The Ecology of Wild Horses and their Environmental ... - Parks Victoria

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The Ecology of Wild Horses and their Environmental Impact in the Victorian Alps May 2013Argentina. The presence of wild horses was associated with an increase in the rate of egg predation, whichvaried from 12.5% within exclosures to 70% in grazed areas. This was directly related to increased visibility ofthe nests and subsequent exposure to predators, nest trampling and reduced nesting sites. In Victoria’sBarmah Forest wild horses are listed as a threat to some ground nesting birds including the AustralasianBittern and Little Bittern (SAC 1997a; 1997b). Due to a lack of research it is unknown if wild horses pose athreat to any birds in the Victorian Alps.Wild horses may potentially cause the collapse of wildlife burrows (Theile & Prober 1999a). In the VictorianAlps many medium and small mammals, such as the bush rat (Rattus fuscipes), swamp rat (R. lutreolus) andbroad-tooothed rat (Mastacomys fuscus), use burrows that may be susceptible to trampling damage. Wombatburrows that, for example are dug in open ground with little protective cover over the burrow may besusceptible to damage by wild horses. In the USA, exclosure plots around springs exhibited more smallmammalburrow entrances than plots at horse-grazed springs (Beever & Brussard 2000).The affect that wild horses have on invertebrate eco-engineers such as ants in the Victorian Alps is currentlyunknown. Beever and Herrick (2006) found a greater abundance of ant mounds in sites where wild horses hadbeen removed over ten years ago. This may have a substantial influence on ecosystem health and the survivalof other species.Extensive damage to the structural complexity of ground-level vegetation and habitat caused by wild horsesthreatens the reptile and amphibian species of the Victorian Alps. Structural complexity of habitat is a criticalrequirement for many reptile species and communities (Clemann et al. 2001). Litter cover, ground cover andthe degree of soil compaction are all important elements for reptiles (e.g. Brown 2001; Jellinek et al. 2004).Similarly, most amphibian species in the Australian Alps rely on the structural complexity formed by vegetationalong the margins of streams and ponds, especially for male calling and female egg laying sites (Osborne 1990;Green & Osborne 1994). Grazing and trampling by ungulates such as wild horses in the alpine region affectshabitat complexity through: direct removal of vegetation; destruction of structural components in the groundlayer; and, soil compaction (Clemann et al. 2001). For example, the alpine water skink (Eulamprus kosciuskoi)(photo 10), alpine she-oak skink (Cyclodomorphus praealtus) and alpine bog skink (Pseudemoia cryodroma) arethreatened by wild horse impacts on peatlands, wet heath and riparian areas, which provide vital habitat forthese species (Clemann et al. 2001; Clemann 2001; Meredith et al. 2003; Steane et al. 2005).As discussed, peatland habitats are extremely vulnerable to wild horse activity. Horse damage to peatlands hasimmediate effects on habitat structure, and leads to hydrological consequences such as deteriorating waterquality and loss of landscape structure. Damage to habitat caused by wild horses is specifically identified as athreat to the federally endangered alpine she-oak skink, which occurs in close proximity to bogs, streams andwet heaths. Recommended actions to mitigate this threat include the development and implementation of amanagement plan for the control of wild horses (DEWHA 2009b).Historically, the threatened alpine tree frog (Litoria verreauxii alpine) was widespread and abundant acrossmost of the high country of mainland south-eastern Australia (Clemann & Gillespie 2004). The species hassince retracted from much of its previous habitat. Populations of the alpine tree frog are known to be subjectto a number of threatening processes, including the trampling of habitat by wild horses (Clemann & Gillespie2004). The threat of further population declines is likely and remains a cause for concern (Brown et al. 2007).Small mammal communities may be affected by grazing ungulates such as wild horses indirectly throughalteration in vegetation structure, water quality and other site characteristics (Giuliano & Homyack 2004).Removal of ungulates (mostly cattle) grazing from streams and associated riparian zones led to an increase inmammalian species richness and abundance within two years in a study in south west Pennsylvania (USA)(Giuliano & Homyack 2004). A similar result was shown in the central Kimberley across several habitats after23
The Ecology of Wild Horses and their Environmental Impact in the Victorian Alps May 2013the removal of cattle (Legge et al. 2007). In a study of horse impacts using exclosures in the USA, qualitativedifferences were observed in small mammal activity (Beever & Brussard 2000).There are a limited number of small mammal species in the Australian Alps, and complexity of vegetationstructure is associated with habitat quality for these species (Carron 1985). The habitat of small mammals inAlpine regions is extremely sensitive and threatened by climate change (Green et al. 2008). A species ofparticular concern is the broad-toothed rat Mastacomys fuscus (Green & Osborne 2003 and references within)whose habitat is shrinking and is likely to face a further declines with climate change (Green et al. 2008). Thebroad-toothed rat inhabits areas of moderate-to-dense grass and/or sedge cover (Green et al. 2008). Thisenvironment is favoured by wild horses for grazing and the habitat can be broken-up through wild horseactivity. O’Brien et al. (2008) found populations of broad-toothed rats that existed in isolated patches werelikely to suffer local extinction due to low levels of success emigrating through unsuitable habitat to reachsuitable habitat.3.6.3 Threatened fauna in the Victorian AlpsHerpetofauna (reptiles and frogs) are central components of the vertebrate fauna of the Victorian Alps andseveral threatened species are endemic to alpine areas (Clemann et al. 2001). A large proportion of this groupare officially listed as threatened at the federal and state level (Appendix 3). Herpetofauna in this region aregenerally dependent on bog, grass and wet heath habitats (Clemann et al. 2001). As previously mentioned,most alpine herpetofauna are intimately linked with habitat structure, particularly close to ground level(Clemann et al. 2001). Wild horses are believed to be one of the processes directly or indirectly threateningthese species (Clemann et al. 2001). For example, the alpine water skink (Eulamprus kosciuskoi) (photo 10) islisted as critically endangered in Victoria and is only found in the high altitude areas of south-eastern Australia(Steane et al. 2005). Steane et al. (2005) found the preferred habitat of alpine water skinks to be mossbeds ingood condition (i.e. high cover of live Sphagnum and low cover of bare-ground, rock and non-mossbedvegetation). Any threat to Sphagnum cover, such as that posed by wild horse grazing and trampling, severelythreatens the habitat of the alpine water skink (Steane et al. 2005). Horses have impacted much habitat thatwas previously occupied by the alpine water skink on Forlorn Hope Plain in the Buchan headwaters.Consequently, to protect threatened alpine water skink populations from trampling by wild horses, wild horseexclusion fences at Davies Plain around two sub-alpine bogs have been constructed (see section 3.3.3).Photo 10: The alpine water skink (Eulamprus kosciuskoi), an endangered species whose habitat is threatenedby wild horses (Source: Nick Clemann).24
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