Environmental Engineering Reference
In-Depth Information
A disturbance opens up a relatively large space
Onlly certain “pioneer” species
are capable of becoming
established in the open space
Individuals of any species in the succession could establish and exist as
adults under the prevailing conditions
Modification of the environment
by the early occupants
has little or
no effect
on the recruitment of
“late-successional” species
Modification of the environment by the early
occupants makes it
less suitable
for the
recruitment of “late-successional” species
Modification of the
environment by the
early occupants makes
it
more suitable
for
the recruitment of
“late-successional”
species
As long as earlier
colonists persist
undamaged or continue to
regenerate vegetatively,
they exclude or suppress
subsequent colonists of all
species
Earlier species are eliminated
througth competition for
resources with established
“late successional” adults
The sequence continues until
the current resident species
no longer facilitate the
invasion and growth of
others species or until no
species exists that can invade
and grow in the present of
the resident
Facilitation Model
If external stresses are
present early colonists
may be damaged (or
killed) and replaced by
species which are
more resistant
Tolerance Model
Inhibition Model
Fig. 3.3 Three different models of ecological succession from J.H. Connell and R.O. Slatyer [
19
]
depicting succession as arising from the resident species at a location encouraging the colonization
of a subsequent set of species (Facilitation Model), by the resident species being relatively neutral
to the other species (Tolerance Model), or by the resident species inhibiting the colonization by
other species (Inhibition Model)
Tansley created the ecosystem concept as in contrast to Phillips' and Clements'
views. It is significant that in his iconoclastic “ecosystem” definition, he
emphasized ecosystems “were of the most various kinds and sizes.” In doing so,
Tansley basically defined an ecosystem as what systems scientists would nowadays
call a
system of definition
: an arbitrary system defined by the specific considerations
for a particular application [
37
]. Tansley's ecosystem definition conforms well to
more mathematical, interactive-system concepts in other sciences. His definition
includes the intrinsic consideration of scale found in other sciences, particularly
physically based sciences.
Tanssley also endorsed the concept of the “polyclimax”, the possibility of
multiple stable ecosystems following from succession and a logical contrast to
the Clements idea of the “monoclimax” as the eventual product of successional
processes working in a region. An example of the polyclimax is shown in
Fig. 3.3
for succession after fires in the boreal forests in the vicinity of Fairbanks, Alaska.
In this case, the pattern of succession following a wildfire on south-facing slopes
