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

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.

Understanding population dynamics of feral horses in the Tuan and Toolara State Forest for successful long-term population management

“…The average foaling rates in feral populations have been reported to be low as 27-36% (Siniff et al 1986;, and moderate 40-65% (Welsh 1975;Keiper and Houpt 1984;Berger 1986;Scorolli and Lopez Cazorla 2010a). Very high rates of 80-90% have been reported in females of prime age ).…”

“…Despite being widely adopted, selective removal practices have an impact on the distribution of age groups, and skew the male to female ratio, which may have consequences on population dynamics Rogers 1991 Scorolli and Lopez Cazorla 2010a), while in the northern hemisphere it is usually between March and September (Keiper and Houpt 1984;Duncan 1992;Greger and Romney 1999).…”

“…Removal of bachelor and sub-adult males often occurs in managed small and confined populations (Duncan 1992), while removal of adult females is common in populations where the main aim is to decrease the population growth rate Rogers 1991). Management practices focusing on removal of both sexes in adults (Scorolli and Lopez Cazorla 2010a) or young horses (Keiper and Houpt 1984), attempt to maintain a balanced sex ratio. Other management methods, which focus on the protection of existing animal numbers and maintaining the population's pure-breed status, rely on the introduction of new individuals of both sex ).…”

“…The removal of adult horses of both sexes is also performed in populations where the aim is to reduce the population size Scorolli and Lopez Cazorla 2010a).…”

“…Juveniles are usually born at a well-defined time of the year, and, if they survive, they are classified as yearlings at the beginning of the next biological year (Gaillard et al 2000a;Scorolli and Lopez Cazorla 2010a). Subsequent age classes are defined as sub-adults (generally 2 -3 years of age), and adults, in which several distinct sub-classes (young adults, prime-aged adults, and senescent-aged adults) can be recognised (Gaillard et al 2000a;).…”