Clutton-Brock 1981)
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Introduction 1 CHAPTER 1. INTRODUCTION 1.1 Context and purpose Horses were domesticated about 2,500 to 5,000-years ago (Clutton-Brock 1981). Domesticated animals that return to a wild state independent from humans and are capable of reproducing and sustaining a population are termed feral animals (McKnight 1976). In the United States of America (USA), feral horses persist because they have (Berger 1986): (1) flexible behavioural and physiological systems that enable them to adapt and reproduce under a broad spectrum of ecological conditions; (2) occupy isolated, remote habitats; and (3) are legally protected and regarded as a ‘national treasure’ that embodies ‘the historic and pioneer spirit of the west’, under the Wild Free-Roaming Horses and Burro Act 1971 (Wagner 1983; Rutberg 2003). In Australia, feral horses inhabit a range of environments from deserts to wetlands, and tend to be most abundant in the remote unfenced cattle production areas of the northern states and in rugged mountainous country (Symanksi 1994; Csurhes et al. 2009). Unlike the USA, feral horses are not protected by legislation and in South Australia, for example, are declared a pest species under the Natural Resources Management Act 2004 (Dawson et al. 2006). The majority of populations exist on public lands of conservation value where government authorities such as the National Parks and Wildlife Service (NPWS) have statutory obligations to reduce population numbers (English 2001b; Edwards et al. 2003). However, many Australians and several public interest groups (e.g. Save the Brumbies, Australian Brumby Alliance) either do not perceive feral horses as a pest species or regard them as an iconic species of cultural and heritage significance that should be managed for preservation in accordance with their status in the USA (Walter 2002; Ballard 2005; Finch and Baxter 2007; Nimmo and Miller 2007; Nimmo et al. 2007). Horse population control thus continues to be one of the most complicated problems for wildlife management agencies worldwide due to multiple, often highly emotional, and polarised attitudes of stakeholders (Wolfe 1980; Rikoon and Albee 1998; Rikoon 2006; Hubert and Klein 2007; Kincaid 2008; Taggart 2008). These contrasting views were epitomised by the public reaction to the aerial (helicopter) cull of 606 horses in Guy Fawkes River National Park (GFRNP), New South Wales (NSW), in October 2000. Ballard (2005) concluded that rarely has a wildlife management issue in Australia received such a sustained level of public acrimony, domestic and international media coverage, and political Introduction 2 involvement as the GFRNP cull. The direct ramifications of the cull were considerable and included an official review of the GFRNP culling operation and horse management in all NSW National Parks (English 2000, 2001b), a NSW Local Court case with 12 counts of animal cruelty filed against the NPWS (NSW NPWS 2002), parliamentary allegations of corruption (Fraser 2002) and a permanent ban on the aerial culling of horses in national parks in NSW (AAP 2000), and the development of a management plan for the remaining horses in GFRNP (English 2001a; NSW NPWS 2003). The critical points of contention in horse control are the heritage value of the Australian ‘brumby’ versus the adverse effects of horses on the environment and native wildlife. To date, no scientific studies on the ecological effects of horses on Australian ecosystems have been published in peer-reviewed literature, despite Australia reputedly having the largest population of feral horses world-wide at an estimated 300,000 to 600,000 individuals (Dobbie et al. 1993; Dawson et al. 2006). After the GFRNP cull, the then NSW Minister for the Environment established a Heritage Working Party (HWP) to determine the heritage value of the horses in GFRNP (Heritage Working Party 2002a). In concordance with the official review (English 2001b), the Minister also directed the NPWS to monitor the environmental impacts in GFRNP associated with horses (Debus 2002). The purpose of this thesis is to independently assess the impact of feral horses on select plant communities, soil properties and grazing native mammals (i.e. macropods) in GFRNP. The results are discussed in relation to the knowledge of horse impacts elsewhere in Australia and overseas and the implications for future management efforts. 1.2 Horse evolution and definitions The mammalian family, Equidae (zebras, asses and horses) includes one genus (Equus) with 18 extant native (wild) species or subspecies that are limited to Africa and Asia, with a recent reintroduction in parts of Eurasia (Oakenfull et al. 2000). The one horse species (Equus ferus) has three subspecies: the tarpan or Eurasian wild horse (Equus ferus ferus), Przewalski’s horse or takhi (Equus ferus przewalskii), which is the only true extant (and critically endangered) native horse species, and the domestic and feral horse (Equus ferus caballus) (ICZN 2003). The other domesticated member of Equus is the domestic donkey or feral ass (Equus africanus or E.asinus) (Oakenfull et al. 2000). The modern genus Equus first appeared some 2 million years ago in North America from whence they spread to Asia, Europe, Africa and South America (Kavar and Dovc 2008) to become one of the top four grazing herbivores of the Pleistocene Mammoth Steppe (Guthrie 1990). Wild equids became Introduction 3 extinct in North and South America some 8,000–12,000 years ago but survived and diversified in Asia, Europe and Africa, for example, into numerous species and subspecies of zebra in Africa inparticular (Martin 1970; Bennet and Hoffmann 1999). In western Europe, rock engravings resembling the takhi have been dated at 11,000–22,000 years of age in Italy, western France and northern Spain (Van Dierendonck and Wallis De Vries 1996). Wild horses are thought to have existed in England until about 700 years ago, in Germany until 2000 years ago, and in Eastern Europe (e.g. Poland, Lithuania and Latvia) until the tarpan became extinct sometime between 1814 and 1879 (Groves 1991; Levin et al. 2002; Kavar and Dovc 2008). Some populations of specific breeds of feral horses have thus had an almost continuous presence in Europe (e.g. Camargue horses in France, Konik polski horses in Latvia and Poland) (Duncan 1992; Schwartz 2005). In North and South America, domestic horses were progressively introduced from the early 1500s to 1700s (Darwin 1962; Berger 1986). In Australia, they were introduced from 1788 onwards and in New Zealand in 1814, with the first feral population in the Kaimanawa mountain region recorded in 1876 (MacDougall 2001; Mincham 2008). Thus, evolutionary history may be generally described as long and continuous in Europe, long but discontinuous in North and South America, and short in Australasia. Differences in evolutionary history between Europe, both the Americas and Australasia was described because those regions are where studies of the ecological impact of feral horses have mostly been conducted (Section 1.4; Appendix 1). The inverse relationship between the degree of exposure over evolutionary periods to large, generalist herbivores and the responses of certain plant communities to grazing is an important explanatory variable in some contemporary global models of grazing impacts (Mack and Thompson 1982; Milchunas et al. 1988; Mack 1989; Milchunas and Lauenroth 1993). The evolutionary history of feral horses thus provides a general background for evaluating the international peer-reviewed literature on the environmental effects of feral horses. It also provides clarification for terms used in this thesis. For example, coinciding with the heritage perspective is an objection to the term ‘feral’, with preferences for ‘brumby’, ‘wild’, or ‘free-ranging horses’. Throughout this thesis the term feral horse will be used to describe Australian populations as this has been independently assessed (English 2001b) as the ecologically correct description of how horse populations originated and remain in the wild. The same rationale applies to other populations arising from domestic introductions (i.e. the Americas and New Zealand). In the interest of brevity, hereafter the term feral horse(s) is also abbreviated to horse(s) with domestic populations referred to specifically in the context Introduction 4 required, for example, stockhorse, recreational riding horse or the generic term of domestic horse. The term ‘wild’ horse is akin to native and also has a specific ecological meaning, namely ‘a species or race thought to have occurred in a geographical area before the Neolithic’ (New Stone Age, circa 9500 B.C.; Manchester and Bullock 2000). This definition only applies to the extant takhi subspecies of horse and the world’s species and subspecies of ‘wild’ equids, of which only the three subspecies of kiang (E. kiang) in Asia and five of the six subspecies of plains zebra (E. burchelli) in Africa remain abundant and widespread (Duncan 1992). 1.3 Feral horse biology in relation to ecological impact potential According to the so-called ‘tens rule’ for statistical regularities in biological invasions, 10% of introduced species become established in the wild, and 10% of those established become a pest (Williamson and Fitter 1996a, b). Pest species are difficult to define (Perrins et al. 1992) because species are pests for individualistic reasons (Williamson and Fitter 1996b). Across New Zealand and Australia, feral horses are considered a pest species and have been assigned the same types of impacts as feral pigs (Sus scrofa) and feral goats (Capra hircus), with the main impact being environmental degradation (Wilson et al. 1992; Cowan and Tyndale-Biscoe 1997). Aspects of the biology and ecology of feral horses that predispose the species to having an adverse affect on the environment are outlined in the following sections. As horse impact studies in Australia are scarce, I also drew on the livestock (i.e. cattle) grazing literature to develop experimental hypotheses about ungulate impacts relevant to Australian ecosystems. Consideration was given to cattle (Bos taurus) to validate the assumption that some environmental impacts associated with cattle in Australia may also apply to horses with qualifications.