In 1944, the British Ecological Society held a symposium on “The Ecology of Closely Allied Species,” at which Lack, Elton, Varley, and others used various lines of evidence to argue that competition is a major factor in structuring plant and animal communities. Others argued to the contrary, with Diver contending that “the mathematical and experimental approaches had been dangerously oversimplified and omitted consideration of many factors [including] sources of energy and their relative availability, predator attack, mobility, population structure and growth, individual growth rate and bulk, relation of life cycle to annual cycle, range of tolerance, means of dispersal, and...

In this paper, I report two studies. The results show the information that may be required for both a dynamically sufficient description of these guilds and an understanding of the laws of transformation that determine their composition. In neither example is the interspecific interaction that determines the laws of transformation obvious from the original description of the community, and I cannot claim that those laws have been specified yet. I do hope to show the scope that they must eventually cover. If guilds are to be considered ecologically meaningful groups of species, some such specification must be sought.

Both studies...

Orthodox community theory is dominated by the assumptions of density-dependence and interspecific competition. Population growth inexorably depletes resources and shortages govern interactions both within and between species in this theory. Here, I summarize research with phytophagous insects of the tropical monocotHeliconia,which coexist in a manner quite opposite to this orthodox theory. Resources are not depleted by these insects. They do not feed from or occupy host plants in a densitydependent manner, and species do not compete among themselves. Although analyses of the forces that do affect these populations are not complete, preliminary observations indicate that natural enemies such...

Considerable ecological theory postulates a central role for interspecific competition as a cause of patterns in resource utilization, distribution, and relative abundance of animal species. Tests for competition frequently have been indirect, relying upon inferences from patterns that are consistent with competition theory but that may have alternative explanations. Continuing calls for more direct tests(e.g.Reynoldson and Bellamy, 1971; Connell, 1975; Colwell and Fuentes, 1975; Pianka, 1976; Wiens, 1977a) reflect an increasing awareness of the value of carefully controlled field experiments. However, field experiments have not been widely used to test for competition in animal communities, probably for two...

A major objective of community ecology is to identify the factors that determine the relative abundances of species in communities and to discern whether recurring predictable patterns of community structure exist. Identification of the structuring factors for one community may lead to high predictability about communities that have similar phyletic components and exist in similar environments. Alternatively, communities from distinct environments or with different species compositions may have differring structuring components. Studies of communities revolve around a search for underlying generalities. Ideally, a model of community structure should give valid predictions for most, if not all communities. No such model...

Whether or not communities of unrelated species converge in structure under similar environmental conditions is of both theoretical and empirical interest. If they do, the likelihood is that interspecific competition for limiting resources, usually food, imposes strong constraints on community structure. If they do not, then belief in strong constraints must inevitably be weakened. Obviously, different sorts of communities need not obey identical rules (MacArthur, 1972). This point has been nicely put by Cody and Mooney (1978), who, summarizing the evidence at their disposal, wrote (p. 268):

Convergent evolution (a) is best observed in patterns at the level of the...

The equilibrium theory of insular biogeography has received a great deal of attention since its introduction in 1963 (MacArthur and Wilson, 1963). The model is conceptually simple, offers mechanistic explanations for a number of natural phenomena that have interested biologists for a long time, and has the potential for a wide range of practical and theoretical applications.

In its simplest form, the model predicts that the number of species on an island or similarly isolated area is a result of a dynamic equilibrium between immigrations and extinctions. Immigration rate is seen as a decreasing function of numbers of species already...

Patterns of species associations, and the processes that generate patterns, are the focal points of community ecology. The study of pattern has been primarily descriptive, delimiting species associations in replicated physical environments (Wiens, 1969), along natural environmental gradients (Whittaker, 1956), or along statistically derived multifactorial gradients (James, 1971; Bloomet al.,1972; Nicholsonet al.,1979). Insights into the processes that form the patterns have come through two types of experimental ecology. Microcosm studies, of varying scale, have investigated the consequences for community structure of various initial arrays of species, densities and environmental conditions (Vandermeer, 1969; Hallet al.,1970;...

Major patterns of species richness in the marine environment include a general increase in species numbers with decreasing latitude and, within the tropics, an increase in regional species number in the order Eastern Atlantic, Western Atlantic ˜ Eastern Pacific, Indo-West Pacific (for a discussion on crustaceans see Abele, 1982). There is no shortage of hypotheses to explain these and other patterns of species richness(e.g.Ricklefs, 1979). The hypothesis considered here suggests that species in diverse environments are more specialized and have narrower niches, resulting in more coexisting species per unit habitat than in less diverse habitats. One of the...

Assembly rules of species communities (Diamond, 1975; Cody, 1978) based on assumed interspecific interactions within guilds have been derived in order to explain details of community structure. In this paper, our approach will be somewhat different. We wish to compare island bird communities on different-sized islands with those on the mainland (or an island that is several orders of magnitude larger), and, on the basis of census information and extensive autecological data available on Northern European land birds, we attempt to answer the following questions:

(1) How much of insular impoverishment can be understood in terms of habitat differences(cf....

Most modern investigations of intertidal ecology search for general mechanisms and processes to account for observed patterns of distribution and abundance of and the intensity of interactions among component species of intertidal communities. The early history of this endeavor consisted of detailed observation of patterns of distribution of intertidal biota (summarized by Lewis, 1964; Rickettset al.,1968; Stephenson and Stephenson, 1972). Some early workers, notably Hatton (1938), set the scene for the modern era of controlled manipulative experimentation, a methodology that has become widespread and highly productive for unraveling the complex interactions among the components of a community(e.g....

This paper is a brief review of mechanisms structuring some marine communities. I address three related questions. First, are the distributions and demographic patterns of the populations regulated by general and/or predictable biological interactions? Specifically, are the habitats, resources, and risks modified in a predictable way by other populations in the community? Second, is the organization of the very different communities structured by similar mechanisms, or are different processes emphasized? Finally, what are the most efficient means of studying these processes?

When appropriate, I will discuss these questions with regard to hard-and soft-bottom and plankton communities. Most of the benthic...

Different views prevail on the extent to which communities can be considered structured, and the role of biotic interactions, most notably competition and predation, in determining community structure. Plant ecologists have been preoccupied with these questions for a long time (Jackson, 1981), but the current concerns of animal ecologists (Connell, 1980) appear to stem from the work of David Lack with Darwin’s Finches (1940, 1945, 1947). Lack made an argument for considering interspecific competition to be an important process in the adaptive radiation of the finches. In this paper we will briefly review Lack’s argument, the work that it stimulated...

Stronget al.(1979) concluded that size ratios of coexisting bird species on islands provide “little, if any, evidence of the phenomenon [communitywide character displacement]” and therefore do not support the hypothesis that interspecific competition was a key determinant of sizes. Grant and Abbott (1980) and Hendrickson (1981) criticize this paper on various grounds, and the former authors find fault with several of my efforts in a similar vein: Simberloff (1970, 1978a) and Connor and Simberloff (1978). Here I reexamine avifaunas of the Tres Marias Islands and Galápagos and attempt to deal with the criticisms and to incorporate the authors’...

Closely related sympatric species, although similar in most ways, can differ strikingly in some aspect of size. Enough such cases were noted by Huxley (1942) and Lack (1944) for them to suggest that the size differences resulted from selection to avoid interspecific competition. Hutchinson (1959), in giving “homage to Santa Rosalia,” began the tradition of computing size ratios between adjacent members of size-ranked associations of sympatric species. He listed a small number of such ratios, most of which fell between 1.2 and 1.4, and wrote that 1.3 “may tentatively be used as an indication of the kind of difference necessary...

The early development of community ecology was stimulated in large part by the suggestion that morphological and geographical patterns among closely related species of terrestrial vertebrates were related to ecological mechanisms of coexistence or competitive exclusion(e.g.Lack, 1947; MacArthur, 1958; and much subsequent work including that of Pianka and Schoener on lizards; Cody, Grant, and Diamond on birds; and Rosenzweig and Brown on mammals). The majority of these studies supported the thesis that interspecific competition plays a major, but not necessarily exclusive, role in determining the organization of guilds of closely related, ecologically similar species(e.g.MacArthur, 1972). In...

Questions of structure lie at the heart of any science. That ecological systems have structure due to their abiotic environments and also due to limitations involving their own internal energy flows has long been recognized. Beyond this, there is a tradition, extending back to Darwin, that interspecific interactions such as competition, predation, parasitism, and mutualism regulate membership and abundance of species in local communities.

During the 1960’s and early 1970’s, the expected character of interspecific competition was given a strong theoretical formulation by the late Robert MacArthur and his associates. Since then, many field and experimental ecologists have described various...

Oddity in ecological nature is relative to expectation. Whether we respond to facets of nature with wide-eyed amazement or knowing nods depends on what our theories and predispositions have led us to expect. Connor and Simberloff (1979) examined biogeographic patterns of species among islands in this spirit. Can we tell when certain patterns imply a specific cause? Questions of this type require one to attempt to identify the possible configurations of species on islands. In particular, Connor and Simberloff (1979) responded to Diamond's assertion (1975) that biogeographic patterns of species’ co-occurrence are shaped largely by competitive interactions between species. We...

We completed the text of our contribution to this volume in September 1981. In March 1982, shortly before this volume was to go to press, the editors sent us a chapter that one of the editors and a colleague wrote to rebut our chapter (Connor and Simberloff, this volume: abbreviated C and S). In November 1982 the editors sent us a further rebuttal, entitled “Rejoinder by Connor and Simberloff” and repeating some of the same arguments unchanged. These two documents have been helpful to us in understanding the style of reasoning preferred by C and S. We appreciate the editors’...

If we could seed a series of virgin, replicate earths with primordial life and set the level of interspecific competition differently in each, could we tell them apart three billion years later by looking at biogeographical patterns? In this paper we present the results of an effort to approximate this experiment by computer simulation, with the purpose of examining the potentials and the pitfalls of several methods of biogeographical analysis. Since we control the intensity of competitive exclusion as a variable in the simulation, biases and limitations in the construction of null models in biogeographical studies can be studied directly....

Ecologists have long been intrigued by patterns in species numbers and relative abundance, and much speculation has been proffered on the extent to which these patterns reveal the underlying processes responsible for community structure. In recent years, Hutchinson’s (1957) reformulation of the niche concept and the suggestion of measures relevant to the field(e.g.Levins, 1968) have given rise to a burgeoning literature stressing the role of competition in organizing communities and describing patterns in resource partitioning among groups of coexisting species(e.g.reviewed by Schoener, 1974b). Despite the popularity of this approach and the great interest it generated, there...

Kuhn (1970) notes that a concern about philosophy and logic often arises when a science enters a state of controversy over the failing of past theory and the appearance of new approaches. This symposium suggests the time is propitious for such a concern in community ecology.

“To do science is to search for repeated patterns.” This quote from Mac Arthur (1972) captures the essence of a common approach taken in community ecology. By examining patterns such as species distributions, we hope to unveil the processes that cause them. Although unequivocal evidence for causal processes will come only through experimentation, ecologists...

Following MacFadyen’s (1963) review of the many possible meanings of the word “community” one might have expected some consensus. Yet, what followed was the addition of a qualifier to the term(e.g.bird communities) and a novel meaning. In the words of McNaughton and Wolf (1973): “community is usually applied to organisms with similar life habits,” and, I might add, to species that are usually on the same trophic level. Most of the papers in this volume reflect this newer definition; mine does not. I shall consider what might be called a “vertical” property of communities (one that runs across...

Analyses of both real and hypothetical ecological communities have demonstrated that any relationships between stability and complexity are more complicated than once presumed and are often contrary to the once fashionable adage that complexity begets stability(e.g.May, 1973; Goodman, 1975; Pimm, 1979a). One current approach to stability and complexity in ecological communities is through food-web analysis. Pimm (1979b, 1980b), Pimm and Lawton (1977, 1978), and Rejmanek and Stary (1979) suggest that dynamical constraints are responsible for many hypothesized food-web properties, such as restrictions on the number of trophic levels and interspecific interactions. Here, I examine the underlying assumptions and...

The mission of community ecology, as of any scientific endeavor, is to detect thepatternsof natural systems, to explain them by discerning the causalprocessesthat underlie them, and to generalize these explanations as far as possible. We usually detect patterns by making comparisons among features of different natural communities, or between the natural systems and logical or theoretical models of community structuring. Finding patterns, we then often derive a process explanation by inference, by constructing logical and realistic scenarios of how pattern and process might be linked, or by accepting the underlying premises of a theoretical model as...

Ecologists generally agree that morphological adaptations are evolutionary responses to ecological conditions, and that if species populations remain fairly stable over time, density-dependent factors are probably involved (Krebs, 1972). When these intraspecific concepts are combined and extended to the community level, the question becomes: “Is there a linkage between the densities of individual species and the morphology of other members of the community?” This question is important because several widely held constructs in community ecology have the underlying assumption that community structure is largely attributable to such a linkage. Although the principle of competitive exclusion can be shown to be...

Many people would see the aim of science as a search for order or regularities in the universe. MacArthur (1972) put it simply and concisely. “To do science is to search for repeated patterns, not simply to accumulate facts.” For him, to do ecology was to search for general patterns in the distribution and abundance of species. While this is an enticingly simple definition of ecology, if taken literally, as it has been by some, as the modus of ecology, it may lead to false conclusions and a lack of the questioning that is one of the hallmarks of science....

Density compensation is the phenomenon by which summed population densities of faunas on islands or insular habitats equal (or, for excess density compensation, exceed) mainland population densities, although number of species is reduced (MacArthuret al.,1972). Density compensation is derived from competition theory. Inherent in most explanations for its occurrence is the idea that for a specified time, community of interacting species, geographic area, and resource base, overall density of individuals is dictated by competition for resources. The species number and actual species composition in the community is deemed relatively unimportant to this optimum density (Crowell, 1962; MacArthuret...

The paradigms of ecology concerning community organization cluster around interspecific competition as a central organizing force. Niche occupancy is defined by the presence of competitors on adjacent parts of environmental gradients. Strong competitors have broad niches, weak competitors narrow niches. Similarity between coexisting species is limited through competition, so communities become tightly packed with a regular spacing of Hutchinsonian distances between species. The gradient in species density from high to low latitudes and the changing environmental conditions result in more competition in the tropics, narrower resource utilization and more overlap between species. Other factors such as predation and disturbance moderate...