Wildlife Research Wildlife Research Society
Ecology, management and conservation in natural and modified habitats

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

Jonathan D. Ballou A F , Kathy Traylor-Holzer B , Allison Turner C , Aurelio F. Malo A , David Powell D , Jesus Maldonado A and Lori Eggert E
+ Author Affiliations
- Author Affiliations

A Center for Conservation and Evolutionary Genetics, National Zoological Park, Smithsonian Institution, Washington, DC 20008, USA.

B IUCN/SSC Conservation Breeding Specialist Group, Apple Valley, MN 55124, USA.

C Assateague Island National Seashore, National Park Service, Berlin, MD 21811, USA.

D Wildlife Conservation Society, Bronx, NY 10460, USA.

E University of Missouri, Columbia, MO 65211, USA.

F Corresponding author. Email: ballouj@si.edu

Wildlife Research 35(6) 502-512 https://doi.org/10.1071/WR07124
Submitted: 30 August 2007  Accepted: 18 March 2008   Published: 22 October 2008


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.


We thank C. Zimmerman and M. Sturm for information about the ASIS horse population and J. Kirkpatrick for valuable comments and suggestions on the modelling and manuscript. The manuscript was improved by comments from two anonymous reviewers.


De Stoppelaire, G. H. , and Gillespie, T. W. (2004). Use of remote sensing techniques to determine the effects of grazing on vegetation cover and dune elevation at Assateague Island National Seashore: impact of horses. Environmental Management 34, 642–649.
CrossRef | PubMed |

Ebert T. A. (1999). ‘Plant and Animal Populations: Methods in Demography.’ (Academic Press: San Diego, CA.)

Frankham R. , Ballou J. D. , and Briscoe D. (2002). ‘Introduction to Conservation Genetics.’ (Cambridge University Press: Cambridge.)

Franklin I. R. (1980). Evolutionary change in small populations. In ‘Conservation Biology: An Evolutionary–Ecological Perspective.’ (Eds M. E. Soulé and B. A. Wilcox.) pp. 135–149. (Sinauer Associates: Sunderland, MA.)

Garrott, R. A. (1991). Feral horse fertility control potential and limitations. Wildlife Society Bulletin 19, 52–58.

Gross, J. E. (2000). A dynamic simulation model for evaluating effects of removal and contraception on genetic variation and demography of Pryor Mountain wild horses. Biological Conservation 96, 319–330.
CrossRef |

ISIS (2005). ‘Single Population Analysis and Records Keeping System (SPARKS). Version 1.52.’ (International Species Information System: Eagan, MN.)

Keiper R. R. (1985). ‘The Assateague Ponies.’ (Tidewater Publishers: Centreville, MD.)

Keiper R. R. , and Zervanos S. M. (1979). Ecological impact and carrying capacity of feral ponies on Assateague Island National Seashore. Final Report to the National Park Service, Contract US Department of the Interior No. PX4000-0-0446. National Park Service, Assateague Island, NS.

Kirkpatrick J. F. (1995). Management of wild horses by fertility control: the Assateague experience. Scientific Monograph NPS/NRASIS/NRSM – 92/26. US Department of the Interior, National Park Service.

Kirkpatrick, J. F. , and Turner, A. (2002). Reversibility of action and safety during pregnancy of immunizing against porcine zona pellucida in wild mares (Equus caballus). Reproduction Supplement 06, 197–202.

Kirkpatrick, J. F. , Liu, I. M. , Turner, J. W. , Naugle, R. , and Keiper, R. (1992). Long-term effects of porcine zonae pellucidae immunocontraception on ovarian function in feral horses (Equus caballus). Journal of Reproduction and Fertility 94, 437–444.
CAS | PubMed |

Lacy, R. C. (1993). Vortex: a computer simulation model for population viability analysis. Wildlife Research 20, 45–65.
CrossRef |

Lacy, R. C. (1997). Importance of genetic variation to the viability of mammalian populations. Journal of Mammalogy 78, 320–335.
CrossRef |

Lacy, R. C. (2000). Structure of the Vortex simulation model for population viability analysis. Ecological Bulletin 48, 191–203.

Lacy R. C. , Borbat M. , and Pollak J. P. (2005). ‘Vortex: A Stochastic Simulation of the Extinction Process. Version 9.60.’ (Chicago Zoological Society: Brookfield, IL.)

MacCluer, J. W. , VandeBerg, J. L. , Read, B. , and Ryder, O. A. (1986). Pedigree analysis by computer simulation. Zoo Biology 5, 147–160.
CrossRef |

McCullough D. (1979). ‘The George Reserve Deer Herd: Population Ecology of a K-selected Species.’ (University of Michigan Press: Ann Arbor, MI.)

Miller P. S. , and Lacy R. C. (2003). ‘Vortex: A Stochastic Simulation of the Extinction Process. Version 9 User’s Manual.’ (IUCN SSC Conservation Breeding Specialist Group: Apple Valley, MN.)

Miller P. S. , Westley F. W. , Byers A. P. , and Lacy R. C. (2007). An experiment in managing the human animal: the PHVA process and its role in conservation decision-making. In ‘Conservation in the 21st Century: Gorillas as a Case Study’. (Eds T. Stoinski, H. D. Steklis and P. Mehlman.) pp. 173–188. (Kluwer Press: New York.)

Pollak J. , Lacy R. C. , and Ballou J. D. (2005). ‘PM2000: Population Management Software. Version 1.211.’ (Chicago Zoological Society: Brookfield, IL.)

Ralls, K. , Ballou, J. D. , and Templeton, A. R. (1988). Estimates of lethal equivalents and the cost of inbreeding in mammals. Conservation Biology 2, 185–193.
CrossRef |

Seagle, S. W. , and Close, J. D. (1996). Modeling white-tailed deer Odocoileus virginianus population control by contraception. Biological Conservation 76, 87–91.
CrossRef |

Soulé M. E. (1980). Thresholds for survival: maintaining fitness and evolutionary potential. In ‘Conservation Biology: An Evolutionary–Ecological Perspective’. (Eds M. E. Soulé and B. A. Wilcox.) pp. 151–169. (Sinauer Associates: Sunderland, MA.)

Starfield A. M. , and Bleloch A. L. (1991). ‘Building Models for Conservation and Wildlife Management.’ (Burgess International Group: Edina, MN.)

Turner, A. , and Kirkpatrick, J. F. (2002). Effects of immunocontraception on population, longevity and body condition in wild mares (Equus caballus). Reproduction Supplement 60, 187–195.

Wright, S. (1931). Evolution in Mendelian populations. Genetics 16, 97–159.
CAS | PubMed |

Wright S. (1977). ‘Evolution and the Genetics of Populations. Vol. 3.’ (University of Chicago Press: Chicago, IL.)

Zhang, Z. (2000). Mathematical models of wildlife management by contraception. Ecological Modelling 132, 105–113.
CrossRef |

Zimmerman C. , Sturm M. , Ballou J. D. , and Traylor-Holzer K. (Eds) (2006). ‘Horses of Assateague Island Population and Habitat Viability Assessment: Final Report.’ (IUCN/SSC Conservation Breeding Specialist Group: Apple Valley, MN.)

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