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

Demography of feral horses (Equus caballus): a long-term study in Tornquist Park, Argentina

Alberto L. Scorolli A B and Andrea C. Lopez Cazorla A

A Departamento de Biología, Bioquímica y Farmacia, Universidad Nacional del Sur, San Juan 670, 8000 ICN, Argentina.

B Corresponding author. Email: scorolli@criba.edu.ar

Wildlife Research 37(3) 207-214 http://dx.doi.org/10.1071/WR09059
Submitted: 15 May 2009  Accepted: 11 March 2010   Published: 18 May 2010

Abstract

Context. All over the world, invasive alien mammals present conservation problems. Feral horses are invasive in many countries and their management is challenging.

Aims. To determine the demography of feral horses in Tornquist Park, Argentina, so as to develop a quantitative basis for their management.

Methods. Ground counts of individually identified female horses were conducted monthly from 1995 to 1997 and from 2000 to 2002, and seasonally in 1998 and 1999. We calculated the age structure, foaling and survival rates of mares and analysed the effects of adult density and rainfall on demographic parameters. The population trend was calculated by fitting a logistic equation to annual adult mare counts.

Key results. The foaling rate averaged 0.55 annually and the survival rate in females averaged 0.89 for adults, 0.91 for juveniles, 0.87 for yearlings and 0.94 for foals. We found adult density, rainfall and the previous year’s rainfall to have a significant effect on fecundity but no effect on adult, juvenile and foal female survival rates. Adult density had a significant negative effect on female yearling survival; however, the effect is probably trivial. The logistic equation parameters gave r = 0.327 and carrying capacity (K) = 240 adult mares.

Conclusions. A density-dependent response is suggested. The population growth is mainly limited by low fecundity, with the reduced adult female survival probably also playing a minor role.

Implications. It is expected that the present long-term demographic study will contribute to the understanding of feral ungulate ecology and improve the management of feral populations.


References

Beever E. A. , and Brussard P. F. (2000). Charismatic megafauna or exotic pest? Interactions between popular perceptions of feral horses (Equus caballus) and their management and research. In ‘Proceedings of the 19th Vertebrate Pest Conference’. (Eds T. P. Salmon and A. C. Crab.) pp. 413–418. (University of California: Davis, CA.)

Berger J. (1986). ‘Wild horses of the Great Basin, Social Competition and Population Size.’ (The University of Chicago Press: Chicago, IL.)

Berman D. McK. (1991). The ecology of feral horses in central Australia. Ph.D. Thesis, University of New England, Armidale, NSW.

Boyd L. (1979) The mare foal demography of feral horses in Wyoming’s Red Desert. In ‘Symposium on the Ecology and Behavior of Wild and Feral Equids’. (Ed. R. H. Denniston.) pp. 185–204. (University of Wyoming: Laramie, WY.)

Bradshaw C. J. A. Field I. C. Bowman D. M. J. S. Haynes C. Brook B. W. 2007 Current and future threats from non-indigenous animals species in northern Australia: a spotlight on World Heritage Area Kakadu National Park. Wildlife Research 34 419 436 doi:10.1071/WR06056

Burgos J. (1968). El clima de la Provincia de Buenos Aires en relación con la vegetación natural y el suelo. In ‘Flora de la Provincia de Buenos Aires’. (Ed. A. L. Cabrera.) pp. 33–100. Colección Científica del INTA 4 (1). (INTA: Buenos Aires.)

Burnham K. P. , and Anderson D. R. (2002). ‘Model Selection and Multimodel Inference: A Practical Information-theoretic Approach.’ 2nd edn. (Springer Verlag: New York.)

Cabrera A. (1945). ‘Caballos de América.’ (Editorial Sudamericana: Buenos Aires.)

Cabrera A. L. (1976). Regiones Fitogeográficas Argentinas. In ‘Enciclopedia Argentina de Agricultura y Ganadería’. Tomo 2, Fascículo 2. (Ediciones Acme: Buenos Aires.)

Cameron E. Z. Linklater W. L. Minot E. O. Stafford K. J. 2001 Population dynamics 1994–1998, and management of Kaimanawa wild horses. Science for Conservation (Wellington) 171 1 165

Crawley M. J. (2005). ‘Statistics: An Introduction Using R.’ (John Wiley and Sons: Chichester, UK.)

Crawley M. J. (2007). ‘The R Book.’ (John Wiley and Sons: New York.)

Dawson M. J. , Lane C. , and Saunders G. (2006). ‘Proceedings of the National Feral Horse Management Workshop.’ (Invasive Animals Cooperative Research Centre: Canberra.)

Dobbie W. R. , Berman D. McK. , and Braysher M. L. (1993). ‘Managing Vertebrate Pests: Feral Horses.’ (Australian Government Publishing Service: Canberra.)

Douglas C. L. , and Leslie D. M. (1996). Feral animals on rangelands. In ‘Rangeland Wildlife’. (Ed. P. R. Krausman.) pp. 281–292. (The Society for Range Management: Denver, CO.)

Duncan P. (1992). ‘Horses and Grasses. The Nutritional Ecology of Equids and Their Impact on the Camargue.’ Ecological Studies 87. (Springer Verlag: Berlin.)

Eberhardt L. L. 1977 Optimal management policies for marine mammals. Wildlife Society Bulletin 5 162 169


Eberhardt L. L. 2002 A paradigm for population analysis of long-lived vertebrates. Ecology 83 2841 2854
doi:10.1890/0012-9658(2002)083[2841:APFPAO]2.0.CO;2

Eberhardt L. L. Majorowicz A. K. Wilcox J. A. 1982 Apparent rates of increase for two feral horse herds. The Journal of Wildlife Management 46 367 374 doi:10.2307/3808648

Eberhardt L. L. Breiwick J. M. Demaster D. P. 2008 Analyzing population growth curves. Oikos 117 1240 1246 doi:10.1111/j.0030-1299.2008.16402.x

Feist J. D. McCullough D. R. 1975 Reproduction in feral horses. Journal of Reproduction and Fertilityt 23 Suppl. 13 18

Fowler C. W. (1987). A review of density dependence in populations of large mammals. In ‘Current Mammalogy’. (Ed. H. H. Genoways.) pp. 401–441. (Plenum Press: New York.)

Frangi J. L. Bottino O. J. 1995 Comunidades vegetales de la Sierra de la Ventana, Provincia de Buenos Aires, Argentina. Revista de la Facultad de Agronomía (La Plata) 71 93 133


Franke Stevens E. 1990 Instability of harems of feral horses in relation to season and presence of subordinate stallions. Behaviour 112 149 161
doi:10.1163/156853990X00167

Frei M. Peterson J. Hall J. 1979 Aerial census of wild horses in western Utah. Journal of Range Management 32 8 11 doi:10.2307/3897377

Gaillard J.-M. Festa-Bianchet M. Yoccoz N. G. 1998 Population dynamics of large herbivores: variable recruitment with constant adult survival. Trends in Ecology & Evolution 13 58 63 doi:10.1016/S0169-5347(97)01237-8

Gaillard J.-M. Festa-Bianchet M. Yoccoz N. G. Loison A. Toïgo C. 2000 Temporal variation in fitness components and population dynamics of large herbivores. Annual Review of Ecology and Systematics 31 367 393 doi:10.1146/annurev.ecolsys.31.1.367

Garrott R. A. Taylor L. 1990 Dynamics of a feral horse population in Montana. The Journal of Wildlife Management 54 603 612 doi:10.2307/3809357

Garrott R. A. Siniff D. B. Eberhardt L. L. 1991 Growth rates of feral horse populations. The Journal of Wildlife Management 55 641 648 doi:10.2307/3809513

Goodloe R. B. Warren R. J. Osborn D. A. Hall C. 2000 Population characteristics of feral horses on Cumberland Island, Georgia and their management implications. The Journal of Wildlife Management 64 114 121 doi:10.2307/3802980

Grange S. Duncan P. Gaillard J.-M. 2009 Poor horse traders: large mammals trade survival for reproduction during the process of feralization. Proceedings of the Royal Society of London. Series B. Biological Sciences 276 1911 1919 doi:10.1098/rspb.2008.1828

Greger P. D. Romney E. M. 1999 High foal mortality limits growth of a desert feral horse population in Nevada. The Great Basin Naturalist 59 374 379

Keiper R. Houpt K. 1984 Reproduction in feral horses: an eight-year study. American Journal of Veterinary Research 45 991 995


Kirkpatrick J. F. Turner J. W. 1991 Compensatory reproduction in feral horses. The Journal of Wildlife Management 55 649 652
doi:10.2307/3809514

Kristensen M. J. Frangi J. L. 1995 Una isla de biodiversidad. Ciencia Hoy 5 25 34

Lever C. (1994). ‘Naturalized Animals.’ (Poyser Natural History: London.)

Linklater W. L. Cameron E. Z. Minot E. O. Stafford K. J. 2004 Feral horse demography and population growth in the Kaimanawa Ranges, New Zealand. Wildlife Research 31 119 128
doi:10.1071/WR02067

Long J. L. (2003). ‘Introduced Mammals of the World. Their History, Distribution and Influence.’ (CSIRO Publishing: Melbourne.)

Loydi A. Zalba S. M. 2009 Feral horses dung piles as potential invasion windows for alien plant species in natural grassland. Plant Ecology 201 471 480 doi:10.1007/s11258-008-9468-0

Monard A.-M. Duncan P. Fritz H. Feh C. 1997 Variations in the birth sex ratio and neonatal mortality in a natural herd of horses. Behavioral Ecology and Sociobiology 41 243 249 doi:10.1007/s002650050385

National Research Council (1980). ‘Wild and Free-roaming Horses and Burros: Current Knowledge and Recommended Research.’ (National Academy of Science: Washington, DC.)

Parkes J. Murphy E. 2003 Management of introduced mammals in New Zealand. New Zealand Journal of Zoology 30 335 359

Salter R. E. Hudson R. J. 1982 Social organization of feral horses in western Canada. Applied Animal Ethology 8 207 223
doi:10.1016/0304-3762(82)90205-X

Scorolli A. L. (1999). Demografía y áreas de actividad de una población de caballos cimarrones en el Parque Tornquist. Magister Thesis, Universidad Nacional del Sur, Argentina.

Scorolli A. L. (2007). Dinámica poblacional y organización social de caballos cimarrones en el Parque Provincial Ernesto Tornquist. Doctoral Thesis. Universidad Nacional del Sur, Argentina.

Scorolli A. L. Lopez Cazorla A. C. Tejera L. A. 2006 Unusual mass mortality of feral horses during a violent rainstorm in Parque Provincial Tornquist, Argentina. Mastozoología Neotropical 13 255 258

Sinclair A. R. E. (1989). Population regulation in animals. In ‘Ecological Concepts’. (Ed. J. M. Cherrett.) pp. 197–241. (Blackwell Scientific Publications: Oxford, UK.)

Siniff D. B. Tester J. R. Mc Mahon G. L. 1986 Foaling rate and survival of feral horses in western Nevada. Journal of Range Management 39 296 297
doi:10.2307/3899765

Tatin L. King S. R. B. Munkhtuya B. Hewison A. J. M. Feh C. 2009 Demography of a socially natural herd of Przewalski’s horses: an example of a small closed population. Journal of Zoology 277 134 140 doi:10.1111/j.1469-7998.2008.00520.x

Turner J. W. Morrison M. L. 2001 Influence of predation by mountain lions on numbers and survivorship of a feral horse population. The Southwestern Naturalist 46 183 190 doi:10.2307/3672527

Tyler S. J. 1972 The behaviour and social organization of the New Forest ponies. Animal Behaviour Monographs 5 85 196

Walter M. J. (2002). The population ecology of wild horses in the Australian Alps. Ph.D. Thesis, University of Canberra, Australia.

Welsh D. A. (1975). Population, behavioral and grazing ecology of the horses of Sable Island, Nova Scotia. Ph.D. Thesis, Dalhousie University, Halifax, Canada.

White P. Ford A. E. S. Clout M. N. Engeman R. M. Roy S. Saunders G. 2008 Alien invasive vertebrates in ecosystems: pattern, process and the social dimension. Wildlife Research 35 171 179
doi:10.1071/WR08058

Witmer G. W. , Pitt W. C. , and Fagerstone K. A. (Eds) (2007). ‘Managing Vertebrate Invasive Species: Proceedings of an International Symposium.’ (USDA–APHIS: Fort Collins, CO.)

Wolfe M. L. 1980 Feral horse demography: a preliminary report. Journal of Range Management 33 354 360 doi:10.2307/3897882

Zalba S. M. Cozzani N. C. 2004 The impact of feral horses on grassland bird communities in Argentina. Animal Conservation 7 35 44 doi:10.1017/S1367943003001094

Zuur A. F. , Ieno E. N. , Walter N. J. , Saveliev A. A. , and Smith G. M. (2009). ‘Mixed Effects Models and Extensions in Ecology with R.’ (Springer: New York.)



Export Citation