Geoscience Reference
In-Depth Information
well known, and are reflected in the changing charac-
teristics 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 succession 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
The unit of the plant community was first studied by the US ecologist Frank Clements at the beginning of
the twentieth century. A group of plants with similar tolerances are able to live together in a definite
association. Some ecologists dispute the existence of communities as major units, and stress the life of
plants as individuals governed by their own limiting factors; in this view, albeit held only by a minority,
any associations are coincidental.
2
The main concepts which govern our knowledge of the nature of vegetation, and which determine the
methods by which it is studied, are six in number. Three of these relate to the units of study: the concepts
of the plant community, the ecosystem and the biome. Three relate to the factors which govern the
distribution of plants as individuals, species or growth forms: the concepts of range, limiting factors and
ecological niche.
3
Climate is the major determinant of Earth's major ecosystems or biomes. Climate brings in its effects
through temperature, precipitation, radiation and seasonality. Famous climatologists such as Köppen and
Thornthwaite have always stressed the links between climate and vegetation. The scheme by the German
biogeographer Walter provides a basis for dividing Earth into twelve major biomes.
4
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 outcompetes and replaces it.
5
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.
6
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.
7
Human actions have to be taken into account when considering the vegetation of any area, be it large or
small. It is doubtful whether any vegetation region on Earth has not been modified to some degree. Human
impacts have been severe where 'development' and agriculture have been important, as, for example in
the psammoseres on sand dunes, and haloseres on salt marshes.
