White-Tailed Deer
Breeding Potential Index (BPI)
Empirical basis and model assumptions:
The white-tailed deer
are modeled, as they are an important prey species for the Florida
panther.
Urban areas are not
good for reproduction for white-tailed deer.
*
In the BPI spatial
cells in urban areas are excluded (the index is set to zero).
The reproductive season is between January 1 and May
31. Ponded water can act as an
impediment to fawning, movement and foraging during the breeding season. If the food supply is interrupted during
this period, which can happen during high water, the health of mother and
offspring may suffer, and fawns are less likely to be recruited into the herd. Elevated water levels can make beds
uninhabitable, and high water can drown young fawns.
*
In the IBP a water
depth of 55 cm is defined to be the depth above which movement and foraging are
assumed to be absolutely precluded.
Water of any depth during this season is assumed to restrict fawning and
slow down movement, and so subtracts from the index. The degree to which water is an impediment is represented by
calculating the ratio of (water-depth days)/(maximum possible water-depth days)
during the reproductive season.
The hydroperiod during the year previous year is an
indication of the quality and availability of forage prior to fawning. This will influence the health of the
females and thus their likelihood to fawn.
Too short a hydroperiod reduces the quality of the forage. Too long a hydroperiod reduces the
availability of forage.
*
The BPI is weighted
by a multiplicative factor that depends on hydroperiod. The values of this discounting factor for
hydroperiods of 0, 1, 2, 3, 4,….. 12-month hydroperiods are {0.3, 0.4, 0.6, 0.9,
1.0, 1.0, 1.0, 0.9, 0.8, 0.6, 0.5, 0.4, 0.3}.
Selected
References:
Fleming, D.M., J. Schortemeyer, and J. Ault. 1997.
Distribution, abundance and demography of white-tailed deer in the
Everglades. Proceedings of the Florida
Panther Conference, Ft. Myers Fla., November 1994, Dennis Jordan, ed., U.S.
Fish and Wildlife Service, pp. 494-503.
Loveless, C.M. 1959.
The Everglades deer herd, life history and management. Tech. Bull. No. 6, Fla. Game and Fresh Water
Fish Comm., Tallahassee, 104 pp.
Flow Chart for Construction of White-tailed Deer Breeding Potential Index
The flow
chart shows the steps in computing an index value for a cell:
Variables
of index computation (top box):
500-m cells that are in urban areas are excluded.
HIWATER - water level at which deer movement and
foraging are absolutely precluded
HpDiscount[ ] - a
function that discounts the value of a cell according to its hydroperiod the
previous
year, which is an indication of foraging quality and availability.
Cycle through days of
year to determine breeding conditions (middle):
For each spatial cell the model computes a daily term, the
ratio of the current ponding depth to the (HIWATER x total days in the breeding
season). When summed over the whole
reproductive period, this sum then is the sum of ratios of (water-depth days in
the reproductive season) to (maximum water-depth days in the reproductive
season), or
I = S
(WaterDepth(x,y)/HIWATER).
This sum of ratios over the days of the breeding season
gives an indication of the difficulty of moving and fawning during the breeding
period.
Calculation
of total BPI (bottom):
The BPI for each cell, termed the IndexMap(x,y), is composed
of two factors. The first factor
measures the favorability of water conditions during the breeding season, and is computed as [1 - I(x,y)/MaxI], where
MaxI is the maximum value that I(x,y) could obtain. This results in an index
between 0 and 1, with 0 representing no breeding potential, and 1 representing
maximum potential (no interference from ponded water). This index is further scaled by a second
factor, which is a measure of habitat quality, using the metric of hydroperiod
(HpDiscount[ ]) for each cell during the previous year as an indication of
forage productivity and availability.