Principles of biogeography - Fundamentals of the Physical Environment - page 494

freshwater hydrosere is called the hydrosere , whereas the
salt-water hydrosere is the halosere .
The principle governing the sequential changes which
occur during a succession is the principle of competitive
replacement , which states that 'a plant community in a
succession creates conditions which are more and more
favourable to more complex and demanding communities
which will outcompete and replace it'. The initial habitat
conditions are very demanding, and only a small number
of pioneer plant species can survive them, the net effect
of which is to create more favourable conditions able to
support a greater diversity of plants. The diversity in
species and in the structure of these seral communities
increases with time until environmental conditions
become stabilized, and a self-perpetuating climax com-
munity is established. An early student of succession was
the US ecologist Frank Clements in 1916. He envisaged
succession as an orderly and predictable evolutionary
process, following a definite pathway to a predictable
climax. He noted five basic processes: nudation,
migration, ecesis, reaction and stabilization. These terms
are defined in Table 20.4 .
Several amendments of Clements's basic model are
now accepted. Three alternative models are illustrated
in Figure 20.8 . The facilitation model mostly follows
Clements and envisages the establishment of plant
communities which modify the physical conditions so that
they become favourable to late successional species. In the
tolerance model successive stages in the succession depend
upon the competitive abilities and life spans of the plant
species, so that, for example, the longer-lived species
associated with later stages will persist in the community.
The third alternative, the inhibition model , envisages the
initial plant cover modifying the physical habitat so that
it is less favourable to colonization by other species, and
succession can occur only when the inhibitory species are
removed.
The classical views of succession have thus been modi-
fied to reflect research since Clements. There are some
elements of orderliness and predictability in successions,

than on a bare sterile surface. The pioneer stage ,when
plants are first establishing a foothold in the prisere, is
much shorter in the secondary succession. Figure 20.7 also
illustrates the importance of human activities in acting
either as an arresting or displacement factor. Grazing by
domestic animals or deliberate burning will maintain a
subclimax vegetation community which would otherwise
be a natural climax . For example, the heather moorlands
of the British uplands are burnt on a 12-year rotation,
when the plant becomes old and woody, in order to restore
young heather plants for grouse and sheep grazing ( Plate
20.3 ). Such communities are designated plagioclimaxes
('changed' climaxes) or, in the American literature,
disclimaxes ('disturbance' climaxes). The succession from
plagioclimax to climax is a type of subsere , or a succession
which proceeds from a subclimax community (say, a
plagioclimax) to climax. Another example would be where
the primary succession is halted by a dominant environ-
mental property (for example, excessive wetness). This
factor maintains the community in a condition which is
subclimax and not yet climax. Over time the influence of
the arresting factor will be lessened or removed, and the
subclimax community will proceed to climax via a
subsere. In order to understand fully the complexity of
vegetation patterns which one meets in the real world, it
is important to know whether a particular community
is seral, subclimax, plagioclimax or climax. This is the
ecological status of the ecological community, and it is not
always an easy attribute to judge. However, it is worth
undertaking an analysis of it, as it indicates the relation
of a particular community to the hierarchy of plant
communities in the ecological succession.

Types and models of succession

A commonly used classification of seres is based on the
nature of the surface from which the primary succession
starts. Those habitats where drought is the main limiting
factor are referred to as xeroseres . Two common situations
are where bare rock dominates as in new volcanic lava or
scree, or where sand dominates as in dunes in coastal,
fluvioglacial or desert location. The former cases are
lithoseres and the successional processes are directed at
the weathering of the bare, consolidated rock, and the
production of a soil upon it. In the latter cases successional
processes strive to stabilize the unstable sandy environ-
ment so that stable plant and soil communities can
develop; these are psammoseres . Other seres start with
almost the opposite type of conditions, unfavourable to
plant growth. These are habitats dominated by water in
lakes or marshes, and hence are termed hydroseres .The

Table 20.4 Fundamental successional processes

Term Process

Nudation Initial creation of bare surface

Immigration Arrival of available propagules

Ecesis Establishment of propagules

Reaction Interaction of plants and of plants and habitat

Stabilization Creation of equilibrium communities

Source: After Clements (1916).