Ecological Niches: Linking Classic and Contemporary Approaches / Edition 2 available in Paperback
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Why do species live where they live? What determines the abundance and diversity of species in a given area? What role do species play in the functioning of entire ecosystems? All of these questions share a single core concept—the ecological niche. Although the niche concept has fallen into disfavor among ecologists in recent years, Jonathan M. Chase and Mathew A. Leibold argue that the niche is an ideal tool with which to unify disparate research and theoretical approaches in contemporary ecology.
Chase and Leibold define the niche as including both what an organism needs from its environment and how that organism's activities shape its environment. Drawing on the theory of consumer-resource interactions, as well as its graphical analysis, they develop a framework for understanding niches that is flexible enough to include a variety of small- and large-scale processes, from resource competition, predation, and stress to community structure, biodiversity, and ecosystem function. Chase and Leibold's synthetic approach will interest ecologists from a wide range of subdisciplines.
About the Author
Jonathan M. Chase is an assistant professor in the Department of Biology at Washington University.
Mathew A. Leibold is an associate professor of integrative biology at The University of Texas at Austin, and was formerly associate professor of ecology and evolutionary biology at The University of Chicago.
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Ecological Niches: Linking Classical and Contemporary Approaches
By Mathew A. Leibold
University of Chicago PressCopyright © 2003 Mathew A. Leibold
All right reserved.
CHAPTER ONE - INTRODUCTION: HISTORY, CONTEXT, AND PURPOSE
The niche concept remains one of the most confusing, and yet important, topics in ecology. --R. B. Root (1967)
I think it is good practice to avoid the term niche whenever possible. --M. H. Williamson (1972)
The niche concept is a very useful addition to the ecologist's tool kit be- cause it combines the best properties of both baling wire and putty; it holds ill-fitting pieces together that would otherwise fall apart and at the same time fills the gaps between them so that poor workmanship may go undetected. --D. Reilloc (cited in Hurlbert 1981)
At its most ambitious, the theory of niche helps us understand funda- mental questions of ecology. --T. W. Schoener (1989)
No concept in ecology has been more variously defined or more univer- sally confused than "niche." --L. A. Real and S. A. Levin (1991)
The concept of niche provides a way of characterizing important ecologi- cal attributes of species while recognizing their uniqueness. --J. H. Brown (1995)
Ecology's love-hate relationship with the nicheconcept has been long and not especially pretty. --N. G. Hairston Jr. (1995)
Studies of the niche have played an important role in the development of community ecology, and are likely to do so in the future. --B. A. Maurer (1999)
I believe that community ecology will have to rethink completely the classical niche-assembly paradigm from first principles. --S. P. Hubbell (2001)1.1. Clearing the Decks and Setting the Stage
This book is directed at reinterpreting and reevaluating the concept of the niche within the framework of recent developments in ecology. The field of ecology has developed tremendously in recent years as theoretical issues have been resolved, as experimental approaches have been refined, and as new questions and approaches have been developed. In this rapidly shifting conceptual landscape, old ideas can often seem to disappear or change in ways that make them unrecognizable. One of these is the concept of the ecological niche, which for now we loosely define as the requirement of a species for existence in a given environment and its impacts on that environment (a more formal definition is at the end of this chapter).
The niche concept is an important element in almost every aspect of ecological thinking, from the study of behavior, morphology, and physiology of individual organisms to approaches that evaluate how species participate in ecosystem functioning. From its inception in the ecological literature, most notably due to Grinnell (1917), Elton (1927), and Gause (1936), use of the niche concept increased steadily in the ecological literature (fig. 1.1). In fact, the term "niche" appeared in more than one-fourth of all papers published in the journal Ecology during its peak usage in the 1960s and 1970s. However, over the past twenty or so years, the niche has declined significantly as an ecological concept. The term seems to be almost avoided by many ecologists, and there is a perception that it is a concept that has outlived its utility. We argue to the contrary that many of the concepts and theoretical tools that provide the deepest insights into ecology are intimately linked to the idea of the niche and that a recognition of this link can help to solidify the conceptual synthesis that ecology so desperately needs.
The niche concept we develop in this book is probably valid for almost any situation of ecological interest. A species is consistently able to exist for long periods of time only when its ecological requirements are met in a local environment. "Sink" habitats, in which a species could not exist unless supported by immigration from a highly productive "source" habitat, are a notable and important exception (Pulliam 1988, 2000), but one that begs understanding of how a species' requirements are met at these sources (see also Loreau and Mouquet 1999; Amarasekare and Nisbet 2001). Further, the niche concept provides a richer and potentially more instructive context when it is used to evaluate species interactions in communities. Our approach is based on the idea that differences among species in their niches (either their requirements or their impacts or both) are important in determining the outcome of species interactions as might be revealed in the their distributions and/or abundances, as well as in their biodiversity and their functional role in ecosystems. There is a very strong historical legacy (Elton 1927; Hutchinson 1957, 1959, 1978; MacArthur 1958, 1972; Williams 1964; Levin 1970) and contemporary focus (Tilman 1982, 1988, 1999; Chesson 1991, 2000a,b; Leibold 1995; Weiher and Keddy 1999; Pulliam 2000) of the niche concept in ecology. However, there is also a considerable amount of debate as to whether the concept of the niche, with particular regards to local interactions and ecological differences among species, is necessary or useful in understanding broad patterns in the diversity, distribution, and abundance of species (Bell 2001; Hubbell 2001).
Recently, Hubbell (2001) has suggested that ideas related to the niche may not be needed at all, and that the classical paradigm based on the niche perspective needs to be completely rethought. To that end, Hubbell has comprehensively developed an alternative approach, called the "unified neutral theory of biogeography," with the stated assumption that all species are identical with respect to their ecological traits, and to "see how far" he could go with it in explaining natural patterns of species diversity and abundance. Hubbell based his neutral theory on a synthesis of ideas and data from island biogeography (MacArthur and Wilson 1967), previous neutral ecological models (Caswell 1976), his own studies in tropical forests (Hubbell 1979; Hubbell and Foster 1986; Hubbell et al. 1999), and a strong analogy with concepts of genetic drift in population genetics and neutral speciation in evolutionary biology. Although the key assumption of neutrality is not likely to be strictly true, as Hubbell freely admits, the theory does a remarkable job of predicting patterns of biodiversity and relative abundance of species in a variety of different ecosystems (see also Bell 2000, 2001). Furthermore, Hubbell's ideas have reinvigorated theoretical and empirical studies aimed at understanding patterns of biodiversity and relative species abundance at larger spatial scales (Bell 2001; Chave et al. 2002; Condit et al. 2002).
The neutral theory is not, however, beyond criticism. First, species do differ in their traits and often show trade-offs that allow them to coexist for long periods of time. Second, many experimental studies and natural disturbances show that when a system is perturbed away from its equilibrium, it often tends to go back to that equilibrium. Third, several recent theoretical models show that the predictions of the neutral theory can be quite tenuous and highly dependent on unrealistic parameter values (Chesson and Huntly 1997; Zhang and Lin 1997; Chesson 2000b). Fourth, predictions from the neutral model are difficult to distinguish from the predictions of models with niche differences (Chave et al. 2002). Finally, many empirical patterns, even within the very tropical forest systems that Hubbell (Hubbell 1979; Hubbell and Foster 1986; Hubbell et al. 1999) used to develop his ideas, do not conform to many of the neutral model's predictions on patterns of biodiversity, species composition, or relative species abundances (Terborgh et al. 1996; Yu et al. 1998; Pitman et al. 2001; Condit et al. 2002).
We agree with Hubbell that the classical niche paradigm, which was often based on phenomenological (e.g., Lotka-Volterra) models of interspecific competition, needs to be rethought from first principles. We do not, however, agree that the solution lies with neutral models. In this book, we attempt to resurrect the concept of niche. That is, we heed Hubbell's suggestion that the niche concept needs to be refined. In doing so, we do not develop entirely new models and ideas, but rather link previously unconnected ideas into a single common framework of both classical and contemporary ideas. In chapters 6 and 11, we return to comparisons between the niche and neutral approaches and suggest avenues where the process of "ecological drift," which arises from the connections among local communities in a region (i.e., a metacommunity), can be fully integrated into a niche-based framework.
In this book we seek to accomplish several things. First, we develop a framework around the niche concept that better accommodates several recent insights about niche relations in ecology. Second, we use this updated niche framework to point the way to new questions, conclusions, and syntheses. Finally, we link our interpretation to some of the insights developed using alternative approaches and identify those areas where the greatest challenges for the field of ecology in relation to the niche concept lie.
1.2. History, Evolution, and Divergence of the Niche Concept in Modern Ecology
We start with a brief history of the ecological niche concept. We broadly focus on the historical background, ignoring many of the finer subtleties of usage and precedence that might be of greater historical interest. The history of the niche in ecology and evolutionary biology has been already reviewed by a number of authors (e.g., Hutchinson 1957, 1978; Udvardy 1959; Whittaker et al. 1973; Vandermeer 1972; Colwell and Fuentes 1975; Whittaker and Levin 1975; Giller 1984; Schoener 1989; Griesemer 1992; Colwell 1992; Leibold 1995).
The word niche (also nitch) has been used in the English language for hundreds of years (see Schoener 1989 for some early uses of the term) and has several related meanings, most of which are not associated with biology at all. The fifth edition of Merriam-Webster's Collegiate Dictionary (1959) defines niche as: "1) a recess in a wall, 2) a covert or retreat, 3) a place, condition of life or employment or the like, suitable for the capabilities or merits of a person or qualities of a thing." The third definition is closest to how biologists view a species' ecological niche. However, in the tenth edition of Merriam-Webster's Collegiate Dictionary (2001) the first definitions remain, but there are two added ecological definitions: fourth, "a habitat supplying the factors necessary for the existence of an organism or species"; and fifth, "the ecological role of an organism in a community esp. in regard to food consumption." It is interesting that this nonscientific dictionary has two distinct definitions of niche related to ecology and evolutionary biology: one referring to the habitat a species needs for survival, and the other referring to a species' role in or impact on the community. In fact, these definitions, although they seem superficially similar, are indicative of a similar dichotomy among ecological definitions of the niche. Specifically, Grinnell's (1917) original description of the niche, as well as Hutchinson's (1957) well-known definition of the niche as an "ndimensional hypervolume," are aligned with the first ecological definition, that of ecological requirements. Alternatively, Elton's (1927) usage, as well as MacArthur and Levins's (1967), was more closely aligned with the second, pertaining to ecological roles (see also Schoener 1989; Colwell 1992; Leibold 1995). These distinct definitions have historically caused confusion among ecologists, and as we discuss below in this chapter and more explicitly in chapter 2, by considering both effects in concert we can gain a more complete view of the niche.
The definition of a species' niche as the habitat or role of a species obviously has roots that predate the modern origin of the concept. Genesis clearly states that each species fills a specific role (see also Patrick 1983). Thus, when Noah was charged to build an ark to save all sorts of species from their peril in the Great Flood, the direct implication was that each species allowed on the ship filled a different role that was needed (at least by humans if not by their ecosystems) in the postdiluvian world. Other religions are also founded on writings that refer to species of plants and animals filling various roles within the environment. More explicitly, the writings of early philosophers such as Aristotle and naturalists such as Linnaeus (1758) obviously refer to concepts of diversity and the ecological niche by defining and describing differences among species' traits.
Darwin's (1859) and Wallace's (1876) writings on natural selection and evolution also implicitly refer to species' roles in much the same context that we use the niche concept today. For example, as cited in Stauffer (1975), Darwin used the term "line of life" (in much the same way as we would use "line of work" about a person's job) to describe the role of a species and explain the great diversity of butterflies discovered by Bates in the Amazon basin. Darwin's "line of life" was used similarly to the way a modern biologist would use "niche."
Following the pioneering works of Darwin and Wallace, several naturalists discussed concepts related to a species' niche (see citations in Hutchinson 1978), but Johnson (1910), in a rather obscure publication on the habits and habitats of ladybird beetles, was the first to use "niche" in an explicitly biological way. Despite Johnson's chronological precedence, however, it is widely accepted that the real founder of the niche concept was Grinnell, who in a series of papers, discussed the niches of a variety of species, including their abiotic requirements, habitat, food, and natural enemy relationships (Grinnell 1914, 1917, 1924; Grinnell and Swarth 1913). Although Grinnell's 1904 paper did not use "niche," it was an earlier formulation of the concept to which he later applied the word.
Grinnell's papers, most notably the now classic "The niche-relation-ships of the California thrasher" (1917), discussed the niche as the place in an environment that a species occupies. Grinnell used the concept to verbally map all of the necessary conditions for a species' existence, including physiological tolerances, morphological limitations, feeding habits, and interactions with other members of the community (notably predators). Grinnell also alluded to some topics of ecological investigation that are not generally accredited to him (Schoener 1989). For example, Grinnell (1904) expanded on the observations of Darwin and Wallace and pioneered the concept that two species must differ in some traits related to their fitness in order to coexist. This "competitive exclusion principle" (Hardin 1960) is generally credited to Gause (1936; see below). By the time he wrote his 1917 paper, Grinnell had discussed this principle as "axiomatic." Grinnell also discussed whether communities were full or whether they were undersaturated, with "empty niches." Many of these questions remain at the forefront of ecological research today.
Elton (1927) provided the second major advance in the use of niche. Specifically, Elton focused on the niche of a species as its functional role within the food chain ("food cycle" in Elton's terminology) and its impact on the environment (e.g., what it eats). The distinction between Elton's focus on the effects of a species on the environment and Grin-nell's focus on the effects of the environment on the species will play an important role as we develop our refined niche concept in chapter 2 (see also Schoener 1989; Colwell 1992; Leibold 1995). Many authors, having found little evidence that Elton was aware of Grinnell's writings, have suggested that Elton's conceptualization of the niche was more or less independent of any influence from Grinnell (Hutchinson 1978; Schoener 1989; Griesemer 1992).
Around the time that Grinnell and Elton were refining their niche concepts in animal community ecology, plant ecologists were formalizing their own ideas and pioneering many similar concepts regarding niche and community structure (but often using different terminology). However, as pointed out by Jackson (1981), many of the concepts from early plant ecologists have been subsequently ignored by the vast majority of modern ecologists. Tansley and Salisbury introduced a variety of concepts that are alarmingly similar to ideas that have generally been attributed to authors decades later. For example, Tansley (1917) performed experiments that showed how plant species competed and coexisted, in a sense vying for shared niche space. Tansley also explicitly contrasted the conditions in which a species could theoretically exist with the actual conditions in which it did exist: ideas generally attributed to Hutchinson (1957) in his discussion of "fundamental" and "realized" niches (see below). Salisbury (1929) furthered this distinction and suggested that the similarity in species requirements was strongly related to the intensity of their competition--much the same concept as appears in the more widely appreciated work of Gause (1936).
Although the term "niche" was not a component of the pioneering theories of competition and predation by Volterra (e.g., 1926) and Lotka (e.g., 1924), the elements of these models, depicting interacting populations as dynamical differential equations, provided much of the foundation for modern explorations into the niches of organisms. This was especially evident during the heyday of "niche theory" pioneered by Robert MacArthur and his colleagues, which we discuss below. Gause (1936) made elegant use of experimental laboratory protist populations to explicitly simulate the dynamics of the Lotka-Volterra models. Gause's experiments are perhaps best known for their part in demonstrating the principle of competitive exclusion, sometimes referred to as the "Volterra-Gause principle" (Hutchinson 1978), which states that no two species with identical competitive effects on one another can coexist locally. Although this principle was evident in Darwin and Wallace's writings and was formalized by Grinnell, it was not until Gause's strong experimental approach that this axiom became widely known. In fact, competitive exclusion remains one of the most fundamental principles in ecology today (e.g., Chesson 1991, 2000b; but see Hubbell 2001 for arguments to the contrary).
Following these early uses and refinements of the niche concept in both animal and plant ecology, Hutchinson provided a major (sometimes called "revolutionary"; see Schoener 1989) step in the definition and quantification of the niche concept. Hutchinson's definition was first presented in a footnote of a limnological paper (Hutchinson 1944, 20 n. 5). Hutchinson stated, "The term niche (in Gause's sense, rather than Elton's) is here defined as the sum of all the environmental factors acting on the organism; the niche thus defined is a region of an n-dimensional hyper-space." Hutchinson's (1944) "n-dimensional hyper-space" transformed to an "n-dimensional hypervolume" in his now classic concluding remarks at the Cold Springs Harbor Symposium of Quantitative Biology (Hutchinson 1957). In this paper, Hutchinson applied a more quantitative approach to the niche concept than had been done previously by Grinnell and Elton. To do this, Hutchinson explicitly defined any number (n) of limiting factors (e.g., temperature, resources) for a given organism. The quantity of each limiting factor a given organism needs to exist can then be plotted in a hypervolume or space of n dimensions. The space occupied within the n-dimensional hypervolume would thus be the range of conditions where a species could exist (see also Hutchinson 1965, 1978) (fig. 1.2). The n-dimen-sional hypervolume approach provided a conceptually quantitative niche concept, which provided a useful way to succinctly define a niche, as well as to terrify math-phobic introductory ecology students (this definition remains the mainstay of modern ecology textbooks: e.g., Pianka 1995; Begon et al. 1996; Krebs 2000). Around the same time, MacFadyen (1957) presented a similar, but less well developed, description of the niche in multiple dimensions.
Hutchinson (1957) took his quantitative formulation of the niche a step further. As both animal and plant ecologists had long realized, the areas and conditions in which a species could feasibly live were often greater than those where the organism actually lived, and this was typically caused by the effects of interactions among co-occurring species. Thus, Hutchinson termed a species' fundamental niche as all aspects of the n-dimensional hypervolume in the absence of other species, and the realized niche as the part of the fundamental niche to which the species was restricted due to interspecific interactions (we return to this distinction in chapter 3). In so doing, Hutchinson revolutionized the niche concept from the vague constructs of Grinnell, Elton, and others into a quantifiable unit that allowed explicit theoretical analyses and prediction (e.g., Levins 1968; MacArthur 1969; Vandermeer 1972; Schoener 1989).
Hutchinson's other major contribution to the niche concept and its role in understanding patterns of species diversity came in his 1959 address to the American Society of Naturalists entitled "Homage to Santa Rosalia, or why are there so many kinds of animals?" (Hutchinson 1959). In this address, Hutchinson weaves a wonderful story, beginning with a visit to a small artificial pond below a cave thought to house the remains of the Italian saint Santa Rosalia. In this pond, as throughout much of Europe, Hutchinson noted the coexistence of two abundant species of Corixidae (Hemiptera) insects (commonly referred to as water boatmen) (fig. 1.3). In reflecting upon why there were only two common species in these ponds, Hutchinson began to wonder about the causes and limits of the number of species we see. Giving rise to what is now a blossoming area of research on determinants and consequences of biological diversity (e.g., Ricklefs and Schluter 1993; Rosenzweig 1995; Gaston 2000; Hubbell 2001), Hutchinson discussed several factors that might either cause or limit the amazing diversity of plants and animals in nature. Armed with his own niche concept (Hutchinson 1957), he set out to determine what limited the similarity of coexisting species. In looking at a variety of size ratios, he found what he termed a tentative relationship--when two similar species coexist, the mean ratio of the size of the larger to the smaller is 1.3:1. This soon became known as the Hutchinsonian ratio (Lewin 1983) and consumed much of the creative interests of both theoretical and empirical ecologists for many years (Karieva 1997).
MacArthur and his collaborators greatly expanded Hutchinson's approach and motivated an extraordinary amount of creative energy by ecologists in the 1960s and 1970s. This led to a large body of work on what is now known as niche theory, and the concept became firmly ensconced in most problems of ecological study. Niche theory was essentially a group of theoretical models designed to investigate how many and how similar coexisting species could be within a given community (MacArthur 1969, 1972; MacArthur and Levins 1967; May and MacArthur 1972; for a review, see Vandermeer 1972). They were invariably based on the Lotka-Volterra equations and were coupled with the widespread view that interspecific competition was very important in structuring natural communities. This theoretical framework drove field ecologists to measure niche breadth (i.e., the variety of resources or habitats used by a given species), niche partitioning (the degree of differential resource use by coexisting species), niche overlap (the overlap of resource use by different species), and niche assembly (the colonization and organization of species with different resource use in new or abandoned habitats) in natural communities (see e.g., MacArthur 1969; Schoener 1968, 1974a; Colwell and Futuyma 1971; Roughgarden 1972; Pianka 1973; Diamond 1975). Niche theory consequently became the focus of a number of studies on topics from the realms of evolutionary ecology, population and community ecology, and biogeography.
Hutchinson, MacArthur, and others used the idea of competition for resources as the primary underlying mechanism driving ecology. Although both Hutchinson and MacArthur also considered many other factors, such as predation and environmental variability, subsequent authors focused on their work on resource competition. The word "niche" became firmly entangled with the notion of interspecific competition (fig. 1.4). Prior to 1965 less than 50 percent of all articles that contained the word "niche" also had "competition" (though during the ten-year period 1936-45, this association did exceed 50 percent but then declined), suggesting that authors used "niche" quite often without considering competition. However, by 1975 this figure increased to around 80 percent, where it has remained. We explored the connection between niche and two other common species interactions, predation and mutualism, and found no similar strong correlations. Given the strong linkage between niche and competition, it is not surprising that as resource competition has fallen out of favor as the overriding factor influencing community structure, so has the niche concept.
1.3. The Downfall of the Niche Concept
The ardent way that many ecologists approached niche and competition theory brought about a revolution in the way that ecological questions are asked and answered. The backlash that resulted from the boom of niche theory in the late 1970s was couched in terms of the philosophy of science, in particular the hypothetico-deductive methodology espoused by Popper (e.g., 1963), which emphasizes the comparison of null hypotheses (i.e., the result expected under random conditions) with the alternative hypotheses of interest (i.e., the result due to some condition of interest). Simberloff, Strong, and their colleagues rightly pointed out that the explosion of studies exploring patterns of competition and niche theory were typically without adequate null hypotheses (e.g., Simberloff 1978; Connor and Simberloff 1978, 1979; Strong et al. 1979; Simberloff and Boecklen 1981). Thus, the validity of scores of studies examining patterns of character displacement, coexistence and diversity, and biogeography based on competition and niche theory were brought into question. Lewin reviewed the professional and personal acrimony that surrounded this debate in a provocative article, "Santa Rosalia was a goat" (1983), the title referring to the discovery that the bones thought to be those of Santa Rosalia, whom Hutchinson (1959) dubbed the patron saint of diversity studies, were actually those of a goat (see Simberloff and Boecklen 1981).
Most researchers eventually conceded that appropriate null hypotheses and experimentation were necessary in order to disclose the importance of competition and niche theory (see, e.g., Gotelli and Graves 1996). However, the debate about the form of null models remains contentious (see, e.g., Losos et al. 1989; Brown et al. 2000; Stone et al. 2000). A particularly caustic exchange between the groups can be seen in the discussion between Connor and Simberloff (1984) and Gil-pin and Diamond (1984), where the line between scientific discourse and name calling became blurred. The backlash created by this exchange arguably changed the face of ecology (e.g., Lewin 1983; Col-well and Winkler 1984; Harvey et al. 1983; Gotelli and Graves 1996; Brown 1997). Among other things, the ubiquity of resource competition has been downplayed for a more pluralistic view that includes the role of other factors, such as predation and abiotic stresses. Furthermore, hypothesis testing and rigorous statistics, as well as a strong emphasis on experimental approaches, have emerged to the forefront of ecology. Despite this positive response, the emphasis on experimental and statistical rigor necessarily created a focus on smaller-scale, shorter-term processes that were amenable to such manipulations. Studies of large-scale diversity, range, abundance, and the like were diminished, while studies of local interactions and processes were emphasized (see, e.g., Brown 1995; Maurer 1999).
Even before traditional tests of niche theory were being criticized in the empirical literature for their lack of statistical rigor and null models, and experiments were pushed as the primary way to discern the importance of competititive interactions and coexistence, theoreticians were refining competitive models to more explicitly incorporate the mechanisms of species interactions. MacArthur (1972) showed that the competition coefficient and carrying capacity of the Lotka-Volterra equations could be linked with more mechanistic consumer-resource models under certain limiting assumptions. Others, including Schoener (1974b) and Abrams (1975, 1983), more specifically discussed niche overlap and the limiting similarity of coexisting species in the context of the linkage between the Lotka-Volterra models and more mechanistic approaches. Although in the limiting case, Lotka-Volterra models and mechanistic consumer resource models converge to similar structure (see also Tilman 1982; Petraitis 1989), theoreticians also began to explore the limitations of the earlier niche theory.
From this, a diverse body of theoretical literature arose that discussed several aspects of species niches and coexistence as it related to competitive interactions (e.g., Abrams 1977, 1980, 1986), the influence of predation (e.g., Holt 1977, 1984, 1985, 1987), and intrinsic and extrinsic spatial and temporal heterogeneity (e.g., Armstrong and McGehee 1976, 1980; Chesson and Warner 1981; Chesson 1985; Warner and Chesson 1985). However, these advances in theory were not well connected with much of the empirical focus. This was most likely the result of two related factors. First, empiricists were focused on testing very basic hypotheses about the presence or absence of the effects of interspecific interactions (mostly competition) in experiments or against statistically rigorous null models. Second, empiricists were skeptical of the utility of theory in the aftermath of the hostile reaction to niche theory discussed above.
A reasonable caricature of how science progresses can be seen in the dialectical approach (e.g., Levins and Lewontin 1980). The dialectical cycle begins with a thesis, followed by skepticism or antithesis, and finally synthesis. We view the niche concept as having progressed in a dialectical manner (fig. 1.5). Beginning with the formulations of Grinnell and Elton, the niche concept became a full-blown thesis with Hutchinson and MacArthur. The flaws of this thesis were pointed out by the antithesis led by Simberloff and Strong, who called for more rigorous null models and experimentation. Following this antithesis, empirical ecologists shifted their focus using strict philosophical and statistical ideals and well-controlled experimentation. However, in the backlash of this antithesis, many questions of broad ecological interest related to the niche concept have fallen to the wayside.
1.4. Revisioning the Niche Concept
The history of the niche concept draws on several important issues that must be resolved in order for it to have a useful, synthetic role in ecology. First, it needs to incorporate processes other than resource competition. While previous concepts of the niche have often had this broader perspective, today the niche concept is often too closely aligned with competition even though theoretical and empirical ecologists have embraced other processes. Second, the concept needs to clarify the distinction between things that describe how an organism responds to the environment (implicit in concepts focused on requirements such as Grinnell's and Hutchinson's) and things that describe how an organism alters its environment (implicit in concepts focused on impacts such as Elton's and MacArthur's). Finally, the idea of the niche needs to be made relevant to multiple spatial scales. Current niche theory is often too narrowly focused on explaining species interactions at the local scale (where population dynamics are the only processes present), but many of the more important and challenging ecological questions occur at larger scales (where colonization dynamics, invasions, and the like become quite important).
In order to put the niche concept into the context of a synthesis for ecology, we propose a revised definition. First, we give a broad definition:
NICHE DEFINITION #1: the joint description of the environmental conditions that allow a species to satisfy its minimum requirements so that the birth rate of a local population is equal to or greater than its death rate along with the set of per capita effects of that species on these environmental conditions.
This definition is a simple joining of the two concepts that we have outlined in our historical review. However, we also want to use this definition to develop a tool that can flexibly address and help synthesize the wide variety of ecological phenomena under current study.
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Table of Contents
Chapter One: Introduction: History, Context, and Purpose
Chapter Two: Revising the Niche Concept: Definitions and Mechanistic Models
Chapter Three: Comparing Classical and Contemporary Niche Theory
Chapter Four: Designs and Limitations of Empirical Approaches to the Niche
Chapter Five: Incorporating Biological Complexities
Chapter Six: Environmental Variability in Time and Space
Chapter Seven: Species Sorting in Communities
Chapter Eight: Community Succession, Assembly, and Biodiversity
Chapter Nine: Niche Relations within Ecosystems
Chapter Ten: The Evolutionary Niche
Chapter Eleven: Conclusions