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15.2.8 Model Assumptions
As with all complex models, a series of assumptions is required. Here are ours:
1. There is no significant difference in the behavior of males and females regarding
interaction outside the family unit and dispersion. The behavior of foxes is not
significantly different when foxes outside the family unit are encountered.
2. Large rivers are not barriers to migration; it is assumed that foxes will use
bridges, swim, or walk across the frozen surface during the winter.
3. Rabies is always fatal; every fox that contracts rabies will die 39 .
4. All rabid foxes are equal sources of infection; all foxes during the rabid phase
will behave similarly.
5. Rabies incubation and infectious periods are the same length of time; there is no
significant difference in the time periods of incubation and being infectious.
6. All surviving juvenile foxes mature to adulthood at 11 months.
7. There is no significant difference in the amount of resources each fox uses, re-
gardless of age.
8. The fox population has a definable maximum. We used the maximum densities
of foxes found in Great Britain 40 .
9. The model starts with 3 rabid foxes along the Illinois/Indiana border.
15.2.9 Georeferencing the Modeling Process
Red foxes live in a variety of habitats 41 . According to the Illinois Natural History
Survey, foxes avoid forested areas and interior urban areas 42 . Foxes use forested
areas for migration but forests are commonly avoided due to coyote predation. In
general, red foxes are found in open croplands, grasslands, or pasture, using sloped
areas for den sites. Urban edge areas and farmsteads are important habitats for the
red fox due to the abundance of prey and forage in these areas. Gosselink estimated
the average east-central Illinois red fox density at three adult foxes per 10 square
miles; a breeding pair, plus one nonbreeding (juvenile) fox 43 . An average litter size
is estimated at six pups, so approximately nine foxes are estimated to occupy every
10 square miles in Illinois.
This estimate of the average healthy carrying capacity in the state helped to create
a habitat-weighted fox carrying capacity map for the state of Illinois. This was done
using data collected by the Illinois Natural History Survey in the Land Cover of
39
Bacon and MacDonald 1980.
40
Anderson et al. 1981.
41
Storm et al. 1976.
42
Gosselink,
T.
March
1988.
Personal
Communication,
Illinois
Natural
History
Survey,
Champaign, IL.
43
Gosselink 1998.
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