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 https://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.


Acknowledgements

We are grateful to the ranger staff at Tornquist Park, in particular Daniel Porte and Facundo Casale Pintos, for their logistical support. Thanks are due to the Ministry of Agrarian Affairs of the Province of Buenos Aires for allowing the research to be carried out and for providing facilities in the reserve. Dr Scorolli thanks Dr Michelle Walter for her constant help and encouragement during the study period. Dr Nélida Winzer and Valdemar Delhey provided key bibliography on R software and GLM/GLMM analysis. We thank Drs Lee Boyd and David Forsyth and M.Sc. Laurent Tatin for their comments on an earlier version of this work. Thanks go to Drs Lee Eberhardt, Sophie Grange and Wayne Linklater for their advice on analyses and other aspects of the manuscript. This study was financed by grants (PGI 24B/074 and 24B/110) from the Secretaría de Ciencia y Tecnología of the Universidad Nacional del Sur, Argentina.


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