Geoscience Reference
In-Depth Information
INITIAL START POINT OF SUCCESSION
Pioneers modify
environment to make
it less suitable to others
Early succession
species modify
environment
Short-lived pioneers
modify the
environment
Modification has no
effect on subsequent
recruitment of
'late succession' species
Late succession
species more adapted
to modified environment
Early colonists
persist and exclude
all other species
Climax species do not
permit invasion of
other species
Climax species do
not permit invasion
of other species
Mature, long-lived
individuals in climax
FACILITATION
MODEL
TOLERANCE
MODEL
INHIBITION
MODEL
but there are also elements of apparent disorderliness and
unpredictability due to factors of habitat variability,
propagule availability and replacement by purely random
chance. Successions are less 'unidirectional' than in the
classical model. Initial sites are usually very variable, and
the availability of propagules to colonize and replace
can differ between sites. Also external environmental
fluctuations can occur through the course of a succession.
Because of the complex nature of competition between a
wide range of species combinations, it is difficult to predict
an outcome. The importance of these plant factors gives
rise to what are termed
allogenic successions
(externally
induced successions), which contrast with
autogenic
successions
(self-induced successions), where distinct
seral changes result from autogenic habitat modification.
Differences between species in seed dispersal, germi-
nation rates, growth rates and longevity are important in
studying successions. Pioneer and early successional
species are usually short-lived 'r species' producing many
easily dispersed seeds which have long viability and
are capable of dispersal over long distances. The shade
tolerance of these plants is low, and growth rates are
rapid. By contrast late successional plants are long-lived
'k species' with slow growth rates, large size and shade
tolerance. Their seeds are also large, of short viability and
capable of dispersal for only short distances.
Lithoseres
The development of vegetation communities over time on
a fresh rock surface is called a lithosere. In some instances
the new unvegetated mineral surfaces are already uncon-
solidated. Many deposits resulting from the glaciations of
the Pleistocene era give mineral landscapes which are
already prepared for the invasion of land plants; they give
media such as glacial tills, fluvioglacial sands and aeolian
loess silts in which plants can readily take root. Similarly
fresh volcanic ash and recently deposited river alluvium
can quickly be covered by land plants. A lithosere on a
hard-rock surface, however, presents very different and
much more hostile conditions. Bare rock is classed as a
xerosere, as the surface is extremely dry, owing to the rapid
flow of any precipitation. The first plants to colonize such
a surface must be able to withstand complete drought and
must be able to cling to a bare rock surface devoid of soil.
This pioneer community typically consists of crustose
lichens which form the white, black or orange growths on
stone walls, gravestones or boulders. These lower plants
are a symbiosis between algae which are photosynthesizers
and fungi which provide nutrients by weathering minerals
and cements in the rock by chelation. Chelation is the
process in which organic molecules from humification,
and from root secretions, form soluble complexes with
