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 2013Photo 3: Trampling at The Playgrounds, ANP (Source: Arn Tolsma 2008)In the Australian Alps wild horse tracks have been found to result in a loss of plant cover, erosion (averagingbetween 40-156cm³/m²), soil compaction, and a loss of soil structural composition (Dyring 1990). Drying(1990) found that the soil on tracks was significantly compacted compared with off-track areas. Compactionwas found to be most severe on dry soils, where 20-50 passes by wild horses resulted in significantlycompacted soils. Compaction was found not to increase substantially with subsequent passes. Therefore anaverage group of four wild horses using a new track twice daily for less than a week will result in significantcompaction. Wild horses were found to have similar usage of sub-alpine and montane areas, with nodifference in compaction or track width demonstrated between these sites (Dyring 1990). Whinam et al.(1994) in their study of horse riding in Tasmanian Alpine environments found that 20-30 passes by horses hassubstantial immediate as well as delayed effects on the soils of shrubland, herbfield and bolster heathcommunities, however effects on dry grassland soils were less evident. Wild horse exclosure experiments inthe eastern Victorian Alps have shown that continued grazing and trampling by wild horses has maintained lowherbfield turfs, where the soil surface is more susceptible to trampling impacts than the grasslands found inthe horse exclosure plots (Wild & Poll 2012; Whinam et al. 1994).Dyring (1990) found that wet soils were less prone to compaction but more susceptible to structural damagethan dry soils. This is supported by Rogers (1994) who demonstrated that dry areas are more resistant tofracturing than wet areas, which were more easily broken up by trampling. The loss of soil structuralcomposition is most pronounced on wet soils because horse trampling and grazing fractures saturated soils(Rogers 1994; Turner 1987; Dyring 1990). Fracturing of water saturated grassland can result in downslopesedimentation, water ponding, and opportunities for the establishment of weeds (Rogers 1994). The gradientof the slope has been found in other studies to directly correspond to the level of erosion. The steeper theslope the more prone it will be to erosion (Dyring 1990).Disturbances to the substrate caused by actions such as wild horse trampling, wallowing and grazing increasethe exposure of soils to the elements (such as wind, rain and needle ice), leads to the removal of vegetationand alters drainage conditions hence increasing susceptibility to erosion (Dyring 1990). In central Australia,wild horses were linked to aggravated gully erosion in areas close to water (Berman & Jarman 1988). Wildhorses caused considerable erosion in a sandy environment in the USA: over a five to seven year period fencedplots (excluding horses) were on average 0.63 m higher than unfenced plots in sand dunes habitats (DeStoppelaire et al. 2004).Pugging of soil (trampling of wet soils leaving a dense mat of deep footprints), in particular around wetlandsand waterways, can change soil nutrient status, and increase water turbidity and sediment loads in adjacentwaterways (O’Connor 2005). Dyring (1990) found that wild horses in the Australian Alps can create nutrient13
The Ecology of Wild Horses and their Environmental Impact in the Victorian Alps May 2013hotspots due to the high levels of nutrients (especially nitrogen) in their dung and urine. Manure in dung pilesincreases the availability of soil nutrients, especially nitrogen and phosphorous, thus creating microhabitatssuitable for weed invasion (Loydi & Zalba 2009).3.3 Impacts on vegetationThe level of impact wild horses have on native vegetation is dependent on the amount and type of use, and,the resilience of the vegetation (Whinam et al. 1994). The most obvious impact wild horses have on vegetationis a reduction in vegetation cover and height, however they may also alter plant species composition, richnessand diversity, and contribute to weed invasion (Turner 1987; Dyring 1990; Rogers 1991; Beever & Brussard2000; De Stoppelaire et al. 2004; Loydi & Zalba 2008; Wild and Poll 2012). Trampling by wild horses also altersvegetation, particularly along tracks and at watering points (Turner 1987; Dyring 1990; Rogers 1991; Beever &Brussard 2000).3.3.1 Removal of native vegetation by grazing, trampling and wallowingVertebrate grazers can negatively affect the cover of herbs in alpine and subalpine regions of Australia (Bridle& Kirkpatrick 2001). Dyring (1990) found that wild horses preferentially graze grasslands and healthlands.Studies in the greater alpine region have demonstrated that a decrease in grazing pressure from introducedherbivores has led to an increase in plant cover or flower stem production (Carr & Turner 1959a; 1959b;Wimbush & Costin 1979a; 1979b; 1979c; Leigh et al. 1991; Wahren et al. 1994). Bridle and Kirkpatrick (2001) intheir study of Tasmanian alpine and subalpine plains found the impacts of domestic stock (i.e. cows, horses,sheep), rabbits and native herbivores on treeless subalpine vegetation were much greater than the effects ofnatives herbivores and rabbits alone (Bridle & Kirkpatrick 1999).McKay (2001) conducted a survey of the impacts of horse-riding and walking on the alpine vegetation ofMount Bogong. Trampling was found to reduce the height, cover and abundance of both shrub and groundflora within track areas and to result in greater exposure of bare ground. Beever et al. (2008) argue that theloss of connectivity in shrub canopy due to rubbing and trampling may increase rates of isolation,evapotranspiration and soil loss at small spatial scales.Results from the Cowombat Flat and Native Cat Flat horse exclosure plot monitoring program have shown thathorse trampling and grazing has resulted in the removal of vegetation and increased bare ground in horseoccupied areas (Wild & Poll 2012). In contrast, fenced horse exclosure plots showed a trend for increasingvegetation cover and decreasing bare ground as they recovered from past horse disturbance over a thirteenyear period (ibid).It is argued that grazing may have an impact on the reproductive success of some flora species by impacting onthe dispersal opportunities of wind-dispersed species as well as their ability to attract pollinators; both ofthese functions are affected by flower height (Bridle & Kirkpatrick 1999). For example, on the AssateagueBarrier Island (USA), a small herbaceous annual, Amaranthus pumilus was once abundant and is now limited toa couple of individuals; its decline is linked to wild horse grazing and trampling (De Stoppelaire et al. 2004).Bridle and Kirkpatrick (1999) found decreased fecundity of herbs with increasing grazing pressures. InAustralia, clipping experiments on alpine herbs have shown that flowering success may be retarded if plantsare clipped early in the growing season or if they are cut more than once (Leigh et al. 1991).3.3.2 Change in vegetation composition and structureGrazing by introduced herbivores can alter the appearance, productivity and composition of vegetationcommunities (Dyring 1990; Hobbs & Hyeneke 1992). This may be due to reduced regeneration/recruitment asa direct result of selective grazing or the physical impacts of trampling and erosion. Wild horses selectivelygraze palatable species and have the potential to change the composition of threatened vegetation14
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