28
2. Modeling in STELLA
RATE at 0 and 0.20, to represent a change in the birth rate when the population is between 0 and 600. Here we are using arbitrary numbers for a
made-up model. Finally, use the mouse arrow to draw a curve from the
maximum birth rate and population of 2 to the point of zero birth rate and
population of 600. For population sizes of 600 and above, the birth rate is
zero.
Suppose a census of the whale population was taken at three points in
time. The curve we just drew goes through all three points (Figure 2.10).
We can assume that, if a census had been taken at other times, it would
show a gradual transition through all the points. This sketch is good
enough for now . Click on OK.
Before we run the model again, let us speculate about our results. Think
of the graph for WHALES through time. Generally, it should rise, but not in
a straight line. At first the rise should be steep: the initial population is only
200, so the initial birth rate should be very high. The number of births
should decrease each time period as new additions to the stock reduce the
rate of reproduction. In the long run, the population should approach 600,
when the population's density would be so great that new births tend to
cease. Run the model. The result is shown as curve 2 in Figure 2.11. You
can compare this directly to the explosive dynamics of the previous section ,
shown as curve 1 in Figure 2.11.
1 0 . 080
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1 0 . 000
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600.00
.
.
2.000
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WHALES
FIGURE 2.10