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 2013communities. In 2002 two wild horse exclusion fences were established on Davies Plain around sub-alpinebogs known to contain several threatened plant species and the threatened Alpine Water Skink. In 2010 anexclusion fence was established at The Playgrounds to protect a population of threatened Marsh Leek Orchid(Prasophyllum niphopedium) (see Photo 5). The aim of all exclusion fences is to protect these species (and theirhabitat in the case of the Alpine Water Skink) from trampling and grazing by wild horses.Photo 5: The Playgrounds wild horse exclusion fence, established to protect a population of threatened Marsh Leek Orchid,an alpine bog community and habitat for the endangered Alpine Water Skink (source: Parks Victoria 2012).3.3.4 Weed dispersal and encouragementWild horses can facilitate weed invasion through dispersal and the creation of a favourable environment forweeds through disturbance. Weed species are dispersed through attachment to the body of the wild horse(epizoochory) or by being ingested and later excreted (endozoochory) (Cambell & Gibson 2001). Thereforewild horses have the potential to disperse weeds both long and short distances and can subsequentlycontribute to the establishment of weed species across several spatial scales (Nimmo & Miller 2007). Weaverand Adams (1996) argue that within their home range, horses are a potentially significant vector in thedispersal of a range of weed species.While wild horses are less likely than domestic horses (i.e. recreational riding horses or horses illegally releasedinto the region) to introduce new weeds into the Victorian Alps, there is potential for this to occur, especiallyconsidering their increasing range and the potential for wild horses to occur across tenures (i.e. farms, stateforests and national park). Many species of seed are transported in the dung of wild horses. Dung can be asource of viable seed taxa not otherwise found in a community (Campbell & Gibson 2001). Campbell andGibson (2001) found that horses pass large numbers of seeds through their digestive tract generally within48hours of consumption (but sometimes longer), and many of these seeds remain viable (Weaver & Adams1996). In some cases the process of digestion scarifies the seed coat, enhancing germination (Campbell &Gibson 2001). Weaver and Adams (1996) investigated the spread of environmental weeds into areas of nativevegetation along horse-riding tracks in three national parks in Victoria (including the ANP). Twenty-ninespecies of weeds were found to be dispersed via horse manure.Wild horse disturbance (i.e. dung, soil disturbance and pugging) can provide favourable environmentalconditions for the germination and colonisation of weed species (Dyring 1990; Rogers 1991; Loydi & Zalba2009). Wild horse dung can result in significant changes in vegetation and can introduce and encourage someinvasive weed species that could eventually colonise more pristine areas (Loydi & Zalba 2009). Nutrients indung can favour weed establishment, with weeds establishing more vigorously in areas both trampled andsubject to deposition of dung. In a study of the potential for horse dung to act as an invasion window inmontane pampas grasslands, Loydi and Zalba (2009) found that the cover of introduced species was higher indung piles than in control plots. Dyring (1990) speculated that the redistribution of nutrients through uneven17
The Ecology of Wild Horses and their Environmental Impact in the Victorian Alps May 2013dung deposits and selective grazing would lead to exotic plants such as White Clover (Trifolium repens) andCats Ears (Hypochoeris radicata), which readily colonise dung and disturbed areas, becoming established indisturbed grassland and healthlands.Wild horse disturbance was found to enhance the spread of smothering weeds in New Zealand’s KaimanawaMountains (Rogers 1991; 1994). In the Australian Alps, exotic species were found to colonise the disturbedarea of wild horse tracks but did not tend to expand into untrampled areas (Dyring 1990). Dyring (1990)argued that the maintenance of tracks by wild horses enables the continued survival of exotic weeds.Horse occupied areas at Cowombat Flat were found to be characterised by a higher abundance of Yorkshirefog grass (Holcus lanatus, an invasive grass species native to Europe) and red fescue grass (Festuca rubra, anexotic grass species native to North America) than adjacent fenced horse exclosure areas (Wild & Poll 2012).In a weed assessment of the peatlands within the Eastern Alps Unit of the ANP for Parks Victoria (Thomas 2010unpublished report) Thomas (2010) made a series of assessments regarding wild horses and their relationshipwith weeds. These observations are unquantified so are only mentioned briefly here:• White clover (Trifolium repens) were found growing out of dung piles;• the spread of Birds foot trefoil (Lotus uliginosus) into peatlands from culverts on Davies Plain Trackappeared to be associated with wild horses;• there was evidence of wild horses grazing on Brown top bent grass (Agrostis capillaris), which may beaiding the spread of this weed; and• wild horses appear to avoid Yorkshire fog grass (Holcus Ianatus), providing this species with acompetitive advantage.3.3.5 Native tree mortalityWild horses have been found to affect native tree mortality in Australia. The chewing and stripping of bark byhorses can reduce regeneration of seedlings and may result in the death of mature trees. A significant negativerelationship was found between bark chewing damage and overall tree health in Guy Fawkes River NationalPark (Schott 2002). Impact assessments identified bark-chewing damage was most severe along drainage-linesand in close proximity with other water sources that are heavily used by wild horses. Schott (2002) found thatwild horses chew bark most intensely during summer and favour particular trees (Eucalyptus amplifolia and E.saligna and E. Moluccana).3.4 Impacts on peatlands (also known as mossbeds, peats, bogs, Sphagnum bogs, wet heathlands &mires)In Australia, peatlands are a rare ecological community, largely restricted to the highlands of Tasmania and thesouth-east corner of the mainland (Tolsma 2008b). “Alpine Sphagnum bogs and associated fens” are listed asan endangered ecological community under federal legislation (Environmental Protection and BiodiversityConservation Act 1999). Alpine bogs are also protected by Victorian legislation, where they are listed as athreatened community under the Flora and Fauna Guarantee Act (1988). These ecosystems are of nationalimportance due to their significance in providing critical habitat for several endemic flora and fauna species(many of which are threatened and several risk extinction) and the important role they play in the regulationof water release and flow downstream to major river systems, such as the Murray (DEWHA 2009a).Sphagnum moss is an important component of peatlands, integral to their structure and function. Wild horsesare known to have a detrimental impact on Sphagnum growth and integrity, with the resulting vegetation lossultimately leading to degradation of this ecological community (Dyring 1990; TSSC 2009). Wild horses aretherefore a major threat to Alpine Sphagnum peatland communities (Whinam & Chilcott 2002; Tolsma 2008b;TSSC 2009; DEWHA 2009a).18
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