Geoscience Reference
In-Depth Information
Figure 20.7 has been adapted from the work of the British
biogeographer Eyre to give the detailed terminology used
in the theory of succession. If succession commences on
a bare surface of either land or water, which has not
previously been occupied by a plant community, it is
called a primary succession or prisere ( Plate 20.1 ).
A new land surface of coastal sand or volcanic lava and
ash is a typical site. Primary successions on new volcanic
surfaces such as Krakatoa in the Indian Ocean, Surtsey
Island in the Atlantic or Mount St Helens in the US state
of Washington are all classic examples. However, where the
community develops in an area which was previously
vegetated, but from which the community was removed,
the sequence is a secondary succession . Examples would
include areas cleared for farming but then abandoned,
areas of clear-felled forest ( Plate 20.2 ), or a plant com-
munity destroyed by a natural disaster such as floods,
tsunami and fire.
Secondary successions are usually more rapid than
primary ones, as seeds and seedlings are present from the
start and initial soil conditions are much more favourable
THEORIES OF VEGETATION
SUCCESSIONS
Ecological succession is the term used to signify the changes
in the composition of a community over time. It refers to
the sequence of communities which replace one another
in a given area. The entire sequence of stages is referred
to as the succession or sere , each temporary stage in the
succession being called a seral stage . Plant species invade
the site when conditions are favourable, and are
eliminated when the succession leads to unfavourable
local conditions. Seral stages are thus defined by the
changing dominance of plant species, together with
associated soils and fauna. Each stage has a distinct
ecology which is ephemeral in the sense that it prepares
the ground for the succeeding ecosystem. The final end
point of the succession is called the climax community .
Considerable attention has been paid to successions in
biogeography because they reflect the dynamic nature of
ecological communities, and illustrate the importance of
the time factor in the development of plant communities.
PLAGIOCLIMAX
COMMUNITY
Grazing,
firing, etc.
relaxed
Grazing,
firing, etc.
&/,0$;
&20081,7<
Arresting factor
removed
ARRESTING
FACTOR
Felling, burning
or cultivation
SUBCLIMAX
COMMUNITY
BARE
SOIL
BARE, INORGANIC
SURFACE
Figure 20.7 Priseres, subseres and plagioclimaxes.
Source: After Eyre (1968)
 
 
 
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