Database Reference
In-Depth Information
Illinois GIS based mapping projects
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. Urban edge cells were assigned a fox habitat
value of 5 (urban edge is defined as any areas within 0.5 kilometers of an urban-non-
urban interface) and all interior urban cells were assigned a value of 0. Forest cells
were assessed a value of 1 and remaining cells, including wetlands, croplands, and
grass/pasture lands were assigned a range of values from 2 to 3 depending on slope.
The Land Cover of Illinois map data was aggregated into 6- by 6-mile cells,
statewide. In this new carrying capacity map, the minimum healthy carrying
capacity cell value is 21.50 foxes per cell and the maximum healthy carrying
capacity cell value is 70.84 foxes per cell. This map was exported into a GRASS
raster based format and imported into the Spatial Modeling Environment (SME) for
use with the STELLA fox model.
15.2.10 Spatial Characteristics
Adult foxes do not always remain in the territory they chose during juvenile disper-
sal. Adults are known to disperse only in the fall and winter, as juveniles do, so it
is possible for incoming juveniles to displace some adult foxes. To simulate these
characteristics and to simplify intercellular movement, the spatial model randomly
assigns the direction of travel and limits travel to the four main compass directions.
A fox that moves into a habitat cell that is at the carrying capacity (as derived from
the map) forces the movement of a fox out of the same cell.
Emigration is a function of the spatial resource limits of each cell. In this model,
the carrying capacity of a cell is a fixed amount and has been determined by land
use cover characteristics. The relative population of foxes in each cell fluctuates
with births, deaths, and immigration/emigration. Foxes emigrate when the calcu-
lated population of foxes in the cell exceeds the carrying capacity of that cell. This
simulates the relationship between fox populations and resource availability. When
the relative fox population of an area exceeds the resources available, there is pres-
sure for a part of the population to move.
Once the total number and type of emigrants for each stock is determined, di-
rectional preferences must be calculated. Foxes sometimes travel great distances to
find suitable and available habitat. It appears however, that in most instances foxes
choose home ranges based on availability and not attractiveness. For this reason, a
directional preference was assigned randomly for each group of emigrants. These
random assignments occur only when certain landscape and time considerations are
met. The directional assignment ratios are then applied to the stocks to determine
the number and direction of foxes moving out of each cell at each time step.
Immigration, driven primarily by the emigration function, is the number of foxes
in each of the four primary stocks that have been added to the cell at each compu-
tational time step. If the incoming foxes, plus foxes in the cell, exceed the carrying
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Luman, D. , M. Joselyn, and L. Suloway. 1996. ”Illinois Scientific Survey Joint Report #3”
Illinois Natural History Survey.
