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I used a discrete metapopulation model to evaluate the importance of the Park’s control program, estimate key population parameters, and to gauge the size of the population in the Park. Vegetation data was used to partition GSMNP into eight regions based on overstory type. To match Park policy, harvesting is modeled on a seasonal basis in which more hogs are removed during the winter months. The primary food source for feral hogs is acorns, the abundance of which significantly influences survival, birth, and movement rates of the population. To model this relationship, mast-dependent parameters vary at each time step. The model accounts for the influence of spatial variation in GSMNP by implementing a seasonal movement pattern and integrating vegetation data with yearly mast data.
I first used harvest data from GSMNP officials to set initial values in the model. The control records were further used to estimate parameters by creating a minimization problem that relates model output to harvest data. I implemented the scheme with the parameters defined as unknowns using fmincon and the MultiStart algorithm from the Optimization Toolbox in MATLAB. This allowed me to impose necessary inequality constraints on the parameters and to test enough starting points to fully explore the parameter space. Parameters that influence population dynamics and historic removal rates were estimated. An individual-based model using similar data was recently constructed for hogs in GSMNP (Salinas et al 2015). However, I more accurately mimicked mast-dependent population dynamics by carefully estimating parameter values. Furthermore, measuring historic control efforts provided me a unique position from which to evaluate the control program and estimate the relative size of the current population.
Model predictions indicate that without a control program in place the population of feral hogs in GSMNP could approach 10,000. However, factoring in control efforts by the Park, the population has been limited to approximately 1,500 hogs. These findings highlight the importance of continuing to remove hogs from GSMNP, and justify funding a control program.