An organism-centered approach to some community and ecosystem concepts

J. theor. Biol. (1981) 88,287-307

An Organism-centered Approach to some Community and Ecosystem Concepts JAMES A. MACMAHON, DAVID J. SCHIMPF,~ DOUGLAS C. ANDERSEN, KIMBERLY G. SMITH,$ AND ROBERT L. BAYN, JR. Department of Biology and Ecology Center, U&K 53, Utah State University, Logan, Utah 84322, U.S.A. (Received 22 July 1980) We present a discussion of the ecological concept of the niche based on the perspective of the individual organism, rather than that of a population or species. This discussion is then expanded to include other related ecological concepts such as guild, environment, habitat and functional group. Using the individual as the focus permits the development of a system of concepts which, we believe, approximate the way that ecological interactions occur in nature. 1. Introduction In a recent attempt to clarify and codify some confusing biological terminology, MacMahon et al. (1978) addressed the meanings of the terms

ecosystem and community in an organism-centered context. The two terms were interpreted to apply to coevolutionarily interacting organisms (communities) and entities related by the flow of matter or energy (ecosystems). Specifically, these authors defined community as “a group of coevolutionarily interacting populations connected by effects of one population on the demography or genetic constitution of the other” and ecosystem as “the plexus composed both of abiotic entities and at least one organism which are united by the exchange of matter and energy”. It should be noted that MacMahon et al. (1978, p. 703) stress two possible interpretations of community: the aforementioned which deals with the community as an aggregation of populations and another interpretation where the community includes only those organisms coevolutionarily affecting, directly or indirectly, a central organism of interest (termed hereafter the reference organism). This latter perspective has different implications from the t Present address: Department of Biology, University of Minnesota, Duluth, MN 55812, U.S.A. $ Present address: University of California, Bodega Marine Laboratory, Bodega Bay, CA 94923, U.S.A. 287

0022-5193/81/020287+21$02.00/0

@ 1981 Academic Press Inc. (London) Ltd.

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former, and is the approach we adopt herein. We emphasize that either approach is appropriate within the scheme presented in our current discussion, with some modification of emphasis. Other recent works have likewise adopted an organism-centered perspective (Lamotte, 1979; Webster, 1979; Wilson, 1980, p. 1). A particular organism may participate in both a community and an ecosystem in a functional sense. Other organisms may be components of both the community and the ecosystem of this individual, but it is at least possible that not all of the organisms connected by the flow of matter or energy are coevolutionarily related. Conversely, organisms can coevolutionarily affect one another without exchanging appreciable quantities of matter or energy. We emphasize the extreme of this definitional system in Fig. 1 where no component-of the ecosystem or community is common to the other, except the reference organism. Community

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If one accepts these terms as defined, it is possible to expand their implications to other aspects of the terminological trauma in ecology. Specifically, we address ourselves herein to the often discussed but seldom defined “structure and function of communities and ecosystems”, and terms related to these concepts such as niche, guild, and functional group. For purposes of discussion we shall address the structure and functioning of communities separately from those of ecosystems. Despite this division we realize that since an organism belongs to both ecosystems and communities, there must be overlap of their structure and function.

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Our goal here is to present to our colleagues an alternative approach to some ecological concepts, based on a perspective centered on the individual. We suggest the usefulness of our approach by presenting examples of its application to diverse examples from extant literature. We take this approach to show the generality of this framework rather than developing a single detailed example. 2. Community

and Ecosystem Structure and Functioning (A)

COMMUNITY

STRUCTURE

The view of community accepted here is one of an aggregate of biotic components, i.e. organisms, demes, or populations, centered on a reference organism. The structure of a community is the pattern in space and time of the attributes of these components. Caswell(l976) uses a parallel definition. Attribute patterns which can be important in a coevolutionary context include those of genotype, age, sex, phenotype, or taxon membership. Often, these patterns will be more meaningful at the population or deme level. For example, the distribution of adrenal X zone cell types in a population of small mammals may be a coevolutionarily important attribute (Christian, 1963), while it is probably a meaningless measure, in the same context, for other animal populations in the community or for the community as a whole. Similarly, the age distribution of plants may not be important in size-related phenomena where two plants of different age are of the same size and have similar coevolutionary effects [e.g. competition for water between plants overlapping in their resource contours (Harper, 1977)]. Meaningful research on community structure will typically emphasize spatial density or dispersion patterns of only some of these attribute categories. For example, many workers have used measures of foliage spatial diversity of perennials as independent axes correlating with some measure of animal species diversity. (B)

COMMUNITY

FUNCTIONING

The processes occurring in a community are the coevolutionary interactions among the community components. In the simplified case organisms have only positive, negative or neutral responses to interactions with one another. Thus a modification of the matrix presented by Odum (197 1) is an appropriate generalization of possible community processes (Fig. 2). Levins (1975) addresses the power of such a simple matrix approach. Note that since completely neutral interactions (neutralism) have no coevolutionary

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FIG. 2. The coevolutionary interactions (sensu MacMahon et al. 1978) between two organisms. Terminology and general matrix approach modified after Odum (1971).

effects we ignore them. We also specifically make no distinction between facultative and obligatory relations. It should be noted that we have avoided the phrase community function because it connotes purpose. Communities in our view have no function per se ; rather, they are functioning because of the various biological processes. (C)

ECOSYSTEM

STRUCTURE

The structure of an ecosystem is defined by the ecosystem components and their patterns in space and time-the “stuff” exchanging matter or energy in nature. Two types of components are present: biotic components, which are organisms linked by the exchange of matter or energy, and abiotic components, which are all the non-living entities that are part of a particular ecosystem. The pattern of ecosystem components refers to the distribution of both the biotic and abiotic components in space and time. Attributes used to describe a biotic pattern may be the same as those previously listed for community structure. Since the biotic components of the community and the ecosystem need not include the same organisms, a study of the patterns of the same attribute in each might produce disparate results. Organisms related to our reference organism by neutralism do not figure in community structure, but could pertain to ecosystem structure. In the case of abiotic components, what constitutes “a component” depends, in part, on the specific ecosystem being studied. For example, the compounds in which a particular element occurs at any given time can be quite important to certain biotic components; the presence of purple bacteria depends largely on the presence of certain sulfur compounds rather than sulfur per se (Stanier, Adelberg & Ingraham, 1976, p. 548). Thus, the nature of the entity one

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chooses to term an abiotic component being addressed. (D)

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is not independent

of the question

FUNCTIONING

The processes in the ecosystem are the transfers of matter or energy involving one or more ecosystem components. These transfers collectively comprise ecosystem functioning. Since ecosystem boundaries are investigator-determined (MacMahon, et al. 1978), functioning may include transfers across those boundaries as well as exchanges within the ecosystem (Fig. 1). We specifically assert that the patterns of such transfers may be of interest. We view the major processes in an ecosystem as involving the uptake and release of matter and energy by the biotic and abiotic ecosystem components. Examples of these processes are given in Fig. 3. For an alternative presentation see Odum (1962). Ecosystem component Biotic

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3. Environment,

Habitat,

Niche and Related Terms

Analysis of community and ecosystem structure and functioning leads directly to a consideration of several other concepts included in the contemporary ecological literature. Herein we attempt, using the constant set of premises outlined above, to develop and examine the paradigm that proceeds from our approach to communities and ecosystems. We contend that by rigidly adhering to first premises (the definitions of community and ecosystem), certain other vexing terms take on clearer and biologically more appropriate meanings.

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ENVIRONMENT

In 1973 Spomer discussed a term, operational environment, originally proposed by Mason & Langenheim (1957). In Spomer’s context the total surroundings of an organism are its general environment and the restricted subset containing all environmental factors directly affecting an organism is its operational environment, an approach we use hereafter. The general environment may include entities outside the ecosystem, depending on the boundaries chosen for the latter (Fig. 4). : .. . . . i -+-

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FIG. 4. The diagrammatic representation of the general (. . .) and operational () environments of an organism. The organism’s ecosystem (- - -) is as defined in the text. Components from other ecosystems (EC’) may be part of an organism’s general environment. N.B., In this figure we have included the operational environment wholly within the boundaries chosen for the organism’s ecosystem, a condition which does not always obtain.

Only those factors sensed by or affecting an organism can be part of its biologically important environment (Maelzer, 1965a, see also Patten, 1978). In essence we may apply source-sink terminology and state that the operational environment of an organism is the set of all entities within the general environment that are a source of, or a sink for, the exchange of matter or energy with that organism (see Fig. 4). Thus, the operational environment includes all things directly impinging on an organism, which may include emergent properties (sensu Salt, 1979). We include emergent properties because in some cases there is a very small exchange of matter or energy between an organism and its ecosystem components, yet arrangement of the components can be important to the organism. A case in point is the effect of locale architecture on bird nest-site selection (James, 197 l), an example in which the pattern of ecosystem components has an effect on an organism disproportionate to the quantity of energy exchanged between them.

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HABITAT

In our view, habitat is an artificial category, used by scientists to describe/predict where organisms might occur (Odum, 1971, p. 234). In practice, habitat becomes a short list of entities thought to correlate, with a high reliability, to the presence or absence of an organism, population, species, life form, etc.-a best guess that most likely includes some parts of the operational environment and may include other ecosystem components and entities of the general environment. It is clearly not an organismcentered term. For example, the statement “this looks like good deer habitat” implies a judgment about an area without direct knowledge about the presence of deer. The Sonoran Desert and the Florida Everglades can both be considered white-tail deer habitat, though the operational environments of deer in those areas differ radically. The arbitrariness of the descriptors chosen is clear in definitions such as that given by Partridge (1978), who uses habitat “. . . to describe the conglomerate of physical and biotic factors which together make up the sort of place in which an animal lives.” In a recent discussion of the relationships among the concepts of niche, habitat and ecotope, Whittaker, Levin & Root (1973) define habitat with respect to actual responses of populations, i.e. habitat is described by the species’ population response to habitat variables (physical and chemical factors). Their concept of habitat, as we interpret it, most closely approaches a subset of the general environment as discussed herein. They later specifically permit a role for competition and other species interactions as factors relating to habitat (Whittaker, Levin & Root, 1975). (C)

NICHE

In textbook definitions to which most ecologists have been exposed, Odum (1971) defines niche as the “profession” of an organism and habitat as “the organism’s address”. Both definitions are clearly organism-centered, and thus differ from more rigorous attempts to remove ecology from a terminological conundrum (e.g. Whittaker, Levin & Root, 1973; Kroes, 1977; Hutchinson, 1978). Throughout the numerous discussions of niche, we perceive two general emphases, which we term the actual and the potential. Using the most common terminology, the potential emphasis identifies the fundamental (pre-interactive) niche, defined as “, . . primarily without reference to competitors, but merely in terms of requirements and tolerances . . .“, while the actual emphasis identifies the realized (post-interactive) niche, which is the niche remaining “When a second competing species has been able to

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take over a part of the niche . . .” (Hutchinson, 1978, p. 210). It is not our purpose here to rehash definitions, details of interpretation, or to correct misunderstandings occurring in the literature. For such information and for historical perspectives, we refer the reader to Vandermeer (1972), Colwell & Fuentes (1975), Whittaker & Levin (1975), and Hutchinson (1978). McIntosh (1970) points out that there are a number of ways to bound niches, each based on a different aspect of a species’ biology. Using what we interpret as a potential-niche emphasis, he recognizes four kinds of bounds: (1) “That dictated by physiological capacity, or all states in which it (the species) can exist-the ‘prospective’ niche of Valentine (1968).” (2) “That in which it can survive . . . in the natural habitat.” (3) “That in which it can reproduce in nature.” (4) “That in which it can maintain itself indefinitely as a component of a community-the ‘fundamental niche’ of Valentine (see Hutchinson, 1957).” Any criterion relating to an organism’s performance may be applied for our discussion of potential niches. All such criteria would lead to response surfaces, often complex, where the organism’s performance is the response to the niche axes, To avoid assessing degree of response or choosing which of the many types of responses to use, we will use the less ambiguous life-death demarcation. We view “niche” as a generic term which may be modified to describe the actual state or several potential states of an organism at an instant in time. By “state” we specifically mean “the sum of the qualities or characters involved in a thing’s existence” (Anon., 1959) and not the internal condition of the organism. [Hutchinson (1957, p. 416) also uses “state” with regard to niche.] The states of an organism are affected by two sets of relationships. One set is determined by the operational environment, as discussed above (see also Fig. 4); the other is determined by the components of the organism’s community which have a direct coevolutionary effect on the reference organism. We term this latter set the “proximate community” (Fig. 5), members of which may also be part of the operational environment. Thus, states of the organism are determined by the components of the organism’s proximate community and operational environment, via the interactions and flows between the organism and those components (Fig. 6). Our approach leads to the belief that there are no “empty niches” awaiting tenants, although we do not imply by our rejection of “empty niches” that the community is closed to invasion. This follows because identification of the proximate community requires a reference organism, and the delineation of the operational environment depends upon the characteristics of that reference organism (Spomer, 1973). Kroes (1977, p. 324) presents a parallel idea for the species rather than the organism. An implication of our view is that niche is an attribute of the organism, a position

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consistent with our organism-centered view and the organism-centered approach to environment of Maelzer (1965b) and Spomer (1973). Previously niche has been used in the context of the population or species (e.g. Hutchinson, 1957,1978; Valentine, 1968,1973; Whittaker, Levin & Root, (1973). Certainly, potential-niche variation exists within species, and may sometimes be significant between conspecific populations in close spatial proximity (e.g. Musselman, Lester & Adams, 1975). For the following discussion we refer to Fig. 7, which depicts the potentialand actual-niches of an organism at two instants during its life. While the Niche determinants

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FIG. 6. The diagrammatic representation of the determinants of an organism’s niche. This is a magnification of the features in Fig 4 and 5 which are relevant to the niche. For the relation of the niche to larger scale systems, refer to Figs 4 and 5.

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FIG. 7. Actual- and potential-niches of an organism at two instants (A and B) in its life, considering two environmental factor axes.

organism’s independent relationships to two environmental factors are illustrated, the organism has simultaneous, not necessarily independent, relationships with a multitude of factors which define a hyperspace. An organism has an hereditarily defined upper and lower limit of tolerance to any environmental factor at any time in its life (Fig. 7). While the most easily visualized examples of tolerance ranges usually involve abiotic factors, such as temperature or concentrations of salts, we emphasize that organisms have analogous tolerance ranges involving any component of the operational environment or community, e.g. physiological or behavioral intolerance to frequent social encounters. Certain maturational events may have irreversibly fixed a new set of tolerance limits within the inherited tolerance extremes (Fig. 7). For example, this pertains to the drought tolerance of plant leaves as a function of past water availability (Cutler, Rains & Loomis, 1977). Reversible physiological or behavioral processes, which we subsume under the term “acclimatization”, may restrict an organism, for a time, to a smaller portion of the maturationally defined range. Thus, if the organism were suddenly displaced to an environment within its developmentally constrained tolerance range, but outside its acclimatized range, it would die. Given a gradual shift to this new value of the environmental variable (i.e. sufficient time to acclimatize), the organism could survive. Such is the case with the general process of “hardening” in plants (Levitt, 1972). Even though at any instant an organism has an hereditarily determined range of potential tolerance, and within that a more limited maturationally

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determined range, and within that an acclimatizationally determined tolerance range, it nonetheless experiences but a single value within that range at any moment (Fig. 7). This single point, in relation to all factors simultaneously affecting the organism, describes the organism’s realized niche. At first blush, the use of realized niche to describe an instant in time may seem to be contrary to tradition, but in his detailed exposition of niche, Hutchinson (1957, p. 417) stated that a limitation of the set-theoretic mode of expression is “3. The model refers to a single instant of time.” Thus, the realized niche is the only actual niche. If one integrates all instants over an organism’s life, a representation such as Fig. 8 emerges. Note that the tolerance ranges of an organism may be hereditarily programmed to vary throughout its life cycle (Fig. 8). For example, the genetically defined temperature and salinity tolerances of a tadpole and the frog into which it metamorphoses may be different. Thus, certain life cycle stages might have narrower ranges of inherited tolerances than other stages, producing ontogenetic bottlenecks. We have depicted such a bottleneck in Fig. 8 (at time B).

FIG. 8. The diagrammatic representation of an organism’s actual- and potential-niches through time, The heavy outer lines bound the prospective niches. The inner light lines bound the maturationally restricted niches. The dotted line depicts the realized niches of the organism through time. Arrows A and B indicate the planes (each perpendicular to the time axis) on which the organism’s instantaneous niches are depicted in Fig. 7. For visual clarity the acclimatizationally restricted niches are not depicted.

The temporal changes in the hereditarily-based tolerance limits to all )z factors affecting the state of the organism during its lifetime determine the shape of a hypervolume of dimension IZ+ 1, the additional dimension being

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time (Harper, 1977, p. 717). Within that hypervolume lie all the states in which the organism might persist. We believe that our concept of this most extensive, hereditarily bounded, niche is somewhat similar to what Valentine (1968,1973) termed prospective niche. Although Valentine referred to populations, we adopt his term for an organism. In Fig. 8, the prospective niches of an organism, integrated throughout its life, are represented by the largest of the hourglass-shaped volumes. The hereditarily bounded tolerance ranges shown in Fig. 7 delineate the prospective niche at two points (A and B) in an organism’s life. The prospective niche is the only aspect of an organism’s potential existence that we could know a priori if we could decode its genetic information. Because of subsequent stochastic tolerance-constraining events, it is perhaps only at syngamy (or other life starting event) that an organism’s realized niche might occur anywhere within its prospective niche. The maturationally constrained tolerance ranges within the prospective niche represent the sets of conditions outside of which an organism could not endure. This region (Figs 7 and 8) is similar to what Kroes (1977) termed ecopotential and what Hutchinson (1978) and we term fundamental niche. Although it is not explicit that these other workers considered the fundamental “niche” capable of change during the organisms’ lifetimes, such a notion is implicit in Hutchinson’s work. The portion of the fundamental niche that could be occupied over any short period (the acclimatization range) apparently has not been named by workers to date. We term this portion the acclimatized niche. As noted above, the realized niche of an organism is represented by the single point (Fig. 7) defined by the state of the organism at any instant during its life. Clearly, the realized niche of an organism varies over time (Fig. 8). The characteristics of the niche of an individual bird migrating between North and South America are different on each continent. Functionally, only the niche occurring “now”, the realized niche, is important to the organism. It has survived past actual-niche associations and has not encountered tomorrow?. Yet for integrative purposes we may want to take all of the year’s realized niche variation into account in our discussion of the biology of that bird. In practice most workers have used realized and potential niche as including aggregates of organisms through time. This is dictated by the difficulties of studying many organisms individually at an instant in time. While cognizant of the practical difficulties of studying the realized niche of an organism, we suggest that assignment of the niche to the organism, rather than the population, results in greater conceptual potential for understandingmulti-organism units [see e.g. Harper (1977, p. 707) for discussion of this

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idea]. Two different populations might have similar means and extremes of tolerance limits, yet one population might be composed of only widetolerance generalists while the other is polymorphic with narrow-tolerance specialists (Roughgarden, 1974). While interest in “between-phenotype” vs. “within-phenotype” components of niche breadth is growing (e.g. Pianka, 1978, p. 256), the traditional population-centered niche concept tends to obscure these phenomena. Examples of intra-population differences in potential niches include those due to sex (Selander, 1966; Freeman, Klikoff & Harper, 1976), age, genetic (Ford, 1975) or somatic (Flint & Palmblad, 1978) polymorphism, and differences among demes (Edmunds & Alstad, 1978) or between parents and offspring (VepsilZinen & Jarvinen, 1979). The organism-centered niche, unlike the populationcentered version, demands that these other members of the organism’s population be represented by a niche axis. Whether or not these differences in potential niche are great, the members of a population will have different actual niches due to environmental heterogeneity. Niche has been an object of interest to community ecologists; we encourage the recent trend which suggests that it is an equally valid province for population biologists. Because we view the community as an interacting assemblage without obvious spatial bounds (MacMahon et al., 1978), we are unable to separate the interactions involving “species” into between- and within-community categories, as has been proposed by Whittaker, Levin & Root (1973). If, as we suggest, habitat is not an organism-centered term, then we see no reason to attempt to conceptually unify niche and habitat, obviating the need for a term such as ecotope (Whittaker, Levin & Root, 1973, 1975; Kulesza, 1975). However, one of the values of approaches such as that of Whittaker, Levin & Root (1973) is an emphasis on population response, an emphasis possible within our framework. Further, our approach de-emphasizes that somewhat nebulous grouping, the species, as Vandermeer (1972) does by using OTU’s. Our scheme permits one to extend the niche concept to groups of interacting organisms. We prefer to speak of the niches (not niche) of a deme, a population, a species, a community, or biotic components of an ecosystem, rather than erecting a hierarchy of niche-related terms which might culminate in Colwell & Fuentes’ (1975) “columbarium”. To place our concept of niche in perspective, we note that previous workers have considered niche in reference to groups of differing sixes and/or over various time periods. A general matrix (Fig. 9) depicts the possible emphases which may be placed on .“niche”. We emphasize the instantaneous relationships of the organism, but our approach does not negate the further subdivision of any of the matrix axes, allowing, for

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FIG. 9. General matrix representing the various emphases that have been placed on the niche concept. The shaded column indicates the approach we emphasize in this paper.

example, division of group (Fig. 9) into demes or populations, or division of integrated time into various life history stages. Such a life history stage division is embodied in the concept of the plant’s “regeneration niche” (Grubb, 1977). An example may show the utility of our niche concept. A turtle ovum is fertilized, covered with a shell and deposited in a nest near a pond or a river in eastern North America. The developing turtle has a certain genetic potential, inherited from its parents. For example, its potential to endure different temperatures may be great and its maturation rate may be such that it reaches sexual maturity in from 3-6 years, etc. This potential for all responses is its hereditarily determined (prospective) niche. The vagaries of the abiotic environment surrounding the nest site, beyond control of the embryo, now restrict that turtle’s potential to a much smaller subset of its genetic potential. A case in point is that the sex of several reptile species, but for our purposes a map turtle (Gruptemys), is determined by the incubation temperature of that nest site (Bull & Vogt, 1979). Whichever sex the temperature favors, that individual, for the rest of its life, can only use that portion of its genetic capacity defined by its sex. The turtle thus has a maturationally defined potential less than its genetic potential. This is a stark case of an ontongenetic bottleneck as depicted by arrow B (Fig. 8). If

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femaleness were fixed, time to sexual maturity might be fixed as twice that of males [6 vs. 3 years (Bury, 1979)] as would total adult body weight. With the greater body weight might come differences in metabolic rate (Bennett and Dawson 1976), feeding habits and competitive advantage (Bury, 1979). Even this metabolic rate will vary, depending on the recent previous thermal history of the individual (the acclimatization niche). In a sense the realized niche is as Lachaise (1979) describes, “, . . the trade-off between selective pressures which act to limit expression of the genetic potentialities of a deme in a definite habitat and at a given time”. We would substitute organism for the word deme. (D)

RELATED

TERMS

A term of recent prominence, used in relation to the concept of niche, is “guild”. Although guild was used by early ecologists in several ways (e.g. Schimper, 1903; Clements, 1905, p. 206), current usage stems from Root’s (1967) application of this term to denote “A . . . group of species that exploit the same cIass of environmental resources in a similar way.” The imprecision of the term “similar way” leads to at least one conceptual problem. If from this term we infer that organisms must use a resource in a behaviorally equivalent manner, then most guilds would include only organisms which are close taxonomic relatives, i.e. taxocenes such as leaf-gleaning birds. On the other hand, if one adopts the broader view that the criterion for “similar way” should be based on the effect of resource use on the state of the resource, then that guild would include all organisms, regardless of taxonomic or behavioral similarities, which remove the invertebrate mesofauna from leaves. Such competition, between distantly related and behaviorally distinct taxa, has been documented by Brown & Davidson (1977). Consistent with the above is a resource-centered guild which includes all organisms using a particular resource class. A resource class does not necessarily correspond to a single niche axis common to all the organisms exploiting that resource. An organism interacting with the resource class through at least one of its niche axes is a member of the guild. Thus, it does not matter whether an organism removes a tree leaf for nesting material, for food, or as substrate to grow fungi which in turn are eaten; the leaf is gone and the leaf users belong to a common guild. We note that while any use of a leaf has an equal effect, i.e. the disappearance of one leaf from the leaf pool, the results of use by various guild members may have differing ecosystem consequences. The leaf eater, via feces, may change nutrient cycling in one way, while the burying of leaves and their decomposition by fungi may have

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different consequences. This emphasis leads one to first choose the resource and then consider all organisms as potential members of the guild centered around that resource, regardless of taxonomic affinities. An arbitrary element in the guild concept arises from defining the resource class. For example, seeds are a resource for a variety of desert consumers. Is the resource class all of the seeds on an area? seeds of only some of the plant species?; seeds of particular sizes?; seeds in particular microsites? Clearly, any of these might be the appropriate resource class, depending on the investigators’ frame of reference. We feel that all organisms using a resource class are included in the guild centered about that resource class, but examination of subsets of that guild may be of value in elucidating community organization. The guild subset including only members having competitive coevolutionary relationships results in each member having the others as community components in the organism-centered community. Less restrictive selection of the functional relationships can provide for memberships from separate communities, but still ensure that members hold at least one component of their operational environments in common, i.e. the resource. It is with this less-restricted criterion that guild subsets generally become interesting to the ecologist, since competitors, potential competitors and descendants of past competitors would be members. Such an approach also precludes the conceptual problem of deciding whether a guild subset based strictly on competitive relationships exists whenever competition for that resource is not occurring. Some workers have equated guild with the term “functional group” (e.g. Root, 1975; MacMahon, 1976). We suggest that a functional group consists of those biotic components of ecosystems which perform the same function or set of functions within the ecosystem. What constitutes “the same” function is decided by the investigator. With this definition, guild may equate to functional group if the function chosen is use of a particular resource. If the function chosen is some other matter/energy transformation, then the functional group may not be equal to a guild. For example, if the function is uptake of nitrate, then one can speak of the nitrate-using guild or functional group. If the function is the conversion of nitrate to other oxides of nitrogen or to dinitrogen gas, then the functional group (denitrifiers) is a subset of the nitrate-using guild. Conversely, guilds which are subsets of a functional group can be envisioned. A functional group may be broadly drawn, such as atrophic level, or quite specific, such as the organisms which concentrate selenium in their tissues. An organism may belong to more than one such group, just as it may be a member of several guilds. Neither guild nor funcational group need include more than one species, and perhaps not even all conspecifics (e.g. male vs.

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female mosquitos). Thus, functional group is analagous to, but not synonymous with guild. Each category has its membership defined by the investigator’s somewhat arbitrary choice of central focus: a resource class for guild and a function for functional group. Whether a given guild is also a complete functional group, a subset of a functional group, contains numerous functional groups or is not related to functional groups at all depends entirely on the investigator’s approach. The functional group concept seems to be useful for examining an ecosystem for its “redundancy” in the performance of what is thought to be an important function (MacMahon, 1976). In a diagram of a compartmentflow model of an ecosystem, a functional group is the set of organisms affecting a common matter/energy flow control point or points. The aggregation of diverse organisms into biologically based “black boxes” for modelling purposes can reduce an ecosystem’s bewildering complexity to “mathematically tractable proportions” (Botkin, 1975). Our meaning of functional group differs from that in Botkin (1975), where a criterion of having similar environmental tolerances was suggested. We prefer not to apply such a standard for functional group membership, since the variety of environmental conditions under which a particular function can be performed would seem to be an important characteristic of a given functional group (MacMahon, 1976). We believe that functional taxonomies which produce aggregates such as guilds and functional groups are desirable. However, we do not view such arbitrary groupings as natural parts of a formal system of ecological concepts, but rather as useful descriptors of subsets of components or relationships within the system. We do not necessarily suggest that workers drop the various other useful terms for lumping certain components or relationships within or between communities or ecosystems. For example, at one scale, “component community” (Root, 1973) has been used to describe highly integrated “within community” aggregations. On a larger scale, “ecological equivalents” (Odum, 1971; Krebs, 1978), inferred to be functionally similar organisms produced through convergent evolution in environmentally similar areas (formations or biomes), conveys a certain amount of ecological information. 4. Concluding

We have reconsidered recent organism-centered et al., 1978). We believe definition of the following consistent manner.

Remarks

several ecological concepts in the context of a approach to biological organization (MacMahon that the application of this system permits the terms in a biologically realistic and internally

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Community. The organisms which affect, directly or indirectly, the expected reproductive success of a reference organism. Proximate community. The organisms which directly affect the expected reproductive success of a reference organism. Community structure. The patterns in space and time of the attributes of the organisms in a community. Community functioning. The coevolutionary interactions among the organisms of a community. Ecosystem. The plexus composed both of abiotic entities and at least one organism, which are united by the exchange of matter and energy. Ecosystem structure. The patterns in space and time of the attributes of the components (biotic and abiotic) of an ecosystem. Ecosystem functioning. The transfers of matter and energy involving ecosystem components. General environment. The entities external to an organism, whether or not they are in that organism’s ecosystem or directly affect the organism. Operational environment. All of the entities of the general environment, including their emergent properties, which are directly exchanging matter or energy with the organism. Habitat. An artificial category used to describe or predict where an organism might occur. Niche. The state of an organism (i.e. the sum of the qualities of characters involved in an organism’s existence) as determined by the organism’s proximate community and operational environment. Prospective niche. The hereditarily defined range of states that an organism could endure. Fundamental niche. The maturationally restricted range of states that an organism could endure. Acclimatized niche. The acclimatizationally restricted range of states that an organism could endure. Realized niche. The state of an organism at any instant during its life. Guild. All organisms which use the same investigator-defined resource class. Functional group. All organisms which perform the same investigatordefined ecosystem function. While many of these definitions apply to individual organisms, investigators may wish to examine aggregations of organisms (e.g. demes, populations, or species) and thus speak, for example, of the population’s niches. Additionally, various time constraints may be used to modify the definitions, e.g. the pocket gopher’s summer niches.

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In conclusion, we discuss further the relationships between the organism’s community and the biotic components of its ecosystem. In many cases an organism will belong to both the community and the ecosystem of the reference organism. Some of the biotic ecosystem components will have no coevolutionary effect through their matter/energy exchanges and hence will not be considered part of the community. Conversely, some of the community members may not exchange measurable matter or energy, and thus would not be biotic components of the ecosystem. This latter situation seems unlikely, in that some exchange of matter and energy usually occurs, even if it is not the prime cause of the coevolutionary effects. We wish to re-emphasize the importance of coevolutionary interactions not measurable as simple exchanges of matter or energy. Large fluxes of matter or energy between organisms will almost always be biologically significant, but immeasurably small fluxes may also be consequential. These more subtle inter-organism transfers are at present measurable only by what is essentially a bioassay technique. Some ecosystem modellers have included these subtle interactions in their models under the epithet “information flows”. Unlike the various categories of matter and energy, what has been termed information transfers are so organism- and situation-specific that they do not seem to qualify as common currencies throughout the ecosystem, despite the fact that they may be ecologically significant. A recent paper suggests that ecosystems are not cybernetic systems, in part, because they do not have global information networks or feedback systems (Engelberg & Boyarsky, 1979). We thank the following individuals for helpful commentary on an earlier draft of the manuscript: Martyn Caldwell, Peter Landres, Robert McIntosh, Gordon Orians, Ivan Palmblad, Donald Phillips, Richard Root, Robert Whittaker, Eric Zurcher. Linda Finchum and Bette Peitersen facilitated production of the various drafts. Support from NSF grant DEB 78-05328 to MacMahon is acknowledged.

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