More detailed analysis of the plant-community reveals very considerable distinctions among commensals. Some relationships are considered in the succeeding paragraphs.
Like commensals.—When a plant-community consists solely of individuals belonging to one species—for example, solely of beech, ling, or Aira flexuosa—then we have the purest example of like commensals. These all make the same demands as regards nutriment, soil, light, and other like conditions; as each species requires a certain amount of space and as there is scarcely ever sufficient nutriment for all the offspring, a struggle for food arises among the plants so soon as the space is occupied by the definite numbers of individuals which, according to the species, can develop thereon. The individuals lodged in unfavorable places and the weaklings are vanquished and exterminated. This competitive struggle takes place in all plant-communities, with perhaps the sole exceptions of sub-glacial communities and in deserts. In these open communities the soil is very often or always so open and so irregularly clothed that there is space for many more individuals than are actually present; the cause for this is obviously to be sought in the climatically unfavorable conditions of life, which either prevent plants from producing seed and other propagative bodies in sufficient numbers to clothe the ground or prevent the development of seedlings. On such soil one can scarcely speak of a competitive struggle for existence; in this case a struggle takes place between the plant and inanimate nature, but to little or no extent between plant and plant.
That a congregation of individuals belonging to one species into one community may be profitable to the species is evident; it may obviously in several ways aid in maintaining the existence of the species, for instance, by facilitating abundant and certain fertilization (especially in anemophilous plants) and maturation of seeds; in addition, the social mode of existence may confer other less-known advantages. But, on the other hand, it brings with it greater danger of serious damage and devastation wrought by parasites.
The bonds that hold like individuals to a like habitat are, as already indicated, identical demands as regards existence, and these demands are satisfied in their precise habitat to such an extent that the species can maintain itself here against rivals. Natural unmixed associations of forest trees are the result of struggles with other species. But there are differences as regards the ease with which a community can arise and establish itself. Some species are more social than others, that is to say, better fitted to form communities. The causes for this are biological, in that some species, like Phragmites, Scirpus lacustris, Psamma (Ammophila) arenaria, Tussilago, Farfara, and Asperula odorata, multiply very readily by means of stolons; or others, such as Cirsium arvense, and Sonchus arvensis, produce buds from their roots; or yet others produce numerous seeds which are easily dispersed and may remain for a long time capable of germinating, as is the case with Calluna, Picea excelsa, and Pinus; or still other species, such as beech and spruce, have the power of enduring shade or even suppressing other species by the shade they cast. A number of species, such as Pteris aquilina, Acorus Calamus, Lemna minor, and Hypnum Schreberi, which are social, and likewise very widely distributed, multiply nearly exclusively by vegetative means, rarely or never producing fruit. On the contrary, certain species, for example, many orchids and Umbelliferae, nearly always grow singly.
In the case of many species certain geological conditions have favored their grouping together into pure communities. The forests of northern Europe are composed of few species, and are not mixed in the same sense as are those in the tropics, or even those in Austria and other southern parts of Europe: the cause for this may be that the soil is geologically very recent, inasmuch as the time that has elapsed since the glacial epoch swept it clear has been too short to permit the immigration of many competitive species.
Unlike commensals.—The case of a community consisting of individuals belonging to one species is, strictly speaking, scarcely ever met with; but the dominant individuals of a community may belong to a single species, as in the case of a beech forest, spruce forest, or ling heath—and only thus far does the case proceed. In general, many species grow side by side, and many different growth-forms and types of symbiosis, in the extended sense, are found collected in a community. For even when one species occupies an area as completely as the nature of the soil will permit, other species can find room and can grow between its individuals; in fact, if the soil is to be completely covered the vegetation must necessarily always be heterogeneous. The greatest aggregate of existence arises where the greatest diversity prevails. The kind of communal life resulting will depend upon the nature of the demands made by the species in regard to conditions of life. As in human communities, so in this case, the struggle between the like is the most severe, that is, between the species making more or less the same demands and wanting the same dishes from the common table. In a tropical mixed forest there are hundreds of species of trees growing together in such profuse variety that the eye can scarce see at one time two individuals of the same species, yet all of them undoubtedly represent tolerable uniformity in the demands they make as regards conditions of life, and in so far they are alike. And among them a severe competition for food must be taking place. In those cases in which certain species readily grow in each other's company—and cases of this kind are familiar to florists—when, for instance, Isoetes, Lobelia Dortmanna, and Litorella lacustris occur together—the common demands made as regards external conditions obviously form the bond that unites them. Between such species a competitive struggle must take place. Which of the species shall be represented by the greatest number of individuals certainly often depends upon casual conditions, a slight change in one direction or the other doubtless often playing a decisive rôle; but apart from this it appears that morphological and biological features, for example, development at a different season, may change the nature of the competition.
Yet there are in every plant-community numerous species which differ widely in the demands they make for light, heat, nutriment, and so on. Between such species there is less competition, the greater the disparity in their wants; the case is quite conceivable in which the one species should require exactly what the other would avoid; the two species would then be complementary to one another in their occupation and utilization of the same soil.
There are also obvious cases in which different species are of service to each other. The carpet of moss in a pine forest, for example, protects the soil from desiccation and is thus useful to the pine; yet, on the other hand, it profits from the shade cast by the latter.
As a rule, limited numbers of definite species are the most potent, and, like absolute monarchs, can hold sway over the whole area; while other species, though possibly present in far greater numbers than these, are subordinate or even dependent on them. This is the case where subordinate species only flourish in the shade or among the fallen fragments of dominant species. Such is obviously the relationship between trees and many plants growing on the ground of high forest, such as mosses, fungi, and other saprophytes, ferns, Oxalis Acetosella, and their associates. In this case, then, there is a commensalism in which individuals feed at the same table but on different fare. An additional factor steps in when species do not absorb their nutriment at the same season of the year. Many spring plants—for instance, Galanthus nivalis, Corydalis solida, and C. cava—have withered before the summer plants commence properly to develop. Certain species of animals are likewise confined to certain plant-communities. But one and the same tall plant may, in different places or soils, have different species of lowly plants as companions; the companion plants of high beech forests depend, for instance, upon climate and upon the nature of the forest soil; Pinus nigra, according to von Beck, can maintain under it in the different parts of Europe a Pontic, a central European, or a Baltic vegetation.
There are certain points of resemblance between communities of plants and those of human beings or animals; one of these is the competition for food which takes place between similar individuals and causes the weaker to be more or less suppressed. But far greater are the distinctions. The plant-community is the lowest form; it is merely a congregation of units, among which there is no co-operation for the common weal, but rather a ceaseless struggle of all against all. Only in a loose sense can we speak of certain individuals protecting others, as for example, when the outermost and most exposed individuals of scrub serve to shelter from the wind others, which consequently become taller and finer; for they do not afford protection from any special motive, such as is met with in some animal communities, nor are they in any way specially adapted to act as guardians against a common foe. In the plant-community egoism reigns supreme. The plant-community has no higher units or personages in the sense employed in connection with human communities, which have their own organizations and their members co-operating, as prescribed by law, for the common good. In plant-communities there is, it is true, often (or always) a certain natural dependence or reciprocal influence of many species upon one another; they give rise to definite organized units of a higher order; but there is no thorough or organized division of labor such as is met with in human and animal communities, where certain individuals or groups of individuals work as organs, in the wide sense of the term, for the benefit of the whole community.