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The Ecology of Wild Horses and their Environmental Impact in the Victorian Alps May 2013forced out of their harems; they generally occur alone or in groups of two or three (Dobbie et al. 1993). Theaverage group size (harem and bachelor groups) from three sites across the Australian Alps was 5.65 (± 0.51SE) (Walter & Hone 2003), while Drying (1990) found that harem group sizes in the Alpine region ranged fromtwo to 11 individuals, with groups typically consisting of one stallion, two mares and one foal. Wild horsesrarely have periods of social isolation during their lifetime except for bachelors in their pre-harem formationstage (van Dierendonck 2006). Harem groups tend to be stable breeding units and generally favour permanentlocations around reliable water sources (Dobbie et al. 1993). Within harem groups, adult mares form a longtermstable nucleus, while the breeding stallion is regularly replaced (van Dierendonck 2006). Bachelor groupsare more mobile and unstable (Dobbie et al. 1993).Groups of wild horses are loyal to undefended home ranges with central core use areas (Linklater 2000). Thehome ranges of groups overlap entirely with other groups and home range size increases with group size(Linklater 2000). Home ranges of wild horses in the Australian Alps have not been studied but there isinformation available from other environments. Home range sizes vary within a region and between regions. InQueensland and central Australia wild horses were estimated to have a home range of approximately 100km²and 70km² respectively (Dobbie et al. 1993). In contrast, wild horses at Kiamanawa in New Zealand, which hasa similar climate to the Australian Alps, had home ranges 0.96 to 17.7 km 2 (Linklater 2000). Given the climaticsimilarities, home range of horses in the Victorian Alps are likely to be similar to Kaimanawa.As prey animals in their native environment, a horse’s primary defence mechanism is rapid flight away fromthe threat of danger. It is therefore advantageous that they identify potential predators as quickly as possible(van Dierendonck 2006). This is aided by their monocular and binocular vision, which enables them to have anextensive view of their surrounds (Dobbie et al. 1993). With well developed hearing any movement is readilydetected (Dobbie et al. 1993).3.5 Wild horse habitat and diet preferencesWild horses occupy a range of habitats across Australia and the world. While they are best adapted to opengrassy plains they will also use rugged country (Norris & Low 2005). Wild horses are present from the highestto the lowest elevations in the Australian Alps (Walter 2002). Some groups may migrate to lower elevations inwinter but many horses maintain a high elevation (1600 metres) home range throughout winter (Dawson,unpublished data). Drying (1990) found that wild horses in Kosciuszko National Park (at a site in NSW sixkilometres from the Victorian border) made extensive use of heaths and grasslands for feeding, whilstavoiding the forests at all times of the year; this preference for open areas was broad-based with nodiscrimination between feeding and other activities. The preference for grasslands over forest is universal forwild horses (Pratt et al. 1986; Keiper & Berger 1982; Berger 1986; Linklater et al. 2000). The only exceptionhas been observed in the middle of the day in summer when wild horses seek refuge from the heat andhorseflies (Tabanidae) in forested areas (Duncan 1983; Dyring 1990; Keiper & Berger 1982; Berger 1986).Wild horses spend most (55-65%) of their time feeding (Duncan 1980). They are generalist grazers with astrong preference for the greatest concentrations of high quality food (green plant matter); when green plantmatter becomes sparse, the horses’ tactic is to search out areas with the greatest concentrations of perennialherbaceous plants, green or dead (Duncan 1983). Their diet mainly includes herbaceous plants (grasses, reeds,sedges and forbs), but they will also eat roots, bark, buds and fruit (Csurhes et al. 2009). Due to their forwardcutfront incisors they are able to graze close to the ground (Dobbie et al. 1993). Horses have a differentdigestive system to most ungulates (hoofed mammals), which enables them to consume large quantities oflow quality food and survive on a lower quality diet than cattle (Janis 2007). Cattle require time to chew theircud so cannot consume such large quantities but tend to select higher quality food (Janis 2007).Horses must drink at least once a day in summer and at least every second day during winter (Norris & Low2005). If food is plentiful horses will graze near water sources (Dobbie et al. 1993).7
The Ecology of Wild Horses and their Environmental Impact in the Victorian Alps May 20133.6 Wild horse mortality factorsWith few predators, unmanaged wild horse populations may increase in size and distribution until theyapproach carrying-capacity; at this point the population size stabilises as a result of a decrease in birth ratesand survival rates caused by limited food availability (Grange et al. 2009; Scorolli & Lopez Cazorla 2010). Forexample, in the Camargue in France, Grange et al. (2009) found that a decrease in available food resources, asa result of increasing density, caused a loss of body condition and the survival of foals and adult femalesdecreased with increasing density.In sub-alpine and montane environments of the Australian Alps, there is evidence that between 1999 and2002, the growth of three wild horse populations was limited by food availability (Dawson & Hone 2012). Thedensity of horses at the three sites was higher than the average determined from aerial surveys (see above).The Cowombat wild horse population had the highest density (6.4 km²), adult horses had the poorestcondition, recruitment (from birth to 3-years-old) was low, pasture biomass was low and population growthwas zero. In general however, unlike the Camargue example above, wild horses in the Australian Alps are notcontained and do not currently occupy their entire potential range (Dawson 2009). Therefore there is scopefor the wild horse population to spread.It is not clear whether wild horse populations across the Australian Alps would reach ‘carrying capacity’because the ability to reach equilibrium density depends on the variability of environmental conditions. Instochastic environments characterised by a high degree of unpredictable environmental variance (e.g. rainfall),an equilibrium is not reached (McLeod 1997). There are several examples of environmental events, inparticular drought, snow and fire, which have lead to dramatic declines in wild horse populations (thuspreventing the population from reaching carrying capacity). The eastern side of the AANPs (e.g. Buchan River,Lower Snowy, Suggan Buggan) lies in a rain-shadow and has limited available water. The drought in 1982-83was reported to have led to a dramatic decline of most wild horses in this area (Walter 2002), similar patternsare observed in central Australia (Dobbie et al. 1993). Drought can affect horses through thirst, starvation andingestion of poisonous plants (Dobbie et al. 1993).At higher elevations wild horse mortality occurs as a result of severe snow events or long periods of snowcover in the alpine area (Walter 2002). In winter wild horses at higher elevations, such as those on the BogongHigh Plains, have to dig through snow to access food for many weeks of the year which leads to a loss of bodycondition in the horses (Dawson unpublished data). In some cases severe snow events have resulted inmortality and in one historic event in the Brindabella’s (Australian Capital Territory (ACT)) an entire populationwas wiped out (Walter 2002).The 2003 fires had a substantial impact on the wild horse population of the Australian Alps, with a sharpdecrease in the wild horse population size following the fires (Figure 2). After the fires, Walter (2003) indicatedthat when wild horse numbers were low, that there was great potential for the population to increasedramatically due to the increased availability of high quality food and reduced population pressure. This wasdemonstrated by the results of the 2009 aerial survey which showed that the population had increased by224% since 2003 (Dawson 2009a). In a slightly different context, Catling (1991) predicted that wild horseswould be advantaged by frequent low intensity fires due to a simplification in forest structure.Additional wild horse mortality factors that should be considered include wild dogs and parasitism. There havebeen reports of wild dogs chasing foals in the Victorian Alps (Walter 2002). Parasitism and disease may also becauses of mortality and/or reduced health. However, neither of these factors has been formally investigated orquantified.8
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