STELLA is a program that allows you to solve systems of Ordinary
Differential Equations without ever seeing the equations (which makes
me sad). It has a slightly peculiar syntax but here we will work
through one problems to show how to solve Equation (2). In
the language of STELLA, our model will be made of *Reservoirs*,
*Flows* and *Converters*. The reservoir holds the
population *N* and is given an initial value (say 5). The flows hold
the actual rate equations and say how the reservoir will change in
time. There are three kinds of flow available: *Inflows* make
the reservoir increase in value, *outflows* decrease the
reservoir and *bidirectional flows* allow material to come in and
out. Converters hold parameters and ancillary equations. To make
everything work, all the variable held in different places need to be
connected by arrows (I will show you how to do all this). Figure
2 shows the Stella version the first BioBomb model given in
Equation (1) and has one reservoir (Population), an inflow
(birth rate) an outflow (death rate) and two converters to hold the
constants *b* and *d*. Obvious... isn't it?

Figure 3 shows the simpler model for the one parameter problem Equation
(2). Note there is only one bi-directional flow which will
let us make *r* be both negative and positive.

**Figure 4:** Example STELLA output for the problem of
exponential growth for *r*=0.2 and initial conditions

Mon Sep 22 21:30:22 EDT 1997