Likewise, if a landslide exposes a surface of rock in the mountains, the surface may be successively occupied by a sparse cover of lichens; a spreading moss mat; grasses, which enter and become a meadow; a shrub thicket, which overtops and suppresses the grasses; a first forest stage of smaller trees, which seed into the shrub thicket, grow through it, and replace it; and a final stage of larger trees, which take dominance from the first trees and may form a larger and potentially permanent forest community.
The changes in the structure and composition of the community are rapid at first, slowing gradually until a point of dynamic equilibrium is reached, and the community is more or less stable.
Such an orderly and progressive replacement of one community by another until a relatively stable community, called the climax community, occupies the area is called ecosystem development (Odum, 1971) or ecological succession (Smith, 1977). In the first example the principal cause of the change in the community was a physical process-the filling in of the lake with silt.
In the second example, a principal cause was the growth of plants on an existing soil. In the third example the succession proceeded by a back-and-forth interplay between organisms and environment: one dominant species modified the soil and microclimate in ways that made possible the entry of a third dominant, which in turn altered its environment. Causes of succession changes are, to varying degrees, external to the community or internal to the community, many successions involve both kinds of causes and reciprocal influences (Whittaker, 1970).
Succession is the “birth” of an ecosystem and subsequent “aging” process of its biotic and biotic features. Odum (1971) has rightly included the following three parameters in his definition of ecological succession:
(1) It is an orderly process of community development that involves changes in species structure and community processes with time; it is reasonably directional and, therefore, predictable.
(2) It results from modification of the physical environment by the community; that is, succession is community-controlled even though the physical environment determines the pattern, the rate of change and often sets limits as to how far development can go.
(3) It culminates in a stabilized ecosystem in which maximum biomass (or high information content) and symbiotic function between organisms is maintained per unit of available energy flow.
In fact, ecological succession is a complex dynamic interaction between the community inhabiting the ecosystem, the geological Factors determining the patterns of materials transfer, the current state of the ecosystem as a whole, and time. Insofar as changes in the general state of the ecosystem are related to the community inhabiting it and alteration of the community is a function of the evolution of the general state of the ecosystem, succession proceeds under feedback control (Fig. 14.1).
The basic idea of succession was first of all forwarded by Anon Kerner (1863) in his book “Plant Life of the Danube Basin” during the description of the regeneration of a swamp forest. However, the term ecological succession was first of all used by Hult (1885) in the study of communities of Southern Sweden. H.C. Cowles (1899) laid the foundation of the succession studies by publishing his classic paper dealing with vegetation of sand dunes of Lake Michigan. The succession of animals on these dunes was studied by Shelford (1913).
Later on, Olson (1958) restudied the ecosystem development on these dunes and has given us updated information about it. Frederick Clements (1907-1936) is credited with elaborating the principles and theory of succession. He proposed the monoclimax hypothesis of succession.
During the later years certain other hypotheses were proposed by various ecologists to explain the nature of climax communities: for example, polyclimax hypothesis by Braun-Blanquet (1932), and Tansley (1939); climax pattern hypothesis by Whittaker (1953), Macintosh (i 958) and Sellack (1960): and stored energy theory or information theory by Fosberg (1965, 1967) and Odum (1969).