Environmental Engineering Reference
In-Depth Information
Plate 21.2
Secondary succession of grasses, herbs, shrubs
and tree seedlings invading a clear-cut made by forestry
operations in British Columbia, Canada.
Photo: Ken Atkinson.
water hydrosere is called the
hydrosere
(p. 434), whereas the salt-water hydrosere is a
halosere
(p. 435). Figure 21.3 illustrates the classification of primary successions. The
general principle governing the sequential changes which occur during a succession
typically follows 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 out-compete and replace it'. The
initial habitat conditions are very demanding, and only a small number of plant species
can survive them. However, the net effect of those
pioneer
plants is to create less severe
conditions which in turn can support a greater diversity of plants. Pioneers arrive and
become established at a site, and with time alter the habitat so that other species can
survive. 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 community is formed. Many of the early studies of succession were carried out by
the US ecologist Clements in 1916. He envisaged succession as an orderly and
predictable evolution, following definite pathways to a predictable climax (p. 437). He
envisaged succession as five basic processes: nudation, migration, ecesis, reaction and
stabilization. These terms are defined in Table 21.1.
