Origins and history

The term “ecology,” which has its root in the Greek word oikos (household or living place), came into use in the latter part of the nineteenth century in the works of zoologists and botanists to describe the study of the ways in which organisms live in their environments. Soon two branches of ecology were distinguished: autecology, the study of the individual organism’s interaction with environment, and synecology, the study of the correlations between the organisms engaged with a given unit of environment. The latter study has prevailed, however, and has become the principal connotation of ecology, since it became evident in numerous field studies that organisms, whether plant or animal, establish viable relationships with environment, not independently but collectively, through the mechanism of a system of relationships. Bioecologists were thus led to employ a set of concepts and techniques of investigation that imparted a markedly sociological coloration to their work.

Origins and history

The ecological approach was introduced as human ecology into the field of sociology at a critical period in the development of the latter discipline. In the 1920s the reformistic phase of sociology was drawing to a close, and the subject was gaining acceptance as a respected discipline in the curricula of American universities. That the transition would have been effected so quickly without the aid of a theoretical framework lending itself to empirical research seems doubtful. Ecology opportunely provided the necessary theory. A period of vigorous research followed that was to prove instrumental in launching sociology on its career as a social science.

Sociologists made free use of analogy as they borrowed heavily from the concepts of plant and animal ecology. The Darwinian notion of animate nature as a web of life became at once a general orienting concept and a basic postulate; it directed attention to the necessary interdependence among men as well as among lower forms of life. A second concept, the balance of nature, denoting a tendency toward stabilization of the relative numbers of diverse organisms within the web of life and of their several claims on the environment, provided human ecology with its characteristic equilibrium position. The more or less balanced web of relationships, when viewed in a specific local area, presented the aspect of community, a concept with obvious appeal for students of human social life.

According to plant and animal ecologists, the community, or ecosystem, is a population comprising a set of species whose reactions to the habitat and coactions between each other constitute an integrated system having some degree of unit character. Coactions involve members behaving both with reference to their similarities in an mtraspecific relationship known as commensalism and with reference to their differences in what is called symbiosis, an interspecific relationship. The community develops from simple to more complex forms through a sequence of stages described as succession. Each stage in the sequence is marked by an invasion of a new species or association of species, the series culminating in a climax stage in which a dominant species appears. The dominant species is related to the environment in such a way that it is able to control and maintain the community indefinitely. The community, then, tends to approximate a self-maintaining, or “closed,” system.

Community and environment

The application of the concepts from plant and animal ecology to the human community carried with it the implication that the community was essentially a natural phenomenon, which meant that it had developed independently of plan or deliberation. From this it was a short, though uncritical, step to the interpretation of human ecology as a study of the biotic or subsocial aspect of human social organization (Park 1936), a view that was elaborated at some length by Quinn (1950). Not only did the subsocial characterization convey an excessively narrow concept of social organization, but it posed an operational problem for which there was no workable solution.

A somewhat different definition of human ecology, which ignored any reference to the cognitive level of events, was enunciated by McKenzie ([1924] 1925, pp. 63–64), whose formulation of the subject as a study of the spatial and temporal relations of human beings, affected by the selective, distributive, and accommodative forces of the environment, was widely accepted as authoritative. Although McKenzie’s definition inspired a large amount of fruitful research effort, it had the unfortunate effect of concentrating attention almost exclusively on spatial distributions and correlations. In consequence, many promising implications of ecological assumptions were neglected.

Hawley, in attempting to restore a conceptual continuity with plant and animal ecologies, advanced the view of human ecology as the study of the form and development of the human community (1950, p. 68). Community, in this connection, is construed as a territorially localized system of relationships among functionally differentiated parts; human ecology, then, is concerned with the general problem of organization conceived as an attribute of a population—a point of view that has been shown to be consistent with a long-standing sociological tradition (Schnore 1958). Although the emphasis is centered on the functional system that develops in a population, it is not intended to exclude concern with spatial and temporal aspects; rather, these aspects are regarded as useful dimensions for the measurement of organization.

A further step in making the orientation of human ecology explicit within the larger context of general ecological theory was made by Duncan (1959, pp. 683–684), who described four principal variables of human ecology—population, organization, environment, and technology—that constitute an ecosystem. In other words, while any one of the four may be treated as a dependent variable for certain purposes, it is also reciprocally connected with each of the other variables. The virtue of this perspective lies in the range of problems it opens to the student of human ecology. Yet it seems unlikely tin various eco-logical studies, although hat that advantage can be fully enjoyed without a clear notion of how organization is constituted.

Other applications

While sociologists were at work defining human ecology for their purposes and pursuing many of its research leads, the concept spread into various other fields of inquiry. Human geographers wondered if the term “human ecology” was not a more apt characterization of their discipline; but their historic preoccupation with the landscape and their general addiction to a macroscopic treatment of occupance led them to discard this notion. Archeologists, in their efforts to reconstruct population distributions, have made use of ecological concepts and techniques, but without attempting to give formal statement to the approach.

Studies of human evolution by anthropologists involved questions of the man-environment relationship, which, in turn, led them to describe their work as human ecology; indeed, social or cultural anthropologists have long engaged in various ecological studies, although not until recently have they so defined their activity. One definition by an anthropologist is that of Steward (1955), for whom human ecology is the study of the adaptation of specific items of culture to particular environments. This conception, which reduces ecology to something akin to a research technique, is shared by a number of Steward’s contemporaries.

The language of human ecology has also made its appearance in economics, psychology, epidemiology, and other fields. In some instances, the term is used merely as a label for an environmental emphasis; in others, it is put forward in an effort to broaden the purview of a discipline.

But in spite of the widespread diffusion of ecological concepts, responsibility for systematic development of human ecology has been left to sociologists, who have drawn heavily on related fields for both theoretical and technical aid. The writings on real estate, finance, public administration, demography, planning, history, and other areas of inquiry, in addition to the literature of the fields previously mentioned, have at one time or another been exploited in the interest of human ecology. The reason for that catholicity of taste is not hard to find; human ecology is concerned with sociological problems in their fullest breadth. It overlaps, therefore, all the spheres of learning that concern the social life of man.

Distinctive features of human ecology

As human ecology has moved toward a major concern with the general problem of social organization and thus closer to the central concern of sociology, it has retained certain distinctive features. Foremost among these is the importance attributed to environment.

The broad, positional hypothesis is that organization arises from the interaction of population and environment. Environment, however, defined as whatever is external to and potentially or actually influential on a phenomenon under investigation, has no fixed content and must be defined anew for each different object of investigation. Environment is seen both as presenting the problem of life and as providing the means for its resolution; to adopt this position is to place the problem in a time-space context.

A second distinguishing characteristic is the emphasis on population as the point of reference; organization, it is contended, is exclusively the property of a population taken as a whole and not of an assemblage of individuals. Obvious as this position may be, it has profound methodological and theoretical implications for the manner in which the ecological problem is put, the variables employed, and the data to be observed. The concreteness of population, however, in contrast with the seemingly ephemeral nature of organization, tends to beguile the student to a view of population as the independent variable in all things pertaining to organization. It is obviously more convenient to proceed from the more accessible to the less accessible by asking how a population makes a unitary response to its environment; yet that is merely a common-sense way of approaching the problem, and investigation soon makes it apparent that population is for many purposes better regarded as the dependent variable, delimited and regulated by organization.

A third characteristic is the treatment of organization as a more or less complete and self-sustaining whole. The interaction of population and environment is seen as culminating in a system of relationships between differentiated parts which gives the population unit character and enables it to maintain its identity. As the property of a population, organization lends itself most readily to a morphological, or structural, analysis. The parts are the units—individuals or clusters of individuals—that perform functions and the relationships by which these units are linked. Differing configurations of unit functions and relationships are expected to occur with differences in relationship to environment and at different stages in development.

A morphological concern does not exclude the problem of development. Presumably, any given form of organization had an earlier form and is capable of having a later form. Every organization has a history, perhaps a natural history, the knowledge of which may shed light on the nature of organizations by indicating, for example, at what points they are vulnerable to change and how change spreads through them.

Related to the conception of organization is a fourth distinctive, although by no means unique, attribute of human ecology: the central position given to an equilibrium assumption. Morphological change is assumed to be a movement toward an equilibrium state, whether through a succession-like sequence of stages or through a process of continuous modification. Unlike the equilibrium notion in some of its other applications, such as in functionalist theory, the ecological usage of the term harbors no teleological overtones; on the contrary, this usage merely implies that as an organization attains completeness, it acquires the capacity for controlling change and for retaining its form through time, although the interval need not be specified. To put it differently, to the extent that an organization possesses unit character, an approximation to equilibrium obtains.

There is a further implication that a stable relationship with environment is contingent on relative stability in the relationships between the parts of an organization. A population always remains open to environment, but the formation of organizations canalizes environmental influences and makes for increasing selectivity of response.

The term “community” has commonly been used to denote the unit of organization for ecological purposes. Operationally defined as that population which carries on its daily life through a given system of relationships, the community is regarded as the smallest microcosm in which all the parameters of society are to be found. For reasons that were largely fortuitous, human ecologists at first focused their attention on the city and its tributary area as the prototype of community; later, in an effort to encompass the antecedents of cities, they broadened their consideration to include all forms of nucleated settlement. The term “community,” however, has the disadvantage of referring to the organization of a more or less localized settlement unit that does not always approximate a self-maintaining whole. For example, in an extensively urbanized society, local settlement units are usually components of more inclusive systems; in that event, the entire system must be treated as the object of study. But the difference between a simple, compact organization and a large, diffuse one is primarily a difference in scale; accordingly, the principles of organization should be the same in each. Since the designation of both simple and complex systems as communities threatens confusion, a more neutral term, such as “social system,” is to be preferred.

Principles of ecological organization

Inasmuch as principles of organization hinge on what is meant by population, it is imperative that the concept be developed more fully.

A human population is an aggregate of individuals who possess the following characteristics. As a living organism, every individual must have access to environment, for that is the only possible source of sustenance. Moreover, the interdependence of individuals is necessary; this condition, which obtains for all forms of life, holds true to an exceptional degree in the case of man, because of the naked state in which he comes into the world and his long period of postnatal maturation. Interdependence is the irreducible connotation of sociality.

The human being also possesses an inherent tendency to preserve and expand his life to the maximum permitted by prevailing circumstances; this is a general motive of which all other motives are special cases. In its most elementary sense, expansion of life refers to the multiplication of man-years through either the leaving of progeny or the extension of longevity; but it also includes all that is involved in the realization of that objective. Another important characteristic is the indeterminacy of the human being’s capacity to adapt; there is no known restriction on the kind or extent of refinement of activity in which he can engage. Finally, the human individual, again like other organisms, is time-bound; the recurring needs for food and rest fix the fundamental rhythm of life and regulate the allocation of time to all other activities. Accordingly, the time available for movement is limited and, in consequence, the space over which activities can be distributed is correspondingly limited.

These several attributes of individuals not only define the kind of population with which human ecology deals, but they also constitute a cardinal set of assumptions from which principles of ecological organization may be deduced. The following are some of the more salient of these principles of organization.

Principle of interdependence

Interdependence develops between units on each of two axes: the symbiotic (on the basis of their complementary differences) and the commensal (on the basis of their supplementary similarities). That is, units that combine in a symbiotic union may also enter into other combinations of a commensal character; the effect of each type of union is to raise the power of action above what it would be were the units to remain apart.

The effect, however, is not the same in each case. The symbiotic union enhances the efficiency of production or creative effort; the commensal union, since its parts are homogeneous, can only react and is suited, therefore, only to protective or conservative actions. Although commensalism is an elemental form of union, it is applicable to a wide array of situations. The point of importance at present is that a population tends to be knit together through an interwoven set of symbiotic and commensal relationships.

It should be evident that interdependence has temporal and spatial implications for the units involved. Relations of functional complementation and supplementation require mutual accessibility among units; since this is contingent on the time available for movement, the distance separating related units is always subject to some limitation. In general, for every set of related units, there should be, other things being equal, an appropriate pattern of distribution in the temporal and spatial dimensions.

Principle of the key function

A second principle may be described as the principle of the key function. That is, in every system of relationships among diverse functions, the connection of the system to its environment is mediated primarily by one or a relatively small number of functions, the latter being known as the key function or functions. To the extent that the principle of the key function does not obtain, the system will be tenuous and incoherent; in the extreme case, in which no system exists, every function has the same relationship to environment. Given a functional system, then, there are always some functions or functional units directly involved with environment and others that secure access to the environment indirectly, through the agency of the key function.

The notion of key function invokes the question of how to define the notion of environment, which can refer to many different kinds of things. For present purposes, these things may be classified in two broad categories: the natural and the social. Although every organized aggregate must contend with both, the relative importance of each may vary over a wide range. In some instances, because of the inaccessibility of the settlement, activities of necessity center on the exploitation of the local natural environment, while influences from the social field are relatively infrequent or of no great consequence. In this event, the key function is the activity that extracts the principal sustenance supply from local resources. But where the product from local resources, or a substantial part of it, is exchanged for other sustenance materials, whether through trade or other distributive mechanisms, the key function is determined by the comparative importance of production and of trade as sources of sustenance.

In many such instances, no distinction is necessary because the producer is also the trader—two functions combined in one functionary. But even before the two functions appear as separate specialties, the requisites of trade or distribution begin to regulate the uses of local resources. As the reliance on exchange advances, the social environment actually displaces the natural environment as the critical set of influences. A population is never emancipated from its dependence on physical and animate matters, but the importance of locale declines with increasing involvement in a network of intersystem relations; the natural environment is extended and diffused, and contacts with it are mediated through a variety of social mechanisms. Hence, the functions that link the local system to the social environment come to occupy the key position.

Principle of differentiation

The extent of functional differentiation varies with the productivity of the key function or functions; this is the principle of differentiation. A corollary is that the size of population supportable by the system varies with the productivity of the key function. For given the simplifying assumption that each unit is fully occupied, the number of people is determined by the number of functions to be performed.

In a hunting and gathering community, for instance, productivity is usually low, even though the physical environment is richly endowed; hence, there is little time or opportunity available for the cultivation of more than a few specialized functions. Nor is it possible to support enough people to staff even a moderate extent of specialization. By contrast, where the key function is devoted to stable agriculture, the range of possibilities is much greater, while in an industrial system productivity is so great that there are no known upper limits on either the number of specializations or the size of the population that can be supported.

The productivity of the key function, then, constitutes the principal limiting condition on the extent to which a system can be elaborated, on the size of population that can be sustained in the system, and on the area or space the system can occupy.

A question of some importance has to do with the relative number of units engaged in each of several interrelated functions. That question remains unanswered at present. It may be suggested, however, that the number of units engaged in any given function is inversely proportional to the productivity of the function and directly proportional to the number of units that utilize the product of the function.

It follows, of course, that functional differentiation involves a differentiation of environmental requirements. As the materials and conditions used by diversely specialized units differ, so also will their needs for location in space and time. In general, units performing key functions have the highest priority of claim on location. Other units tend to distribute themselves about the key function units, their distances away corresponding to the number of degrees of removal separating their functions from direct relation with the key functions. The temporal spacing may be expected to reveal a similar pattern. Special location requirements, however, as for type of soil or other resource, may obscure the tendency to a symmetrical arrangement of functions by degree of indirectness of relationship with key functions.

An important implication of the principles of the key function and of differentiation is that of transitivity in the relation among functional units. Relations with environment are necessarily transitive for some units. By the same token, relations among many units are transitive. The advance of specialization increases transitive relations more than proportionally and lengthens the transitive sequences. Thus, it is possible for functional units widely removed from one another so far as direct encounters are concerned to be inextricably linked through their respective linkages with one or more units performing intervening functions. All functions, regardless of kind, are subject to the environmental nexus. They differ only in the immediacy with which influences reach them. Those which operate at or near the ends of chains of relationships may appear to have large degrees of independence of environment. The appearance is illusory, however. It is due, rather, to the time required for effects to reach them and, in complex systems, to their having positions in two or more relational sequences which expose them to countervailing influences.

Principle of dominance

Given the principles stated above, it is a simple inferential step to a principle of dominance. According to this principle, functional units having direct relations with environment, and thus performing the key function, determine or regulate the conditions essential to the functions of units having indirect relations with environment. Dominance, in other words, is an attribute of function.

But while the power inherent in a system is unevenly distributed, it is not confined to the key function unit. Power is held in varying degrees by all other units, in measures that vary inversely with the number of steps of removal from direct relation with the unit performing the key function. A single power gradient running through all the units involved in a system assumes a very simple situation, of course, one in which a single unit occupies a key function position. Before introducing complications into this overly simplified conception, it is opportune to note some further implications of the dominance principle.

Where the progress of differentiation has distinguished a relatively large number of units, they tend to form clusters or complex units. A corollary to the dominance principle sheds light on how that comes about: the greater the extent to which variously specialized units are subject to the environmental conditions mediated by some one unit, the greater their tendency to coalesce in a corporate body. The interdependence among such units is manifestly close, and their requirements for mutual accessibility are correspondingly acute. A hierarchic pattern emerges in which the number of strata corresponds to the number of degrees that the parts are removed from direct relations with the key function; thus, symbiotic subsystems appear as components of a parent system. The nuclear family, although its origins are obscure, fits this principle. But instances with more proximate origins are found in the combinations of specialists to form producing enterprises, welfare agencies, governing bodies, etc., and again in the combinations of producing enterprises, retail establishments, or governments to constitute larger, more complex units. There is no restriction of scale or complexity in the formation of symbiotic or corporate units.

On the other hand, units that are of a given functional type and therefore occupy equivalent positions in the power hierarchy may raise their power potential by the formation of categoric unions. Any threat to a function or to the conditions of its performance can provoke such a response; groups of elders, the medieval guilds, labor unions, professional associations, councils of churches, retail associations, and associations of manufacturers are examples of categoric units. A social class is at most a loose form of categoric unit.

As with the corporate union, the categoric union may be composed of units of any size or kind. It may appear as a federation of categoric units or as an association of corporate units. So long as it retains its pure categoric form, however, such a unit can do little more than react to circumstances affecting it. Nor can it have more than a transitory existence, since in order to engage in positive action of any kind and to attain some measure of permanence, it must develop at least a core of specialists. Although the categoric unit tends to assume the characteristics of the corporate unit, it remains distinctive as long as its criterion for membership is possession of a given, common characteristic. In any event, the categoric unit is a source of rigidity in a social system, and once formed, its effect is to preserve the position of a functional category in the system.

The concept of dominance has been widely employed for the purpose of delimiting the boundaries of systems. It is argued that centers of settlement, such as cities, exercise dominance over their surrounding areas, in diminishing degrees as the distance from the center increases; the margin of influence marks the boundary of the system. Empirical support for this proposition is provided by the evidence of nonrandom distribution of related functions and by the gradient pattern that appears in the frequencies with which outlying functions are involved with central functions.

Two qualifications of this conclusion are needed; first, dominance is exercised from and not by centers, since it resides in functional units rather than in the places where they are located. Further, the apparent decline of dominance results less from an effect of distance in reducing control over related functions than from the increased difficulty of establishing dominance uniformly over an exponentially widening area.

Isomorphism. A principle of isomorphism has been implied in much that has been said and needs now to be stated. Units subject to the same environmental conditions, or to environmental conditions as mediated through a given key unit, acquire a similar form of organization. They must submit to standard terms of communication and to standard procedures in consequence of which they develop similar internal arrangements within limits imposed by their respective sizes. Each unit, then, tends to become a replica of every other unit and of the parent system in which it is a subsystem.

Since small units cannot acquire the elaborate organizations of which large units are capable, they jointly support specialized functions that complement their meager organizations. For example, whereas a large unit may include among its functionaries accountants, lawyers, engineers, public relations experts, and other specialists, the small unit must purchase comparable services from units specializing in each of the relevant functions. The principle of isomorphism also applies to the size of units, at least as a tendency; that is, all units tend toward a size that enables them to maintain contact with all relevant sectors of the parent system.

Closure and social change

Operation of the several principles mentioned thus far moves a system toward a state of closure. This term must be employed here with circumspection, for it cannot have its usual connotation of independence of environment. Closure can only mean that development has terminated in a more or less complete system that is capable of sustaining a given relationship to environment indefinitely. For closure to be realized, it is required not only that the differentiation of function supportable by the productivity of the key function has attained its maximum but also that the various functions have been gathered into corporate and categoric subsystems; moreover, the performance of the key function should have been reduced to one unit or to a number of units united in a categoric federation. Then a system is highly selective of its membership and capable of exercising some control over factors that threaten change in the system.

Under these circumstances, certain conditions of equilibrium are held to obtain: the functions involved are mutually complementary and collectively provide the conditions essential to the continuation of each; the number of individuals engaged in each function is just sufficient to maintain the relations of the functions to each other and to all other functions; and the various units are arranged in time and space so that the accessibility of one to others bears a direct relation to the frequency of exchanges between them. Needless to say, equilibrium as thus defined is a logical construct; the conditions express an expected outcome if the principles of organization are allowed to operate without any external disturbances.

Origins and effects of change

Every social system is continuously subject to change, for since the environment is always in some state of flux, the equilibrium that can be attained is seldom more than partial. A system founded on nonreplaceable resources is faced with “immanent change”; sooner or later it will either pass into decline or shift perforce to a different resource base. Such, for instance, has been the experience of innumerable mining communities; in a similar manner agricultural communities often alter the soil composition of their lands by the uses they practice, with the ultimate result that the lands will no longer support the systems as they are constituted. Instances of maladaptation, such as the reliance of the Irish on the potato as a food staple in the nineteenth century, can lead to catastrophic consequences.

In general, however, change has an external origin, a proposition that follows by definition from an equilibrium position. Some influences emanate from the physical or biotic environment, such as variations in the growing season or invasion by parasites. To the extent that episodic occurrences of that order fail to modify one or more functions comprised in the system, their effects are transitory; the system returns to its original form. But where a function, particularly a key function, is substantially modified, the system must be reconstituted. The disappearance of a game supply, the silting of an estuary, or the eruption of a volcano may render a key function inoperative in its usual location. The population must relocate and work out a new system of activities. Unless there is an increase of productivity, new ways of acting will merely displace old ways, and the change will not be cumulative.

Cumulative change

Cumulative change, or growth of the system, presupposes an increase in the productivity of a key function. Only in this way is it possible to multiply specialization, to employ a greater variety of techniques, and to support a larger population. The probability of the occurrence of disturbances having that effect rises with the number of points of contact with a social environment; location, therefore, is an important factor. A site on a traveled route is more exposed to external influences than one situated at a distance from an avenue of movement; a site at a conflux of routes is much more vulnerable to disturbances from without. Any location that fosters frequent meetings of people from diverse backgrounds is a gateway for the infusion of alien experiences and techniques into the social system centered there. Whether change is released through a deposit of numerous small additions to the culture or through a simple, dramatic innovation is immaterial.

The process of cumulative change may be generalized as a principle of expansion. Expansion is a twofold process involving, on the one hand, the growth of a center of activity from which dominance is exercised and, on the other hand, an enlargement of the scope of the center’s influence. The process entails the absorption and redistribution of the functions formerly carried on in outlying areas, a centralization of mediating and control functions, an increase in the number and variety of territorially extended relationships, a growth of population to man a more elaborate set of activities, and an accumulation of culture together with a leveling of cultural differences over the expanding domain. After the revival of trade in Europe, from the tenth century onward, the favorably situated village with its narrow vicinage grew into a market town that served as a center of an enlarged territorial organization. That gave way, in turn, to the emergent city capable of exercising an integrating influence over an area of regional scope. Most recently, the metropolis has superseded the city and has brought under its dominion a vast interregional territory. Although the process is as old as recorded history, it has not advanced in a simple linear progression. It has moved and then stalled in one place, only to surge ahead in another; it has faltered and even on occasion has seemed to turn back upon itself, but it has always resumed its course with renewed vigor.

The limits to expansion are sometimes fixed by the facility of movement between center and periphery—by the maximum distance over which the exercise of dominance is feasible. More often than not, however, the limits are drawn at the points of juncture with the expanding domains of social systems in neighboring regions. As a system encounters its limits, however they are fixed, it loses capacity to absorb further change, and equilibrium tendencies begin to assert themselves once more.

In many instances, however, the first symptoms of change are experienced at the periphery of a system. Since the effects of dominance grow more uneven with increased distance from a center, boundaries are apt to be permeable at many points. Yet, in the degree to which a system is integrated, events at the boundary are transmitted directly to the key unit, from which they are communicated to ancillary units; it is always at the boundary that one system begins its absorption of another. Expansion may be resumed at any moment and in any of the systems that hem in one another. An innovation, even though it might present itself to all systems about the same time, gains admission to one or another by virtue of a more favorable location for its use, a more appropriate organization for its acceptance, or some other local advantage. The renewed expansion encroaches upon the territories of adjoining systems, sometimes reducing them to mere components of a single, greatly enlarged system.

Under conditions of closure, environmental effects, and particularly cultural innovations, would be expected to enter a system through the key function, for the obvious reason that it has the most direct connection with environment. And, presumably, change would spread through the system by affecting units successively in the order in which they are removed from direct relation with a key function. But in a system centered on a convergence of routes, many units may have direct, although not equal, access to the outside world. Change may therefore enter the system at many points, at least until its structure is fully developed with the parts systematically arrayed relative to a key unit. An expanding system, in other words, is an open system, and it remains so until the limits to expansion are reached.

Burgess’ hypothesis of city growth

A hypothesis of city growth, stated by E. W. Burgess ([1925] 1961, pp. 37–44), pertains to a special case of the more general principle. According to that proposal, city growth takes the form of expansion from a zone centered on a highly accessible location. Growth involves increasing density of occupance of the central zone and, at the same time, a redistribution of activities or land uses scattered around the center to conform to a gradient pattern of variation of intensity of land use according to distances from the center. Redistribution results from increasing pressure at the center and a consequent encroachment by high-intensity uses into the spaces occupied by lower-intensity uses in a succession-like manner. Alternating periods of redistribution and stabilization of distribution, and those of growth and partial equilibrium, create a wavelike effect more or less visible in a set of concentric zones. By venturing to describe, in rather specific terms, the content of the zones and by thus reifying a set of statistical constructs, Burgess diverted attention from a growth process and caused it to be fixed on a specious distribution pattern. His argument, therefore, seemed to acquire a historical limitation that it need not have had.

The miscarriage of the import of the Burgess hypothesis is evident in criticisms that have opposed the preindustrial city to the industrial city as a qualitatively different phenomenon. Whereas Burgess suggested that the social-economic status of residents is higher in each successive concentric zone, critics have shown that in the preindustrial city the social-economic gradient runs in the opposite direction. Useful as that finding may be, it misses the essential point. That is, the significant gradient is one of dominance; units tend to distribute themselves over space in a way that reflects their relationships to the dominant unit. In this respect, both industrial and preindustrial cities are similar. The qualitative difference lies in the kinds of units that exercise dominance. In the preindus-trial city, all functions are carried in familial or household units, and power is unevenly distributed among them. But in the industrial society, the household unit has been relegated to a minor position; specialized functions are performed by extra-familial units, and the separation of functions from the household has involved a spatial separation as well.

Furthermore, the notion of a monocentered system is applicable only to the simplest instances. All others include a constellation of settlement nuclei, that is to say, subsidiary service centers within cities, and villages, towns, or cities within hinterlands. Each serves as a locus of influence over a localized area, varying in scope with the types of functions centered in the nucleus. Thus, as Christaller observed, the constellation of nuclei forms a hierarchy by size and number of places and by order of functions performed (1933). Small places provide low-order, or ubiquitous, functions, whereas each larger place performs, in addition to low-order functions, higher-order functions for broader domains. At the apogee of the hierarchy is the metropolis, in which the integrating and coordinating functions for the entire system are domiciled. Thus, dominance is exercised downward and outward through nested sets of subsidiary centers

The least satisfactory aspect of the theory of change concerns its temporal incidence. The idea of succession, borrowed initially from bioecology, lingers in the dictionary of human ecology. Change as cyclical in form, consisting in movements between equilibrium stages, is clearly the most intelligible conception. Nevertheless, apart from the difficulty in empirically identifying an equilibrium stage, there are unsolved conceptual problems of the spacing of stages, and of the factors governing the intervals between stages. Thus far, succession has been applied only in retrospect. Its utility will remain uncertain until it can be projected into prediction. Quite possibly, that may have to wait for more extensive work on social system taxonomy.

The limits of human ecology

Human ecology has progressed since its inception from an effort to apply the concepts of plant and animal ecology to human collective life, through an extended period of preoccupation with spatial configurations, to an increasing concern with the form and development of territorially based social systems. In the last phase, human ecologists have sought to clarify the assumptions of ecology and to draw out their implications for organization. Although the results of that work are far from complete, it seems clear that they indicate the direction in which human ecology will continue to develop.

As with most approaches in social science, human ecology has limited objectives. It seeks knowl-ledge about the structure of a social system and the manner in which the structure develops. Hence, it is not prepared to provide explanations for all of the manifold interactions, frictions, and collisions that occur within the bounds of a social system. The findings of human ecology, however, define the context in which all such phenomena take place, and which is therefore pertinent to their full understanding.

Human ecology is not qualified to deal with the normative order in a social system. Yet consistent with its position is the expectation that a normative order corresponds to and reflects the functional order. The two are different abstractions from the same reality.