system, and ultimately it is the confederate of the causally-oriented theorist.
Real-world systems typically contain both reinforcing and balancing processes,
often with their strengths varying over time.
Variation in feedback processes can be brought about by nonlinear relationships.
Such nonlinear relationships are present if a control variable does not depend on
other variables in a linear fashion but changes, for example, with the square root of
some other variable. As a result of nonlinear feedback processes, systems can
exhibit complex dynamic behavior. Similarly, adjustments in system components
to influences on them may not occur instantly but be lagged in time. Consequently,
system modelers must pay special attention to nonlinearities and lagged effects that
describe the relationships among models components.
Throughout this book we encounter a variety of nonlinear feedback processes
that give rise to complex system behavior. Let us develop a simple model to
illustrate the concepts of state variables, flows, and feedback processes. We will
then return to discuss some “principles of modeling” that will help you to structure
the model building process in a set of steps.
1.5 Modeling in STELLA
To explore modeling with STELLA, we will develop a basic model of the dynamics
of a fish population. Assume you are the sole owner of a pond that is stocked with
200 fish that all reproduce at a fixed rate of 5 % per year. For simplicity, assume also
that none of the fish die. How many fish will you own after 20 years?
A run-time version of STELLA can
be downloaded for free at the
following internet site:
www.iseesystems.com/modelingdynamicbiologicalsystems.
There you will also find the models
developed and described in this book.
The models are also available at:
www.iseesystems.com/modelingdynamicbiologicalsystems.
In building the model, we will utilize all four of the graphical “tools” for
programming in STELLA. On opening STELLA, you will be faced with a user
interface that shows, on the left side, a series of tabs. Click on the Model tab and
you should see a window as in Fig. 1.1. The left-hand side can be used to develop
the model, get an overview over its structure (that is the mapping layer), build a user
interface for it, or display the equations, initial conditions and such, on which the
1.5 Modeling in STELLA
11
Real-world systems typically contain both reinforcing and balancing processes,
often with their strengths varying over time.
Variation in feedback processes can be brought about by nonlinear relationships.
Such nonlinear relationships are present if a control variable does not depend on
other variables in a linear fashion but changes, for example, with the square root of
some other variable. As a result of nonlinear feedback processes, systems can
exhibit complex dynamic behavior. Similarly, adjustments in system components
to influences on them may not occur instantly but be lagged in time. Consequently,
system modelers must pay special attention to nonlinearities and lagged effects that
describe the relationships among models components.
Throughout this book we encounter a variety of nonlinear feedback processes
that give rise to complex system behavior. Let us develop a simple model to
illustrate the concepts of state variables, flows, and feedback processes. We will
then return to discuss some “principles of modeling” that will help you to structure
the model building process in a set of steps.
1.5 Modeling in STELLA
To explore modeling with STELLA, we will develop a basic model of the dynamics
of a fish population. Assume you are the sole owner of a pond that is stocked with
200 fish that all reproduce at a fixed rate of 5 % per year. For simplicity, assume also
that none of the fish die. How many fish will you own after 20 years?
A run-time version of STELLA can
be downloaded for free at the
following internet site:
www.iseesystems.com/modelingdynamicbiologicalsystems.
There you will also find the models
developed and described in this book.
The models are also available at:
www.iseesystems.com/modelingdynamicbiologicalsystems.
In building the model, we will utilize all four of the graphical “tools” for
programming in STELLA. On opening STELLA, you will be faced with a user
interface that shows, on the left side, a series of tabs. Click on the Model tab and
you should see a window as in Fig. 1.1. The left-hand side can be used to develop
the model, get an overview over its structure (that is the mapping layer), build a user
interface for it, or display the equations, initial conditions and such, on which the
1.5 Modeling in STELLA
11