The young sciences of plant and animal ecology have become fairly well established. Their respective fields are apparently quite well defined, and a set of concepts for analysis is becoming rather generally accepted. The subject of human ecology, however, is still practically an unsurveyed field, that is, so far as a systematic and scientific approach is concerned. To be sure, hosts of studies have been made which touch the field of human ecology in one or another of its varied aspects, but there has developed no science of human ecology which is comparable in precision of observation or in method of analysis with the recent sciences of plant and animal ecology.

I. THE RELATION OF HUMAN ECOLOGY TO PLANT AND ANIMAL ECOLOGY

Ecology has been defined as “that phase of biology that considers plants and animals as they exist in nature, and studies their interdependence, and the relation of each kind and individual to its environment.”^[31] This definition is not sufficiently comprehensive to include all the elements that logically fall within the range of human ecology. In the absence of any precedent let us tentatively define human ecology as a study of the spatial and temporal^[32] relations of human beings as affected by the selective, distributive, and accommodative forces of the environment. Human ecology is fundamentally interested in the effect of position,^[33] in both time and space, upon human institutions and human behavior. “Society is made up of individuals spatially separated, territorially distributed, and capable of independent locomotion.”^[34] These spatial relationships of human beings are the products of competition and selection, and are continuously in process of change as new factors enter to disturb the competitive relations or to facilitate mobility. Human institutions and human nature itself become accommodated to certain spatial relationships of human beings. As these spatial relationships change, the physical basis of social relations is altered, thereby producing social and political problems.

A great deal has been written about the biological, economic, and social aspects of competition and selection, but little attention has been given to the distributive and spatial aspects of these processes. The plant ecologist is aware of the effect of the struggle for space, food, and light upon the nature of a plant formation, but the sociologist has failed to recognize that the same processes of competition and accommodation are at work determining the size and ecological organization of the human community.

The essential difference between the plant and animal organism is that the animal has the power of locomotion which enables it to gather nutriment from a wider environment, but, in addition to the power to move in space, the human animal has the ability to contrive and adapt the environment to his needs. In a word, the human community differs from the plant community in the two dominant characteristics of mobility and purpose, that is, in the power to select a habitat and in the ability to control or modify the conditions of the habitat. On first consideration this might seem to indicate that human ecology could have nothing in common with plant ecology where the processes of association and adjustment result from natural unmodifiable reactions, but closer examination and investigation make it obvious that human communities are not so much the products of artifact or design as many hero-worshipers suppose.^[35]

The human community has its inception in the traits of human nature and the needs of human beings. Man is a gregarious animal: he cannot live alone; he is relatively weak and needs not only the company of other human associates but shelter and protection from the elements as well. Brunhes says there are three essentials to the inception of the human community: the house, the road, and water.^[36] Food may be transported more easily than shelter or water; the latter two therefore constitute, even under the most nomadic conditions, the essential elements in giving a location and a spatial fixity to human relations.^[37] This is exemplified under our present régime of automobile tourist life, where water and shelter become the determining factors in the location of the camp.

The size and stability of the human community is however a function of the food supply and of the rôle played in the wider ecological process of production and distribution of commodities. When man makes his living from hunting or fishing, the community is small and of but temporary duration; when agriculture becomes the chief source of sustenance, the community is still small but assumes a more permanent character; when trade and commerce develop, larger communities arise at points of break in conveyance, that is, at the mouths of rivers, junctions of streams, at waterfalls, and shallows where streams are forded. As new forms of transportation arise, new points of concentration occur and old points become accentuated or reduced. Again, as goods for trade are made in communities, still other points of concentration come into existence, determined largely by sources of power and raw material.^[38]

II. ECOLOGICAL CLASSIFICATION OF COMMUNITIES

From the standpoint of ecology, communities may be divided into four general types: first, the primary service community, such as the agricultural town, the fishing, mining, or lumbering community which serves as the first step in the distributive process of the outgoing basic commodity and as the last stage in the distributive process of the product finished for consumption. The size of such communities depends entirely upon the nature and form of utilization of the extractive industry concerned, together with the extent of the surrounding trade area. The community responds in size to any element that affects the productivity of the economic base or the extent of the area from which it draws its sustenance. But, in any event, so long as such a community does not assume any other function in the larger ecological process, it cannot grow in population beyond a few thousand inhabitants.

The next type of community is the one that fulfils the secondary function in the distributive process of commodities. It collects the basic materials from the surrounding primary communities and distributes them in the wider markets of the world. On the other hand, it redistributes the products coming from other parts of the world to the primary service communities for final consumption. This is commonly called the commercial community; it may, however, combine other functions as well. The size of this type of community depends upon the extent of its distributive functions. It may vary from a small wholesale town in the center of an agricultural plain to that of a great port city whose hinterland extends halfway across the continent. Growth depends upon the comparative advantages of the site location.

The third type of community is the industrial town. It serves as the locus for the manufacturing of commodities. In addition it may combine the functions of the primary service and the commercial types. It may have its local trade area and it may also be the distributing center for the surrounding hinterland. The type is characterized merely by the relative dominance of industry over the other forms of service. There is practically no limit to the size to which an industrial community may develop. Growth is dependent upon the scope and market organization of the particular industries which happen to be located within its boundaries. Industrial communities are of two general types: first, those that have diversified and multiple industries organized on a local sale of products, and, second, those that are dominated by one or two highly developed industries organized on a national or world-sale of products.

The fourth type of community is one which is lacking in a specific economic base. It draws its economic sustenance from other parts of the world, and may serve no function in the production or distribution of commodities. Such communities are exemplified in our recreational resorts, political and educational centers, communities of defense, penal or charitable colonies. From the standpoint of growth or decline such communities are not subject to the same laws that govern the development of towns that play a part in the larger productive and distributive processes.^[39] They are much more subject to the vicissitudes of human fancies and decrees than are the basic types of human communities. Of course, any community may and usually does have accretions added to its population as a result of such service. It may, for instance, be the seat of a university, of a state prison, or it may be a recreational resort for at least certain seasons of the year.

III. DETERMINING ECOLOGICAL FACTORS IN THE GROWTH OR DECLINE OF COMMUNITY

The human community tends to develop in cyclic fashion. Under a given state of natural resources and in a given condition of the arts the community tends to increase in size and structure until it reaches the point of population adjustment to the economic base. In an agricultural community, under present conditions of production and transportation, the point of maximum population seldom exceeds 5,000.^[40] The point of maximum development may be termed the point of culmination or climax, to use the term of the plant ecologist.^[41] The community tends to remain in this condition of balance between population and resources until some new element enters to disturb the status quo, such as the introduction of a new system of communication, a new type of industry, or a different form of utilization of the existing economic base. Whatever the innovation may be that disturbs the equilibrium of the community, there is a tendency toward a new cycle of adjustment. This may act in either a positive or negative manner. It may serve as a release to the community, making for another cycle of growth and differentiation, or it may have a retractive influence, necessitating emigration and readjustment to a more circumscribed base.

In earlier conditions of life, population was kept down to the community balance by variations in the death-rate, or, as in the case of Greek cities, the surplus population emigrated in groups to establish new colonies—offshoots of the mother-city. Under modern conditions of communication and transportation, population adjustment is maintained by a ceaseless process of individual migrations. As a result of the dynamic conditions prevailing throughout the civilized world during the last fifty years, many communities have passed through swift successive cycles of growth or decline, the determining factors being changes in forms and routes of transportation and communication and the rise of new industries.

Some advantage in transportation is the most fundamental and most important of the causes determining the location of a distributing center. It may almost be said to be the only cause for the formation of such centers. For some reason or reasons a particular place is more conveniently and cheaply reached by many people than any surrounding point; and, as a result, they naturally exchange commodities there. The country store is located at the crossing of roads. There also is the village. In a mountain country the market town is at the junction of two, or, still better, of three valleys. Another favorite location is the end of a mountain pass, or a gap that is a thoroughfare between two valleys. If rivers are difficult to cross, settlements will spring up at the safest ferries or fords. In a level plain, a town will be near its center, and a focus of roads or railroads in such a plain, fertile and populous, will almost surely make a city.^[42]

It is the railroad and the steamship that determine where a new business shall be developed, quite as often as the government policy. The grant of special rates and privileges to shippers is nowadays the most efficient kind of protection.

It is this quickening and cheapening of transportation that has given such stimulus in the present day to the growth of large cities. It enables them to draw cheap food from a far larger territory and it causes business to locate where the widest business connection is to be had, rather than where the goods or raw materials are most easily produced. And the perfection of the means of communication, the post-office and the telegraph, intensifies the same result.^[43]

The entire net increase of the population of 1870 to 1890 in Illinois, Wisconsin, Iowa, and Minnesota was in cities and towns possessing competitive rates, while those having non-competitive rates decreased in population, and in Iowa it is the general belief that the absence of large cities is due to the earlier policy of the railways giving Chicago discriminating rates.^[44]

The advent of the trolley line and more recently of the automobile has produced still further disturbing elements in the growth of human communities. Their effect has been chiefly to modify the life of the small town or village, causing the decline of some and the sudden growth of others. The introduction of these two forms of transportation, more particularly of the automobile, has been the most potent force in our recent American history in affecting redistribution of our population and in the disorganization of our rural and small-town institutions which grew up on the basis of a horse-and-vehicle type of mobility.^[45]

The evolution of new types of industry is another feature that becomes a determining factor in the redistribution of the country’s population. As we review our census reports we see the emergence each decade of one or more important industries; first, the textile industry causing concentrations of population in the eastern states, then the development of the iron and steel industry with its center of operations gradually shifting farther and farther west, and more recently the advent of the automobile and oil industries making for enormous concentration of population in certain states of the Union, also the motion picture industry with its concentrated center in southern California. The emergence of a new industry has a far-reaching effect in disturbing the status quo of communal life. Competition soon forces the new industry to concentrate its productive enterprises in one or two communities; these communities then serve as great magnets drawing to themselves the appropriate population elements from communities far and near.

Population migrations resulting from such sudden pulls as are the outcomes of unusual forms of release in community growth may cause an expansion in the community’s development far beyond the natural culmination point of its cyclic development, resulting in a crisis situation, a sudden relapse, disorganization, or even panic. So-called “boom towns” are towns that have experienced herd movements of population beyond the natural point of culmination.

On the other hand, a community which has reached the point of culmination and which has experienced no form of release is likely to settle into a condition of stagnation. Its natural surplus of population is forced to emigrate. This type of emigration tends to occasion folk-depletion in the parent community. The younger and more enterprising population elements respond most sensitively to the absence of opportunities in their home town. This is particularly true when the community has but a single economic base, such as agriculture, lumbering, mining. Reformers try in vain to induce the young people to remain on the farms or in their native villages, little realizing that they are working in opposition to the general principles of the ecological order.

Again, when a community starts to decline in population due to a weakening of the economic base, disorganization and social unrest follow.^[46] Competition becomes keener within the community, and the weaker elements either are forced into a lower economic level or are compelled to withdraw from the community entirely. There are, of course, periodic and temporary fluctuations in the economic balance, due either to circumstances which affect the entire economic order or to the vicissitudes of the particular industry from which the community draws its sustenance. These temporary fluctuations, however, while important from the standpoint of social well-being, do not comprise the basic determinants of community development.

The introduction of an innovating element into the adjustment of a community may be designated as the initial stage of an invasion which may make for a complete change in the structure and organization of the community. The introduction of a new mode of transportation, for instance, may transform the economic organization of a community and make for a change in population type.

Thus the Harlem Railroad transformed Quaker Hill from a community of diversified farming, producing, manufacturing, selling, consuming, sufficient unto itself, into a locality of specialized farming. Its market had been Poughkeepsie, twenty-eight miles away, over high hills and indifferent roads. Its metropolis became New York, sixty-two miles away by rail and four to eight miles by wagon-road.

With the railroad’s coming, the isolated homogeneous community scattered. The sons of the Quakers emigrated. Laborers from Ireland and other European lands, even negroes from Virginia, took their places. New Yorkers became residents on the Hill, which became the farthest terminus of suburban travel.^[47]

The establishment of a new industry, especially if it displaces the previous economic base, may also make for a more or less complete change of population without greatly modifying the size of the community. This condition is exemplified in many of the small towns of the state of Washington which have changed from lumbering to agriculture or from one type of agriculture to another. In many cases few of the previous inhabitants remained after the invasion of the new economic base.

As a community increases in size, however, it becomes better able to accommodate itself to invasions and to sudden changes in number of inhabitants. The city tends to become the reservoir into which the surplus population drains from the smaller communities round about.

In the process of community growth there is a development from the simple to the complex, from the general to the specialized; first to increasing centralization and later to a decentralization process. In the small town or village the primary universal needs are satisfied by a few general stores and a few simple institutions such as church, school, and home. As the community increases in size specialization takes place both in the type of service provided and in the location of the place of service. The sequence of development may be somewhat as follows: first the grocery store, sometimes carrying a few of the more staple dry goods, then the restaurant, poolroom, barber shop, drug store, dry-goods store, and later bank, haberdashery, millinery, and other specialized lines of service.^[48]

The axial or skeletal structure of a community is determined by the course of the first routes of travel and traffic.^[49] Houses and shops are constructed near the road, usually parallel with it. The road may be a trail, public highway, railroad, river, or ocean harbor, but, in any case, the community usually starts in parallel relation to the first main highway. With the accumulation of population and utilities the community takes form, first along one side of the highway and later on both sides. The point of junction or crossing of two main highways, as a rule, serves as the initial center of the community.

As the community grows there is not merely a multiplication of houses and roads but a process of differentiation and segregation takes place as well. Residences and institutions spread out in centrifugal fashion from the central point of the community, while business concentrates more and more around the spot of highest land values. Each cyclic increase of population is accompanied by greater differentiation in both service and location. There is a struggle among utilities for the vantage-points of position. This makes for increasing value of land and increasing height of buildings at the geographic center of the community. As competition for advantageous sites becomes keener with the growth of population, the first and economically weaker types of utilities are forced out to less accessible and lower-priced areas. By the time the community has reached a population of about ten or twelve thousand, a fairly well-differentiated structure is attained. The central part is a clearly defined business area with the bank, the drugstore, the department store, and the hotel holding the sites of highest land value. Industries and factories usually comprise independent formations within the city, grouping around railroad tracks and routes of water traffic. Residence sections become established, segregated into two or more types, depending upon the economic and racial composition of the population.

The structural growth of community takes place in successional sequence not unlike the successional stages in the development of the plant formation. Certain specialized forms of utilities and uses do not appear in the human community until a certain stage of development has been attained, just as the beech or pine forest is preceded by successional dominance of other plant species. And just as in plant communities successions are the products of invasion, so also in the human community the formations, segregations, and associations that appear constitute the outcome of a series of invasions.^[50]

There are many kinds of intra-community invasions, but in general they may be grouped into two main classes: those resulting in change in use of land, and those which introduce merely change in type of occupant. By the former is meant change from one general use to another, such as of a residential area into a business area or of a business into an industrial district. The latter embraces all changes of type within a particular use area, such as the changes which constantly take place in the racial and economic complexion of residence neighborhoods, or of the type of service utility within a business section. Invasions produce successional stages of different qualitative significance, that is, the economic character of the district may rise or fall as the result of certain types of invasion. This qualitative aspect is reflected in the fluctuations of land or rental values.

The conditions which initiate invasions are legion. The following are some of the more important: (1) changes in forms and routes of transportation;^[51] (2) obsolescence resulting from physical deterioration or from changes in use or fashion; (3) the erection of important public or private structures, buildings, bridges, institutions, which have either attractive or repellent significance; (4) the introduction of new types of industry, or even a change in the organization of existing industries; (5) changes in the economic base which make for redistribution of income, thus necessitating change of residence; (6) real estate promotion creating sudden demands for special location sites, etc.

Invasions may be classified according to stage of development into (a) initial stage, (b) secondary or developmental stage, (c) climax. The initial stage of an invasion has to do with the point of entry, the resistance or inducement offered the invader by the prior inhabitants of the area, the effect upon land values and rentals. The invasion, of course, may be into an unoccupied territory or into territory with various degrees of occupancy. The resistance to invasion depends upon the type of the invader together with the degree of solidarity of the present occupants. The undesirable invader, whether in population type or in use form, usually makes entry (that is, within an area already completely occupied) at the point of greatest mobility. It is a common observation that foreign races and other undesirable invaders, with few exceptions, take up residence near the business center of the community or at other points of high mobility and low resistance. Once established they gradually push their way out along business or transportation thoroughfares to the periphery of the community.

The commencement of an invasion tends to be reflected in changes in land value. If the invasion is one of change in use the value of the land generally advances and the value of the building declines. This condition furnishes the basis for disorganization. The normal improvements and repairs are, as a rule, omitted, and the owner is placed under the economic urge of renting his property to parasitic and transitory services which may be economically strong but socially disreputable and therefore able and obliged to pay higher rentals than the legitimate utilities can afford. It is a well-known fact that the vices under the surveillance of the police usually segregate in such transitional areas.^[52]

During the course of development of an invasion into a new area, either of use or type, there takes place a process of displacement and selection determined by the character of the invader and of the area invaded. The early stages are usually marked by keenness of competition which frequently manifests itself in outward clashes. Business failures are common in such areas and the rules of competition are violated. As the process continues, competition forces associational groupings. Utilities making similar or complementary demands of the area tend to group in close proximity to one another, giving rise to subformations with definite service functions. Such associations as amusement areas, retail districts, market sections, financial sections, and automobile rows are examples of this tendency.

The climax stage is reached in the invasion process, once the dominant type of ecological organization emerges which is able to withstand the intrusions of other forms of invasion. For example, in the development of a residential district, when it is not controlled in advance by building restrictions, the early stages of growth are usually marked by wide variations in the type and value of buildings constructed. But, in the process of development, a uniform cost type of structure tends to dominate, gradually eliminating all other types that vary widely from the norm, so that it is customary to find a considerable degree of economic homogeneity in all established residential districts. The same process operates in areas devoted to business uses, competition segregates utilities of similar economic strength into areas of corresponding land values, and at the same time forces into close proximity those particular forms of service which profit from mutual association such as financial establishments or automobile display-rooms. Once a dominant use becomes established within an area, competition becomes less ruthless among the associational units, rules of control emerge, and invasion of a different use is for a time obstructed.

The general effect of the continuous processes of invasions and accommodations is to give to the developed community well-defined areas, each having its own peculiar selective and cultural characteristics. Such units of communal life may be termed “natural areas,”^[53] or formations, to use the term of the plant ecologist. In any case, these areas of selection and function may comprise many subformations or associations which become part of the organic structure of the district or of the community as a whole. It has been suggested that these natural areas or formations may be defined in terms of land values,^[54] the point of highest land value representing the center or head of the formation (not necessarily the geographic center but the economic or cultural center), while the points of lowest land value represent the periphery of the formation or boundary line between two adjacent formations.

Each formation or ecological organization within a community serves as a selective or magnetic force attracting to itself appropriate population elements and repelling incongruous units, thus making for biological and cultural subdivisions of a city’s population. Everyone knows how racial and linguistic colonies develop in all of our large cities, but the age and sex segregations which take place are not quite so obvious to common perception. In the city of Seattle, which has in general a sex composition of 113 males to 100 females, the downtown district, comprising an area inscribed by a radius of half a mile or so, has from 300 to 500 males to every 100 females. But in the outlying districts of the city, except in one or two industrial sections, these ratios are reversed. Females predominate in numbers over males in all the residential neighborhoods and in the suburbs of the city. This same condition is true with regard to the age distribution of population. The school census shows an absolute decline in the number of children of school age in the central districts of the city although the total population for this area has shown an increase for each decade. It is obvious, then, that the settler type of population, the married couples with children, withdraw from the center of the city while the more mobile and less responsible adults herd together in the hotel and apartment regions near the heart of the community.

This process of population-sifting produces not only increasing mobility with approach from the periphery to the center of the formation, but also different cultural areas representing different mores, attitudes, and degrees of civic interest. The neighborhoods in which the settler type of population resides, with their preponderance of women and children, serve as the custodians of the stabilizing and repressive mores. It is in the Seattle neighborhoods, especially those on the hill-tops, that the conservative, law-abiding, civic-minded population elements dwell. The downtown section and the valleys, which are usually industrial sites, are populated by a class of people who are not only more mobile but whose mores and attitudes, as tested by voting habits, are more vagrant and radical.