Essay on Opposing Views Concerning the Nature of the Community
1. The Organismic School:
This approach is developed by Clements (I916) and supported by ecologists such as Daubenmire (1966, 1968) and Langford and Buell (1969). The organism viewpoint regards the community as a sort of superior organism, the highest stage in the organization of the living world—rising from cell to tissue, organs, organ systems, organisms, population, and community.
The whole is more than the sum of its parts. Just as tissues have certain characteristics and function above and beyond those of the cells that comprise them, so the biotic community has characteristics and functions above and beyond the various populations it embodies.
The distribution and abundance of one species in the community are determined by the species interaction with the same community. Species making up the plant community are organised into discrete groups. Groups of stands similar to one another form associations. Stands of one association are clearly distinct from stands of other associations thus; the community acts as a unit in seasonal activity, in competition with other communities, in trophic functions, in succession, etc.
The organism school also argues that stands studied by gradient analysis are disturbed stands or ones that are not in equilibrium. If undisturbed, all stands develop to an endpoint in a few hundred years; and stand heading for an end point will naturally show a continuum of species.
In section 13-6, we have already observed, that the technique of gradient analysis forces data into a continuum. These gradients are usually based on one variable such as moisture; but it is impossible to restrict a continuum to one variable alone because many interacting variables influencing plant distribution are involved. The continuum approach assumes that all species are equal when in fact some are dominant.
2. The Individualistic School:
This approach is advanced by Gleason (1926) and further developed by Curtis (1959), McIntosh (1958, 1967), and Whittaker (1962, 1970).
Fig. 13.12. Four models of distribution of species along environmental gradients. A—The abundance of one species on an environmental gradient is independent of the others. Thus the association of several species along the gradient changes with the response of the individual species to that gradient. B—The abundance of one species is associated with that of another. The two or more species are always found in association with each other. C—The distribution of one species is independent of another in an environmental gradient, but the abundance and distribution of each species is sharply restricted at some point on the gradient by inter specific competition. D—The distribution of a species is sharply restricted by a change in some environmental variable. This occurs in ecotone (after Smith, 1977).
The individualistic viewpoint emphasizes the species rather than the community as the essential unit in the analysis of interrelationships and distribution. Species respond independently to the physiological and biotic environment according to their own genetic characteristics. They are not bound together into groups of associates that must appear together.
Instead, when species populations are plotted along an environmental gradient, long or short, the resulting graph suggests a normal or bell-shaped curve (Fig. 13.12). The curves of many species overlap in a heterogeneous manner In this way, the vegetation and its associated animal life show a gradient or continuum from one extreme (e.g., dry conditions) to another (e.g., wet conditions).
In this view the community is regarded as a collection of populations of species requiring the same environmental conditions. It is a continuous variable, not an integrated unit like the superior organism.
The supporters of individualistic approach suggest that community discontinuities do exist but that these reflect environmental discontinuities, such as breaks in soil types or sharp changes in moisture and salinity. If stands are as distinct as the organism group considers them to be, then the boundaries between associations should be distinct. But they rarely are.
Except where man or some environmental catastrophe such as fire has interfered, it is rare for the vegetation to be a mosaic of discontinuous units. Instead, boundaries between units are more or less diffuse. If separate stands of an association are similar, all such associations should have similar distribution, and the plants that comprise such associations should have distribution locally and over continental limits. If the association is a natural unit, then the species that comprise it should be bound together by obligate inter-relationships. But most species are not obligatory.
R. L. Smith (1977) has suggested that both of these schools may be partly correct. As the species that collectively form the biotic community may respond individually to the environment; yet, each community may also operate as a functional unit, especially in regard to the flow of energy and the cycling of nutrients.