261
Wisheu and Keddy ( 1992 ) combined aspects
of both Grime’s and Tilman’s models to propose
a model of centrifugal organisation of plant
communities. This model describes the distribution of species and vegetation types along standing-crop gradients caused by combinations of
environmental constraints.
17.4 Models of Wetland
Community Development
There are some models which try to describe the
wetland community development. Some of these
are briefl y given below.
17.4.1 Functional Guild Model
Historically, the community concept has been of
immense value in ecology. However, it has been
criticised for being imprecise and not subject to
accurate predictive models for ecological communities. A number of ecologists have addressed this
problem in different ways. One approach is to
describe communities in terms of functional
‘guilds’ which could be defi ned by measurable
traits. A ‘guild’ may be defi ned as a group of functionally similar species in a community (Pianka
1983 ). It is important to note here that the guild
concept has at least three important advantages
over the generalised community concept. These are
as follows: (a) It collates (collapses) the large
number of species in a community to a manageable subset, (b) it defi nes guilds in terms of measurable functional properties and (c) it enables the
prediction of what guilds will be found given specifi c environmental conditions. Further, the application of guild concept could be found in the study
of birds and mammals (Simberloff and Dayan
1991 ), in the functional classifi cation of wetland
plants (Boutin and Keddy 1993 ), and so on.
17.4.2 Environmental Sieve Model
It was advocated by van der Valk ( 1981 ).
According to this model, the presence and
abundance of each species depend on its life
history and its adaptation to the environment of
a site. In van der Valk’s model, all plant species
are classifi ed into life history types, based on
potential lifespan, propagule longevity and
propagule establishment requirements. Each
life history type has a unique set of characteristics, and, thus, potential behaviour in response
to controlling environmental factors, such as
water level changes. These environmental factors comprise the ‘environmental sieve’ in van
der Valk’s model. As the environment changes,
so does the sieve and, hence, the species present. Smith and Kadlec ( 1985 ) had tested the
model’s ability to predict species composition
in a fresh marsh after a fi re and were satisfi ed
with the qualitative results.
17.4.3 Gap Dynamic Model
Chen and Twilley ( 1998 ) had used a gap dynamic
computerised model to simulate the growth and
composition of a mangrove forest in South
Florida. It is an individual-based model which
tracks the growth of each tree in a forest gap of
defi ned size, based on species-specifi c life history traits and limitations of resource availability
on the individual. This model has been used
extensively to model temperate and boreal forests and to simulate terrestrial forest dynamics
(Shugart et al. 1992 ).
17.5 Self-Organisation and
Self-Design
Self-design (Mitsch and Wilson 1996 ; Metzker
and Mitsch 1997 ) and the related concept of selforganisation (H.T. Odum 1989 ) are important
concepts in wetland ecosystem development.
Most wetland ecosystems are continually open to
atmospheric, hydrologic and biotic inputs of
propagules of plants, animals and microbes. It
may be noted here that ‘self-organisation’ manifests itself in both microcosms and newly created
ecosystems, showing that, after the fi rst period of
competitive colonisation, the species prevailing
17.5 Self-Organisation and Self-Design
Wisheu and Keddy ( 1992 ) combined aspects
of both Grime’s and Tilman’s models to propose
a model of centrifugal organisation of plant
communities. This model describes the distribution of species and vegetation types along standing-crop gradients caused by combinations of
environmental constraints.
17.4 Models of Wetland
Community Development
There are some models which try to describe the
wetland community development. Some of these
are briefl y given below.
17.4.1 Functional Guild Model
Historically, the community concept has been of
immense value in ecology. However, it has been
criticised for being imprecise and not subject to
accurate predictive models for ecological communities. A number of ecologists have addressed this
problem in different ways. One approach is to
describe communities in terms of functional
‘guilds’ which could be defi ned by measurable
traits. A ‘guild’ may be defi ned as a group of functionally similar species in a community (Pianka
1983 ). It is important to note here that the guild
concept has at least three important advantages
over the generalised community concept. These are
as follows: (a) It collates (collapses) the large
number of species in a community to a manageable subset, (b) it defi nes guilds in terms of measurable functional properties and (c) it enables the
prediction of what guilds will be found given specifi c environmental conditions. Further, the application of guild concept could be found in the study
of birds and mammals (Simberloff and Dayan
1991 ), in the functional classifi cation of wetland
plants (Boutin and Keddy 1993 ), and so on.
17.4.2 Environmental Sieve Model
It was advocated by van der Valk ( 1981 ).
According to this model, the presence and
abundance of each species depend on its life
history and its adaptation to the environment of
a site. In van der Valk’s model, all plant species
are classifi ed into life history types, based on
potential lifespan, propagule longevity and
propagule establishment requirements. Each
life history type has a unique set of characteristics, and, thus, potential behaviour in response
to controlling environmental factors, such as
water level changes. These environmental factors comprise the ‘environmental sieve’ in van
der Valk’s model. As the environment changes,
so does the sieve and, hence, the species present. Smith and Kadlec ( 1985 ) had tested the
model’s ability to predict species composition
in a fresh marsh after a fi re and were satisfi ed
with the qualitative results.
17.4.3 Gap Dynamic Model
Chen and Twilley ( 1998 ) had used a gap dynamic
computerised model to simulate the growth and
composition of a mangrove forest in South
Florida. It is an individual-based model which
tracks the growth of each tree in a forest gap of
defi ned size, based on species-specifi c life history traits and limitations of resource availability
on the individual. This model has been used
extensively to model temperate and boreal forests and to simulate terrestrial forest dynamics
(Shugart et al. 1992 ).
17.5 Self-Organisation and
Self-Design
Self-design (Mitsch and Wilson 1996 ; Metzker
and Mitsch 1997 ) and the related concept of selforganisation (H.T. Odum 1989 ) are important
concepts in wetland ecosystem development.
Most wetland ecosystems are continually open to
atmospheric, hydrologic and biotic inputs of
propagules of plants, animals and microbes. It
may be noted here that ‘self-organisation’ manifests itself in both microcosms and newly created
ecosystems, showing that, after the fi rst period of
competitive colonisation, the species prevailing
17.5 Self-Organisation and Self-Design