficult time with evolutionary thinking. Elton's chapter on evolution in his brilliant 1927 book is rampantly group selectionist, and he questioned the application of natural selection to many ecological problems. His 1929 paper on copepods, which helped inspire Hutchinson's (1951) strongly evolutionary "Copepodology for the ornithologist," is a distribution and abundance paper lacking in evolutionary argument. Even so, it too was forgotten. We have already seen that Lack's (1944) change of heart about competition was primarily stimulated by Huxley (1942) and the modern synthesis and only secondarily by Gause. For most ecologists the modern synthesis was essential for development of ideas of life histories, and even afterwards progress was slow. Cole's paper was not until 1954, and r- and K-selection and the flood of interest in life histories not until the 1960s. Only with understanding of how phenomena such as character displacement and niche diversification might occur were ecologists to become comfortable with evolutionary thinking. Thus by injecting evolutionary perspective, niche theory profoundly influenced ecology despite the fact that most of its basic tenets had been developed decades before. ACKNOWLEDGMENTS I dedicate this paper to my father Mel who taught me to suspect the arrogance of the present. The ideas for this paper germinated in a seminar on the history of ecology with D. J. Haraway, C. 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