Environmental Engineering Reference
In-Depth Information
CONCLUSION
The study of ecological successions has formed an important part of twentieth-century
biogeography. From the stimulus provided by the early work of Clements have come
classic studies by Tansley, Hutchinson, Whittaker and Aubréville, among others. The
processes operating in primary and secondary successions are quite well known, and are
reflected in the changing characteristics of soils and plant communities with time. The
overall strategy of successional development, and the nature of the climax community in
it, are less well understood. There are four main theories of climax vegetation, and each
may have some relevance in a particular situation. Owing to clear differences in growing
conditions we must not expect successions to have the same overall strategies in tropical,
temperate and polar climates. Overall, however, the polyclimax theory of Tansley has
many supporters.
Changes in soils and plant communities during the course of a succession not only
involve changing soils and plant species but also are accompanied by changes in
ecological processes relating to energy flow, diversity creation, nutrient cycling and
ecological stability. Later stages of successions show increases in species biomass, in the
structural complexity of the community and in the efficient utilization of energy and
nutrient resources. Structure and function become increasingly self-regulated in climax
communities, though species diversity usually declines in climax communities as poorly
adapted species are eliminated.
KEY POINTS
1
Primary successions or priseres start when new habitats on land and water become
available for colonization. Successive plant communities occupy the sites, starting
with pioneer communities and finishing with climax vegetation. The principle of
competitive exclusion operates, whereby each community creates conditions
favourable to a succeeding community, which eventually out-competes and replaces it.
2
Secondary successions occur when a land surface utilized by human activity is made
available to recolonization when the land use is abandoned. Thus old farmland
becomes reinvaded by a pioneer community, or a forest clearing used for farming, as
in tropical shifting cultivation, is abandoned.
3
Climax vegetation marks the end point of succession. It is the most stable,
conservational and massive ecological community. Over the years of studying climax
vegetation, four views of climax have arisen, namely monoclimax, polyclimax, mosaic
climax and cyclical climax. Mosaic and cyclical climaxes have been reported from
particular biomes, but the polycyclical theory appears to be the most widely relevant
on a global scale.
