White-tailed Deer Breeding Potential Index
Basic Model Description
Jane Comiskey and Louis Gross
The Institute for Environmental Modeling
University of Tennessee
Knoxville, TN 37996-1610
(Copyright University of Tennessee -- 1997)
The white-tailed deer is the largest herbivore in the Everglades and a
major prey source for the endangered Florida panther. Since the early
1960's, when intensive water management began, the greater Everglades
and Big Cypress deer population has declined by almost 75%, from a high
of 25-30,000 deer. Changing water management strategies for south Florida
have impacted deer in several ways, affecting reproductive success and
recruitment, movement and foraging, and forage production and
availability (Fleming 1997). During wet years, extended periods of
inundation with water depths over 2-ft. are common in the impounded
marshes of the northern Everglades. During these high water
events, deer move to elevated sites such as tree islands, where they
often suffer deterioration of physical condition and increased
susceptibility to parasites and disease as food stores became
depleted. Does and fawns are particularly susceptible to the effects
of prolonged high water.
As part of the Central and Southern Florida Comprehensive Study Review
(Restudy), the ecological impacts of a series of proposed alternative
water management regimes will be evaluated. Each of these scenarios
will affect potential breeding and foraging activity of deer across
the landscape. The ATLSS White-tailed Deer Breeding Potential Index
(BPI) uses knowledge of how hydrologic factors affect the production
and availability of food resources and the availability of dry bedding
sites during the breeding season to compute a BPI for deer. We express
the effects of proposed scenarios as changes in the spatial pattern of
breeding potential over the model area. Our sub-area reporting units
are based on a combination of public area, drainage basin, and management
unit subregion maps (see REPUNITS.PDF).
SFWMM restoration scenario hydrology output is used to make spatially
explicit estimates of surface area with water in the depth ranges which
restrict and preclude deer movement and fawning success.
The deer breeding cycle consists of mating, gestation, birth,
lactation/nursing and fawn growth and maturation. Peak fawning occurs
during the dry season (February -- March), when uninundated bedding
sites are available. Fawns born in late winter to early spring will
have grown enough to move about on the landscape when the wet seasons
starts in May-June. The BPI focuses on the depth of ponded water which
would serve as an impediment to fawning, movement and foraging during
the breeding season. If the food supply is interrupted during this
period, 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.
For each landscape grid cell in the SFWMM model area, we compute a
daily feedback term, the ratio of the current ponding depth to a
threshold depth above which deer movement and foraging is precluded.
These daily feedback terms are summed over the time period from January 1
to May 31. The Breeding Potential Index is computed as 1 - ((feedback
sum)/(days in summing period)). 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 measure of habitat quality, using a metric of
hydroperiod for each cell during the previous year as an indication of
forage productivity and availability. Deer densities have been
reported to be highest in areas with moderate hydroperiods (Fleming
1997.) Overdrained marshes with annual hydroperiods of less than 2-3
months do not support quality forage production, especially for females
with young. When the ATLSS high-resolution forage model has been
calibrated, the BPI Model will use forage estimates simulated for each
grid cell rather than hydroperiod metrics.
The White-tailed Deer BPI model was initially run on SFWMM
calibration/validation data for 1979-1995. Index values for
subregions were compared to reported densities for deer in those
years to determine optimal settings for water depth thresholds and
habitat effects. A number of related indices and metrics, including
yearly movement potential indices, are being computed to aid in
scenario evaluation. Landscape grid cell index values will be
represented pictorially as 3-panel difference maps, and spatial
averages computed over landscape subregions will be reported in tabular
The Deer BPI is a composite index of spatial and temporal patterns.
Spatial patterns will be computed based on ATLSS High Resolution
Hydrology (28.5 x 28.5 meter units). This resolution captures
fine-scale spatial heterogeneity of the South Florida wetlands and
permits model representation of the elevated tree island habitats that
are critical for deer survival during extended periods of high water.
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.