25
Ecosystem Modeling
Herman H. Shugart
Introduction
In the usual usage, ecological models are mathematical expressions developed to be analogous, in
some sense, with an ecosystem of interest (Table
25.1). The models of principal interest here are
those are used to integrate information and to produce predictions of responses of ecosystems to
change. With an abruptly increasing availability of
computer power in terms of speed, cost, and magnitude of computation over the past two decades,
there has been an explosive development of the application of computer models in ecology as well as
in other sciences. New computer software allows
scientists to explore complex dynamic equations
using small (but powerful) computers in the same
manner that an earlier generation of ecologists used
paper as "scratch pads" to sketch data patterns and
dynamic interrelations. The mathematical techniques used in developing and analyzing ecological
models have been treated in several books (e.g.,
Caswell et al. 1972; Smith 1974; Odum 1983; Jj/lrgensen 1986; Beltrami 1987; Yodzis 1989; Shugart
1998) and are the focus of several ecological
journals.
Dale (1970) enumerated four phases of ecosystem analysis:
1. Lexical phase-determining system entities or
parts.
2. Parsing phase-choosing the types of relationships among the parts that are of interest.
3. Modeling phase-specifying the mechanisms
by which these interrelationships take place.
4. Analysis phase-investigating the properties of
the model so produced (including validation of
the model of the ecosystem).
These can be used as a framework to discuss the
broad topic of ecosystem modeling.
Lexical Phase
The determination of the parts of an ecosystem is
often considered the simplest aspect of ecosystem
studies involving modeling, but it often proves to
be critical to later applications of models. The parts
of an ecosystem can be classified taxonomically
into categories such as individuals, populations,
species, genera, etc. (Dale 1970). They can also be
grouped according to structural criteria such as lifeform criteria (e.g., trees, shrubs, grass) or functional
criteria (herbivores, carnivores, decomposers).
Considerations in defining the parts of the system
are involved with the objectives of a given study
and the intent of other phases of the systems analysis process. A principal simplification for the
mathematical models used in ecology involves ignoring sources of system heterogeneity. For example, models of population growth typically use
the numbers of individuals in a popUlation as a state
variable and ignore the statistics of the ages and
sexes in the population. This is done even in the
face of commonsense intuition that the ages and
sexes in a population should have important consequences on birth and death rates. Similarly, models of element cycles ignore the spatial heterogeneity that seems a dominant feature of terrestrial
373
Ecosystem Modeling
Herman H. Shugart
Introduction
In the usual usage, ecological models are mathematical expressions developed to be analogous, in
some sense, with an ecosystem of interest (Table
25.1). The models of principal interest here are
those are used to integrate information and to produce predictions of responses of ecosystems to
change. With an abruptly increasing availability of
computer power in terms of speed, cost, and magnitude of computation over the past two decades,
there has been an explosive development of the application of computer models in ecology as well as
in other sciences. New computer software allows
scientists to explore complex dynamic equations
using small (but powerful) computers in the same
manner that an earlier generation of ecologists used
paper as "scratch pads" to sketch data patterns and
dynamic interrelations. The mathematical techniques used in developing and analyzing ecological
models have been treated in several books (e.g.,
Caswell et al. 1972; Smith 1974; Odum 1983; Jj/lrgensen 1986; Beltrami 1987; Yodzis 1989; Shugart
1998) and are the focus of several ecological
journals.
Dale (1970) enumerated four phases of ecosystem analysis:
1. Lexical phase-determining system entities or
parts.
2. Parsing phase-choosing the types of relationships among the parts that are of interest.
3. Modeling phase-specifying the mechanisms
by which these interrelationships take place.
4. Analysis phase-investigating the properties of
the model so produced (including validation of
the model of the ecosystem).
These can be used as a framework to discuss the
broad topic of ecosystem modeling.
Lexical Phase
The determination of the parts of an ecosystem is
often considered the simplest aspect of ecosystem
studies involving modeling, but it often proves to
be critical to later applications of models. The parts
of an ecosystem can be classified taxonomically
into categories such as individuals, populations,
species, genera, etc. (Dale 1970). They can also be
grouped according to structural criteria such as lifeform criteria (e.g., trees, shrubs, grass) or functional
criteria (herbivores, carnivores, decomposers).
Considerations in defining the parts of the system
are involved with the objectives of a given study
and the intent of other phases of the systems analysis process. A principal simplification for the
mathematical models used in ecology involves ignoring sources of system heterogeneity. For example, models of population growth typically use
the numbers of individuals in a popUlation as a state
variable and ignore the statistics of the ages and
sexes in the population. This is done even in the
face of commonsense intuition that the ages and
sexes in a population should have important consequences on birth and death rates. Similarly, models of element cycles ignore the spatial heterogeneity that seems a dominant feature of terrestrial
373