Simulation model for contraceptive management of the Assateague Island feral horse population using individual-based data

The National Park Service (NPS) manages a culturally significant population of feral horses (Equus caballus) inhabiting the Maryland portion of Assateague Island, a barrier island in the eastern United States. Rapid growth of this population over the past few decades from 28 to 166 horses negatively impacts native species and ecological processes on the island. Since 1994, contraception via porcine zona pellucida vaccine has been used to control horse numbers, although herd reduction has been slower than initially expected, leading NPS to consider other management options. An individual-based stochastic simulation model was developed using the Vortex software program to examine the effects of different management strategies on the population. Data from the managed population were used to populate the model parameters. Model projections over the next 50 years using current management practices show an average rate of population decline of 13% per year, suggesting that the population will reach the management target of 80-100 horses in 5-8 years. The effectiveness of contraception to reduce the herd and maintain it at various target sizes of 20-100 horses was also assessed. The accumulation of inbreeding at each target population size was also modelled.