Research methods in ecology

by Frederic E. Clements

The position of the competing individuals is of the greatest importance. The distance between the plants affects directly the degree of competition, while their arrangement, whether in groups according to species or singly, exerts a marked influence by determining that the contest shall be between like forms, or unlike forms. Position is controlled primarily by the relation existing between seed-production and dissemination. It is of course influenced in large measure by the initial position taken by the invaders into a nudate area, but this is itself a result of the same phenomena. The individuals of species with great seed-production and little or no mobility usually occur in dense stands. In these, the competition is fierce, for the two reasons of similarity and density, and the result is that the plants fall far below the normal in height and width. This is an extreme example of the group arrangement. When the seed-production is small, the mobility may be great or little without seriously affecting the result. The individuals of a species of this kind will be scattered among those of other species, and the closeness of competition will depend largely upon the similarity existing between the two. The arrangement in such cases is sparse. A species with great seed-production and great mobility usually shows both kinds of arrangement, the position of the individuals and the competition between them varying accordingly. This is due to the intermittent action of distributing agents, making it possible for the seeds to fall directly to the ground during the times that winds, etc., are absent. The three types of arrangement indicated above are termed gregarious, copious, and gregario-copious. They furnish the basis for the investigation of abundance which deals essentially with the number and arrangement of the individuals of competing species. The effect of distance, i. e., the interval between individuals, upon competition is fundamental. The competition increases as the interval diminishes, and the reverse.

The view here advanced, i. e., that competition is purely physical in nature, renders untenable the current conceptions of vegetation pressure, occupation, etc. Masses of vegetation are thought to force the weaker species toward the edge, thus initiating an outward or forward pressure. As has been shown above, no such phenomenon occurs in vegetation. This movement is nothing but simple migration, followed by ecesis, and has no connection with “weaker” species, or the development of a vital pressure. The direction taken by the migrating disseminules is essentially indeterminate. Migration seems to be outward, or away from the mass, merely because the ecesis is greater at the edge, where the increased dissimilarity between plant forms diminishes the competition. The actual movement is outward, but it takes place through the normal operation of competition. In this connection, it should be pointed out that the common view that plants require room is inexact, if not erroneous. This is difficult of proof, as it is impossible to distinguish room as such from the factors normally present, light, heat, water, and nutrient salts, but it seems obvious that the available amounts of these will determine the space occupied by a plant, irrespective of the room adjacent plants may allow it. The explanation of competition upon physical grounds likewise invalidates the view that plants possess spheres of influence other than the areas within which they exert a demonstrable reaction upon the physical factors present.

Competition plays a very important role in alternation. It produces minor examples of alternation in the physical units of an asymmetrical series. Its greatest influence, however, is exerted in modifying the effects of asymmetry. The reaction of occupants emphasizes or reduces the effect of asymmetry, and has a corresponding action upon alternation. This result of competition is typical of succession, in which the sequence of stages arises from the interaction of occupant and invader.

345. Kinds of alternation. Alternation involves two ideas, viz., the alternation of different species or formations with each other, and the alternation of the same species or formation in similar but separate situations. This is the evident result of asymmetry, in response to which contiguous areas are dissimilar and remote ones often similar. Individuals of the same species or examples of the same formation may be said to alternate between two or more similar situations, while different species or formations are said to alternate with each other, occurring usually in situations different in character. From the nature of alternation, the two phenomena are invariably found together.

It is possible to distinguish three kinds of alternation: (1) of a formation, consocies, layer, facies, or species in similar situations; (2) of similar or corresponding formations, species, etc., in similar situations; (3) of facies and other species with respect to number. The last two are merely variations of the first, arising out of slight differences in the physical factors of the alternating areas, the adjacent flora, or the course of competition. The alternation of different examples of the same formation is a significant feature of greatly diversified areas, such as mountains. It is naturally much less characteristic of lands physiographically more uniform. A xerophytic formation will alternate from ridge to ridge, a mesophytic formation between the intermediate valleys; aquatic vegetation will alternate from pond to pond, or stream to stream. The appearance of new or denuded soils upon which successions establish themselves is the most important cause of the alternation of formations. The weathering of rocks in different areas of the same region produces in each a sequence of similar or identical formations. The same statement is true in general of other causes of succession, such as erosion, flooding, burning, cultivation, etc., wherever they operate upon areas physically similar and surrounded by the same type of vegetation. The areas of more or less heterogeneous formations characterized by major physical differences are occupied by consocies. In an extensive formation, the same consocies alternates from one to another of these areas that are similar. When the formation is interrupted and occurs here and there in separate examples, a consocies often alternates from one to another of these. A consocies regularly derives its character from the fact that one or more of the facies of the formation is more intimately connected with certain areas of the latter than with others. This explains why the alternations of consocies and facies are usually identical. Layers sometimes alternate between different examples of the same forest or thicket formation, when they are suppressed in some by the diffuseness of the light.

The alternation of species is a typical feature of formations; it is absent only in those rare cases where the latter consist of a single species. The areas of a habitat which show minor physical or historical (i. e., competitive) differences are occupied by groups of individuals belonging to one or more species responsive to these differences. Each of these groups will recur in all areas essentially similar, the intervals being occupied of course by slightly different groups. Such groups are constituted by gregarious or copious species of restricted adjustability. Sparse plants likewise alternate, but they necessarily play a much less conspicuous part. In habitats not too heterogeneous, a large number of species are sufficiently adjustable to the slight differences so that they occur throughout the formation. Often, to be sure, they show a characteristic response, expressed in the size or number. This is illustrated by the facies and many of the principal species of the prairie formation. Festuca, Koelera, Panicum, and Andropogon occur throughout, except in the moist ravines which are practically meadows. Astragalus, Psoralea, Erigeron, and Aster grow everywhere on slopes and crests, but they are much more abundant in certain situations. Other plants, Lomatium, Meriolix, Anemone, Pentstemon, etc., recur in similar or identical situations upon different hills. Lomatium alternates between sandy or sandstone crests, Meriolix and Pentstemon occur together upon dry upper slopes, while Anemone alternates between dry slopes and crests.

Owing to the accidents of migration and competition, similar areas within a habitat are not occupied by the same species, or group of species. A species found in one area will be replaced in another by a different one of the same or a different genus. The controlling factors of the area render imperative an essential identity of vegetation and habitat form, though in systematic position the plants may be very diverse. Such genera and species may be termed corresponding. The relation between such plants is essentially alternation; it should, perhaps, be distinguished from alternation proper as corresponsive. The prairie formation furnishes a good example of this on exposed sandy crests, upon which Lomatium, Comandra, and Pentstemon alternate. Formations exhibit a similar correspondence.

Fig. 74. Numerical alternation of Pinus and Pseudotsuga upon east and west slopes.

All species that alternate show a variation in abundance from one area to another. Frequently, the difference is slight, and may be ignored, except in determining abundance. Very often, however, the variation is so great that a facies may be reduced, numerically, to the rank of a principal species, or one of the latter to a secondary species. This phenomenon is distinguished as numerical alternation. It arises from the fact that the similar areas are sufficiently different to affect the abundance, without producing complete suppression. It is probable that this result is due almost entirely to competition. Astragalus crassicarpus grows on all the slopes of the prairie formation, but on some it has the abundance of a facies, while on others it is represented by a few scattered individuals. This difference is much more striking in separate examples of the same formation, particularly when a normal facies is reduced to the numerical value of a secondary species. This is a matter of great importance in the study of formations, for it has doubtless often resulted in mistaking a consocies for a formation.