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CHAPTER TWO Concepts of Niches It has often been pointed out that the term “niche” disguises several concepts under a single label (Whittaker et al. 1973, Colwell 1992, Leibold 1995, Chase and Leibold 2003, Odling-Smee et al. 2003). Some authors, perhaps overwhelmed by the broad variety and subtle shades of meaning assigned to the word, have advised that “niche is perhaps a term best left undefined” (Bell 1982). We disagree: in science, arguments benefit from precise and consistent usage of key concepts; otherwise, clear thinking is hindered. Using the same word to refer to different ideas leads to confusion, as the picturesque history of the word “niche” amply demonstrates. Besides, different senses of niche are appropriate to deal with different biological problems. As a consequence, several niche concepts exist, and our first task is thus to clarify and specify which concept we will be using and why. We will choose terminology and ideas best suited to the problem in which we are interested—namely, that of estimating and understanding areas of distribution of species. One reason why “niche” has acquired a veritable bush of meanings is that, since the first time it was used, ecologists have applied the term to analyze a very complex question: what combinations of environmental factors allow a species to exist in a given geographic region or in a given biotic community, and what effects does the species have on those environmental factors? Not only does the preceding statement refer to an intrinsically complex set of problems , but several of its terms can be interpreted and measured in a variety of ways. Moreover, the concept has been used at both geographic and local scales, most often assuming that the ensuing complications and differences should be obvious. For example, Grinnell (1917), studying the niche of the California Thrasher (Toxostoma redivivum) in relation to its area of distribution (this idea will be discussed later as the “existence” of the species), meant by the term niche the thrasher’s climatic and habitat requirements (the environmental factors) expressed geographically (figure 2.1). In a contrasting interpretation, Elton (1927) viewed the niche as the functional role of an animal on a community (its local effects): the existence was taken for granted, but the emphasis was placed on the “impacts.” These two early views of niche illustrate one of the main 8 CHAPTER 2 causes of ambiguity of concept: stress on requirements at geographic scales versus stress on impacts at local scales (Chase and Leibold 2003). After reviewing the history of niche concepts, Chase and Leibold (2003) concluded that much of the confusion surrounding the term results because “previous authors have not consistently distinguished between the responses of organisms to their environment and the effects of organisms on their environFigure 2.1. Distribution of the California Thrasher (Toxostoma redivivum) in California, from Grinnell (1917). Approximate distributional limits of the three subspecies are shown with different shadings, and occurrences are shown as dots (for specimens) or triangles (for published records). Specimans examined Published records 1. Toxostoma redivivum sonomae 2. Toxostoma r. revivium 3. Toxostoma r. pasadenense CONCEPTS OF NICHES 9 ment.” Indeed, this difference has both deep implications about the actual mathematical form of a multivariate niche definition and serious operational consequences, since certain variables related to requirements can be measured easily, whereas most variables related to impact require ad hoc experimental efforts. Although attempting an exhaustive classification of niche concepts based on the preceding ideas would be interesting, that task is not our purpose in this chapter. Rather, we intend to propose a formal and operational definition of a particular niche concept (which is naturally related to the problem of estimating areas of distribution), offer approaches to characterize and measure it, and use it as a conceptual and terminological basis for describing and understanding much of the related practices of ecological niche modeling and species distribution modeling (Peterson 2006c). In this chapter, we explain the reasons for our choice of emphases, leading to a particular meaning and usage of niche. To accomplish this goal, we review briefly the themes most important in understanding niche concepts, highlighting the meanings most appropriate to the purpose of this book. MAJOR THEMES IN NICHE CONCEPTS Recall Hutchinson’s (1957) definition of the fundamental niche of a species: a hypervolume of environmental variables, “every point of which corresponds to a state of the environment which would permit the species to exist indefinitely.” Most differences in niche concepts depend on the formulation and relative importance given to...


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