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]]