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capacity of the cell then the emigration function is re-activated and the movement
process begins anew. If the carrying capacity has not been exceeded then there is no
pressure for any foxes to leave the base cell and none will emigrate.
15.2.11 Model Constraints
The following are the key constraints imposed on our model:
A. There is no interaction with the external geography of the model; foxes may not
come or go beyond the model's defined boundaries.
B. Foxes may only migrate to another cell in the four cardinal directions.
C. The rabies disease may only be transmitted to another cell by a migrating fox.
This implies that home ranges may only exist wholly within a cell, and may not
overlap cell boundaries. There are no fractional home ranges within a cell.
D. There is no edge interaction between cells other than migration. Unlike, adja-
cent home ranges within cells, adjacent home ranges that happen to be in two
different cells do not interact, unless migration occurs.
15.2.12 Model Results
The spatial dynamics of the spread of the fox rabies disease in Illinois provides an
interesting picture. Figure 15.8 displays four maps produced from the full 25-year
run of the model. The maps show the spread of the disease among the originally
healthy, un-hunted fox population.
Since the disease has yet to hit the state, the only model calibration available
was the historic rate of spread of the disease, about 24 miles per year 45 . The critical
Fig. 15.8
45
Gosselink (1998).
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