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 (Dyring 1990). Redistribution of nutrients through dung can also contribute to the changes invegetation patterns (Dyring 1990).Trampling can create openings in vegetation that provide opportunities for new plants to become established.Different vegetation types demonstrate differing levels of resistance (tolerance) to trampling. Within some,non-tolerant vegetation types, trampling can slow the growth of dominant species sufficiently to allow thepersistence of less vigorous species (Hobbs & Hyeneke 1992).Dyring (1990) found trampled sites (and areas adjacent to track systems) had lower native plant diversity and ahigher abundance of exotic species. Plants found on wild horse tracks were characteristically non-woodyprostrate fast-growing annuals and grasses, with hemicryptophitic life-forms (species with renewal buds nearthe surface), which tolerated trampling better than upright plants (Dyring 1990). Similarly, horse occupied sitesat Cowombat Flat and Native Cat Flat exhibited a predominance of hemicryptophitic species in contrast tohorse exclosure plots, which were dominated by grasses and sedges (Wild & Poll 2012). Grasslands are moreresilient to the impact of wild horses than other communities where ferns, mosses and shrubs are importantcomponents of the vegetation (Whinam et al. 1994; Venn et al. 2009). Whinam and Chilott (1999) found thatshrubs and shrubland communities were more vulnerable to trampling than other life-forms or vegetationtypes in central Tasmanian alpine vegetation.As well as affecting vegetation community composition, wild horse activity can affect the physical structure ofvegetation communities. Exclosure experiments have shown that vegetation cover and height is far greater inhorse free sites (Turner 1987; Beever & Brussard 2000; De Stoppelaire et al. 2004; Prober & Theile 2007; Wild& Poll 2012). Lower plant biomass was found in the Australian Alps where wild horse densities were higher(Walter 2002). In the Cowombat Flat and Native Cat Flat wild horse exclosure plot monitoring program, thefollowing results occurred in the wild horse exclosures (see also Photo 4):• a significant increase in the average height of vegetation;• a significant increase in litter cover;• no significant effect in species richness at Cowombat Flat;• significantly lower native species richness at Native Cat Flat, due to the gradual competitive exclusionof some lower stature species by dense swords of sedges and grasses;• no indication of an increase in weed richness/abundance due to exclosure (Prober & Theile 2007,Wild & Poll 2012).While the effect of wild horses on vegetation structure and composition was less consistent than the effect ofhorses on vegetation height, plots inside horse exclosures were often characterised by dense swards of sedgesand grasses associated with the competitive exclusion of some lower stature species (Wild & Poll 2012). Incontrast, horse occupied areas tended to be characterised by low herbfield turfs, likely to be maintained bythe preferential grazing of grasses and sedges by horses (ibid). Wild and Poll (2012) suggest that theresurgence of dense sedges and grasses and competitive exclusion of some lower stature species may indicaterestoration to a previous, more natural state and possibly towards a peatland environment.Studies in the USA have shown that wild horses can lower vegetation cover, abundance and flora speciesrichness as well as alter the species composition and structure of the vegetation (by increasing thepredominance of grazing resistant forbs and exotic plants and creating a less continuous shrub canopy)(Beever et al. 2003; Beever and Herrick 2006; Stoppelaire et al. 2004; Beever & Brussard 2000). Using a seriesof monitoring and exclosure plots in the Kaimanawa Mountains, New Zealand, Rogers (1991; 1994), found thatwild horses severely disrupted the composition of the native vegetation. In the grazed plots, species biomassand stature was low for all potentially taller, palatable grasses (Rogers 1991).15
The Ecology of Wild Horses and their Environmental Impact in the Victorian Alps May 2013Photo 4: Cowombat Flat wild horse exclusion plots from the air (source: Geoff Robinson).3.3.3 Threatened vegetationIn the Victorian Alps, wild horses are considered to be one of the major threats to alpine ecosystems (Tolsma2008b). Wild horses are considered to be a serious threat to at least five plant communities listed in theVictorian Flora and Fauna Guarantee Act 1988 (FFG) (Appendix 1), and numerous plant species (Appendix 2).The threat that wild horses pose to threatened species and communities is recognised in the listing of“degradation and loss of habitats caused by wild horses” as a potentially threatening process under the FFGAct. It is likely that wild horses threaten many other species or communities not yet identified or investigated.Trampling by ungulates (hoofed animals) has been considered one of the major threats to several FFG listedalpine vegetation communities. Within the Caltha introloba Herbland Community, cushions of tuft-rush(Oreobolus), which play an important role in reducing the erosive forces of flowing water, may be dislodged bytrampling, or their regeneration disrupted (McDougall 1982; McDougall & Walsh 2007). Similarly, the AlpineSnowpatch Community, situated on steep sheltered slopes, is subject to constant irrigation during the thawwhich renders them particularly susceptible to soil loss following damage to the vegetation by trampling(McDougall 1982; Wahren et al. 2001a; McDougall & Walsh 2007). Montane Swamp, because of its position inthe landscape, is another listed community likely to be susceptible to the impact of wild horses (Dawson 2009).Ecological communities which have been listed under the Federal Environment Protection and BiodiversityConservation Act 1999 (EPBC) and/or the Victorian FFG Act 1988, and that are potentially at risk from wildhorses, are presented in Appendix 2.In order to mitigate the threat that wild horses pose to threatened flora species in the Victorian Alps, ParksVictoria has established three wild horse exclusion fences around particularly threatened and sensitive16
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