ENVIRONMENTAL SCIENCE : ECOSYSTEM 131
spans and that account for much larger changes in community composition and structure.
These include ecological succession and community evolution.
It is evident from the above said that the term succession denotes a sequence of changes
in the species composition of a community, which is generally associated with a sequence
of changes in its structural and functional properties. The term is generally used for temporal
sequence (in terms of years, decades or centuries) of vegetation on a site; although only
short term changes can be observed directly and the long term ones are inferred from
spatial sequences.
The changes associated with succession are usually progressive or directional. This fact
enables one to predict which species are likely to replace other in the course of a succession.
Sucession tends to continue until the species combinations best suited to the regional climate
and the particular site are established.
Historical Background
The basic idea of succession was in the beginning forwarded by Anon Kerner (1863) in
his book “Plant Life of the Danube Basin” during the description of the regeneration of a
swamp forest. The term ecological succession was first of all used by Hult (1885) in the study
of communities of Southern Sweden. H.C. Cowles held that communities are not static but
dynamic. This changed understanding be visualized as an orderly, directional and predictable
phenomenon. It was added that succession is autogenic i.e. regulated by biotic interactions
within the community. The central foundation of the classical theory was that early
communities alter the environment to their detriment and favour later successional
communities. It was revealed by the later studies that allogenesis was perhaps more common
and dominant than autogenesis; allogenesis means the control of community dynamics by
factors originating outside the community boundaries.
The succession of animals on these dunes was studied by ‘Shelford (1913). Later on,
Olson (1958) restudies the ecosystem development on these dunes and has given us an
updated information about it. Federick Clements (1907-1936) elaborates the principles and
theory of succession. He proposed the monoclimax hypotesis of succession. During the later
years certain other hypotheses were proposed by various ecologists to explain the nature of
climax communities: for example, polyclimx hypothesis by Braun-Blanquet (1932) and Tansley
(1939): climax pattern hypothesis by Whittaker (1953), Mac intosh (1958) and Sellack (1960):
and stored energy theory of information theory by Fosberg (1965, 1967) and Odum (1969).
Odum (1969) defined succession in terms of 3 parameters, viz.:
(1) Succession is an orderly process of reasonable directional and fairly predictable
community development;
(2) Succession results from modification of the physical environment by a community,
i.e. succession is largely community controlled.
(3) Succession culminates in a stabilized ecosystem in which maximum biomass and
symbiotic function between organisms are maintained per unit of available energy
flow. Whittaker (1975), held that through the course of succession community
production, height and mass, species-diversity, relative stability, and soil depth and
differentiation generally all tend to increase. The culminating point of succession
is a climax community of relatively stable species composition and steady-state
function, It is adapted to its habitat. It is permanent in its habitat if it is not
disturbed.