Model Features
The DMS is a general, yet powerful simulation tool designed to assist
natural resource specialists attempting to manage problems relating to overabundant
white-tailed deer populations on national parks units. This simulation environment
integrates available information on deer population dispersion, productivity
and size with GIS databases. Simulations assist the user in identifying
the set of circumstances and conditions necessary for a particular management
strategy to succeed in accomplishing its objectives. The flexibility of
the DMS makes it ideal for conducting sensitivity analysis on input variables
and for evaluating assumptions about the understanding of local deer herd
demography and habitat use patterns.
Importing GIS Maps
The DMS utilizes GIS map image files containing landscape attributes as
a spatially-explicit database for its deer population module, and as the
background matrix for its simulation module. Thus, users must be prepared
to provide this map (hereafter referred to as habitat or landscape map)
and the associated map grid coordinates for the management area they will
be evaluating. The landscape map can be any existing raster-based GIS image
(e.g., IDRISI *.IMG file) or
it can be a hypothetical landscape constructed arbitrarily by the user.
This flexibility gives the user the opportunity to evaluate how the composition
and spatial arrangement of habitat types influences animal dispersion, or
to analyze the effects of user-specified treatments on the demographics
and dispersion of the local deer population. The user also is required to
provide a vector-based boundary file, which delineates the study area on
the landscape map. The DMS overlays this boundary file onto output maps
created by simulations to help orient the user.
Interactive Editing of Map Features
The DMS permits users to alter characteristics of the landscape map and
to delineate specific areas for censusing or treating the deer population.
Drawing tools allow the user to digitize lines as barriers, and polygons
or shapes as census and/or treatment masks using the landscape map as a
spatial reference. Once delineated, these masks can be individually saved
and recalled later for use while evaluating management scenarios. The DMS
also allows users to reclassify existing habitat types over the entire landscape
image, or within masks. These features allow users to investigate how habitat
modifications (e.g., a clearcut) or disturbances may impact the local deer
population.
Individual-based Deer Population Model
The centerpiece of the DMS is a spatially-explicit, individual-based deer
population model. This model interfaces with raster-based, GIS map images
and simulates individual animal movements in response to landscape features.
Initial population characteristics are input by the user through responses
to menu queries. Users have the option of defining the deer population size
explicitly, or specifying a deer density (e.g., deer/mi2) and
have the DMS calculate population size based on area. The user also specifies
the sex and age group composition (i.e., fawns, yearlings, adults), and
provides estimates of reproduction (i.e., conception and fetal rates) and
mortality for each group. In the event that local data are unavailable,
suggested values are provided based on reviews of published studies of white-tailed
deer. Current values for all deer population parameters can be reviewed
and edited by the user at any time.
The DMS creates the specified deer population file, which maintains information
on the status of each individual and distributes them over the landscape.
Initially, the placement of individuals on the landscape is random (with
the exception of fawns, which remain near their mothers until they are 1-year
old). However, this initial placement is adjusted prior to simulation by
allowing animals to move and interact with landscape habitat features for
one annual cycle (i.e., 365 days). This initialization produces a more realistic
dispersion of animals that is dependent on the spatial characteristics of
landscape features and their relative value to deer. In the DMS, the direction
and extent of movements by each deer is calculated on a daily time step
and is a function of the quality of the habitat within each map pixel. Deer
encountering higher quality habitats are displaced (spatially) less than
deer encountering lower quality habitats.
Data Evaluation
The DMS permits the user to observe animal dispersion and movements in response
to assigned habitat values on the computer's screen as the model steps through
the days of the year. At any point during the simulation, the user can use
"hot-keys" to obtain details of the number of deer occurring within
each habitat type and a ratio of habitat use versus availability. These
options allow users to assess how the relative habitat affinity values assigned
for each season influence deer dispersion on the landscape. If results are
inconsistent with expectations, the user may choose to reconsider the basis
for assigning these values prior to running any management scenarios. Additional
options include the ability to count deer within masks and the ability to
observe the accumulation of deer spatial use (i.e., residence time in pixels)
of habitats on the landscape during user-specified time intervals. The resulting
spatial response maps are especially helpful for identifying areas receiving
high or low seasonal or annual use, including potential movement corridors.
Simulating Management Scenarios
The DMS the user to design treatments and to evaluate their potential efficacy
for managing overabundant deer populations. Treatments are designed by selecting
options regarding the type of treatment to employ, and by specifying the
intensity, duration and location of treatment. Available treatment options
include: 1) removals (e.g., translocating or culling); 2) sterilization
(either sex); 3) the installation of a physical barrier to animal movement
(e.g., deer-proof fencing), and 4) the "do nothing" or control
option. The control is included for purposes of comparing and assessing
the impacts that other treatment options have on the deer population's spatial
and temporal dynamics. Additional treatment scenarios can be created by
using combinations of these basic options. The user specifies the location
where a treatment is applied by creating a treatment mask and by indicating
whether the treatment is to be applied inside or outside of the mask.
Because certain elements controlling individual deer movement in the DMS
are stochastic, the population response (i.e., the sum of individual responses)
to any treatment will vary between simulation runs. Just as in nature, stochastic
elements affecting the movement of individual deer in the simulation can
result in significantly different dispersion patterns among runs of the
model despite identical initial population characteristics and treatments.
Differences in animal dispersion among runs can produce different levels
of exposure to treatments experienced by individual deer, and ultimately
result in different rates of reproduction and survival in the population.
Furthermore, differences among runs within a given year may be amplified
during subsequent years of the same simulation. To accommodate this uncertainty,
the user has the option of running multiple iterations (up to 30 replicates
per simulation year) of each treatment for periods of up to 10 consecutive
years. Multiple iterations allow the user to make a probabilistic evaluation
of the effects of various management scenarios on the deer population.
As is the case with any dynamic simulation model, the ultimate utility of
the DMS will depend upon the user's ability to construct a realistic model
that reflects animal responses to temporal changes in habitat conditions.
The DMS is a individually-based model so the observed population response
to a user-defined treatment or habitat alteration is generated by the cumulative
responses of individual deer. In developing the DMS, we have avoided imposing
assumptions regarding explicit deer behavior in order to preserve model
flexibility. Although the DMS was developed specifically to address white-tailed
deer management issues and problems, the structure of the model permits
its use for addressing management concerns of other ungulate and wildlife
species.
Model Output
The DMS generates output in both numerical and graphical forms. Numerical
responses (i.e., size and composition) of the deer population are stored
in standard ASCII text files that can be easily imported into most spreadsheet
and statistical programs for graphing or further analysis. Spatial responses
of the deer population are captured as color spectral map files and make
it possible to visualize how treatments impact the spatial-temporal patterns
of deer use of the landscape map. Spatial response maps are saved in specially
formatted renderings for rapid viewing and as IDRISI raster image files
(*.IMG) to facilitate spatial analyses of simulation results. From IDRISI,
image files can be easily converted for use with other GIS applications.
Both numerical and graphical output files can be reviewed from within the
DMS.
