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140000
no sick foxes
rabies virus introduced
120000
100000
80000
60000
40000
20000
0
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year
Fig. 15.9
15.2.13 Rabies Pressure
Although hunting pressure now exists on an apparently declining Illinois fox popu-
lation, we thought the presentation of our results would be clearer if we started the
model with the fox population at or near the 1970 estimated mean level of about
88,000. We lowered the initial population in the model, but the long-term results of
subsequent runs were unaffected by this kind of change in the initial conditions. For
the first part of our model runs, we choose to suspend the effects of hunting to allow
us to gauge the impact of the disease alone.
Healthy fox populations were set initially at 80% of the carrying capacity of
the cell. The model results show that the healthy, un-hunted population settles to
a reasonable fluctuation between 40,000 and 118,000, with a mean level of 79,000
(Figure 15.4). The total population cycle shown in this figure is caused by the birth
of a large number of juveniles every March and the automatic redefinition of the
surviving juveniles to adults in the ensuing January.
15.2.14 The Effects of Disease Alone
Our next step was to introduce the disease to a healthy, cyclically stable, un-hunted
fox population. Since rabies is noted to be spreading westward from Pennsylvania,
we choose to introduce three rabid adult foxes in the first month of the 25-year run.
 
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