260
17.2 Autogenic Versus Allogenic
Processes
In the classical view of succession, wetlands are
considered as transient stages in the ‘hydrarch
development’ of a terrestrial forested climax community from a shallow lake. According to this
view, lakes gradually fi ll in as organic materials
from dying plants accumulate and minerals are carried in from upslope. At fi rst, change is slow
because the source of organic material is singlecelled plankton. When the lake becomes shallow
enough to support rooted aquatic plants, however,
the pace of organic deposition increases. Eventually,
the water becomes shallow enough to support
emergent marsh vegetation. This continues to build
a peat mat. Eventually, shrubs and small trees also
appear. They continue to transform the site to a terrestrial one. This is done not only by adding organic
matter to the soil but also by drying it through
enhanced evapotranspiration. Finally, a climax terrestrial forest occupies the site (Cooper 1913 ). It is
important to note here that forests do occur on the
sites of former lakes, a fact, which has been certainly documented (Larsen 1982 ).
Notwithstanding the above, the wetlands are in
the centre of dispute about the importance of autogenic versus allogenic processes because of their
transitional nature. In addition to being ‘seres’,
wetlands are often described as being ‘ecotones’.
The latter are transitional spatial gradients between
adjacent aquatic and terrestrial environments.
Thus, wetlands could be considered as transitional
in both space and time. Further, as ecotones, wetlands usually interact strongly to varying (allogenic) forces from both ends of the ecotone. These
forces may push a wetland towards its terrestrial
neighbour, if, for example, regional water levels
fall, or towards its aquatic neighbour, if water levels rise. Alternatively, production of organic matter by plants may raise the level of wetland
(thereby, decreasing its depth) and result in a drier
environment, in which different species succeed.
Further information on these aspects may be
obtained from Cowles ( 1899 , 1911 ), Shelford
( 1911 ), Wilcox and Simonin ( 1987 ), Singer et al.
( 1996 ) and so on.
17.3 Community Concept
and the Continuum Idea
The identifi cation of a ‘community’ is a conceptual
issue. It is confused by the scale of perception. A
community may be regarded as a population of living organisms having homogeneity. Homogeneity is
an index of community. Further, supporters of continuum concept may argue that the scale dependence of plant associations illustrates that individual
species may simply respond to subtle environmental cues, implying little, if anything, about ‘communities’. Also that plant zonation simply indicates an
environmental gradient to which individual species
may respond. The possible reason for sharp zonation
in many wetlands could be that the environmental
gradients are ‘ecologically’ steep. Also, groups of
species, which have fairly similar tolerances, tend to
group on these gradients.
In contrast to above, one major difference
between the classical community ecologists and
proponents of the continuum idea is the greater
emphasis put on allogenic processes by the latter.
In wetlands, abiotic environmental factors often
seem to overwhelm biotic factors. Under these
circumstances, the response of the vegetation is
determined by these abiotic factors. However, the
wetlands are often in dynamic equilibrium with
the abiotic forces. Such an equilibrium is often
called ‘pulse stability’ (E.P. Odum 1971 ).
17.3.1 Centrifugal Organisation
Concept
A number of other models of community change
have been developed, although a few have been
applied to wetlands. Grime ( 1979 ) had proposed
that changes in species composition and richness
of herbaceous plants were related to the gradients
of disturbance and stress factors, which reduce
biomass and determine which functional plant
strategies would work best. Tilman ( 1982 ) suggested that competition amongst plants controls
community plant distribution, with each species
limited by different ratio of resources and spatial
heterogeneity of resources.
17 Wetland Ecosystem Development
17.2 Autogenic Versus Allogenic
Processes
In the classical view of succession, wetlands are
considered as transient stages in the ‘hydrarch
development’ of a terrestrial forested climax community from a shallow lake. According to this
view, lakes gradually fi ll in as organic materials
from dying plants accumulate and minerals are carried in from upslope. At fi rst, change is slow
because the source of organic material is singlecelled plankton. When the lake becomes shallow
enough to support rooted aquatic plants, however,
the pace of organic deposition increases. Eventually,
the water becomes shallow enough to support
emergent marsh vegetation. This continues to build
a peat mat. Eventually, shrubs and small trees also
appear. They continue to transform the site to a terrestrial one. This is done not only by adding organic
matter to the soil but also by drying it through
enhanced evapotranspiration. Finally, a climax terrestrial forest occupies the site (Cooper 1913 ). It is
important to note here that forests do occur on the
sites of former lakes, a fact, which has been certainly documented (Larsen 1982 ).
Notwithstanding the above, the wetlands are in
the centre of dispute about the importance of autogenic versus allogenic processes because of their
transitional nature. In addition to being ‘seres’,
wetlands are often described as being ‘ecotones’.
The latter are transitional spatial gradients between
adjacent aquatic and terrestrial environments.
Thus, wetlands could be considered as transitional
in both space and time. Further, as ecotones, wetlands usually interact strongly to varying (allogenic) forces from both ends of the ecotone. These
forces may push a wetland towards its terrestrial
neighbour, if, for example, regional water levels
fall, or towards its aquatic neighbour, if water levels rise. Alternatively, production of organic matter by plants may raise the level of wetland
(thereby, decreasing its depth) and result in a drier
environment, in which different species succeed.
Further information on these aspects may be
obtained from Cowles ( 1899 , 1911 ), Shelford
( 1911 ), Wilcox and Simonin ( 1987 ), Singer et al.
( 1996 ) and so on.
17.3 Community Concept
and the Continuum Idea
The identifi cation of a ‘community’ is a conceptual
issue. It is confused by the scale of perception. A
community may be regarded as a population of living organisms having homogeneity. Homogeneity is
an index of community. Further, supporters of continuum concept may argue that the scale dependence of plant associations illustrates that individual
species may simply respond to subtle environmental cues, implying little, if anything, about ‘communities’. Also that plant zonation simply indicates an
environmental gradient to which individual species
may respond. The possible reason for sharp zonation
in many wetlands could be that the environmental
gradients are ‘ecologically’ steep. Also, groups of
species, which have fairly similar tolerances, tend to
group on these gradients.
In contrast to above, one major difference
between the classical community ecologists and
proponents of the continuum idea is the greater
emphasis put on allogenic processes by the latter.
In wetlands, abiotic environmental factors often
seem to overwhelm biotic factors. Under these
circumstances, the response of the vegetation is
determined by these abiotic factors. However, the
wetlands are often in dynamic equilibrium with
the abiotic forces. Such an equilibrium is often
called ‘pulse stability’ (E.P. Odum 1971 ).
17.3.1 Centrifugal Organisation
Concept
A number of other models of community change
have been developed, although a few have been
applied to wetlands. Grime ( 1979 ) had proposed
that changes in species composition and richness
of herbaceous plants were related to the gradients
of disturbance and stress factors, which reduce
biomass and determine which functional plant
strategies would work best. Tilman ( 1982 ) suggested that competition amongst plants controls
community plant distribution, with each species
limited by different ratio of resources and spatial
heterogeneity of resources.
17 Wetland Ecosystem Development