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 2013Bogs and peatlands are highly sensitive to damage by trampling and grazing. Preferential grazing of palatableherbs and grasses in Sphagnum bogs, combined with browsing of new growth of shrubs can lead to increaseddominance by unpalatable species (Whinam & Chilcott 2002). Sphagnum is easily crushed and broken up bywild horses trampling the delicate vegetation, selectively grazing and wallowing in pools and waterways (TSSC2009). Trampling may lead to the drying out of bogs, the loss of Sphagnum cover as well as changes to othervegetation, and creation of bare pavements (Theile & Prober 1999a; TSSC 2009). Once Sphagnum cover is lost,alpine soils and peat environments are very susceptible to desiccation, incision and soil erosion (TSSC 2009).In New Zealand wild horses have been found to severely degrade bog and wetland areas through tramplingand grazing, irrespective of local variations in horse density and habitat abundance (Rogers 1991). Thevulnerability of bogs in New Zealand was also found to increase with wetness (Rogers 1991). Whinam andChilcott (2002) found most Sphagnum peatland communities investigated in the ACT and New South Wales(NSW) were degraded by wild horses and other agents (i.e. fire, feral pigs, grazing and clearing). In 2008 thestatus of peatland systems in the Victorian Alps was investigated by Tolsma (2008a; 2008b). In the East AlpsUnit of the ANP, 97% (n=63) of peatland systems were found to be impacted (i.e. compacted, trampled orpugged) by wild horses (Tolsma 2008b). Only two peatlands assessed by Tolsma (2008b) in this unit of the ANPdid not show any signs of current wild horse activity: a fenced peatland at Davies Plain and an isolated systemon the south-east edge of the Unit. Whinam and Chilcott (2002) argue that wild horses are one of the greatestthreats to the survival of peatlands.Channelling is a form of erosion and water diversion which can result from wild horse activity. Trampling andwallowing cause channels to form in the disturbed Sphagnum. Channelling leads to changes in water flow, aswater exits the disturbed bog more rapidly. This action may completely alter the drainage pattern and result indrier conditions (Whinam & Chilcott 2002; TSSC 2009). The formation of channels is detrimental to the fensassociated with the bogs because the fens drain directly into the stream system and dry-up (TSSC 2009).Due to the access provided to peatlands following extensive bushfires throughout the Victorian Alps (primarilyin the 2003 fires) Tolsma (2008b) found that wild horse activity in burnt sub-alpine to alpine peatlands in theVictorian Alps varied from minor trampling at peatland margins to extensive pugging across almost the entireburnt peatland surface. On average, 25% of all peatland surface area assessed in the East Alps Unit of the ANPand 16% of peatland area assessed in State forest to the south was impacted by trampling (Tolsma 2008a) 1(see Photo 6). Trampling by wild horses restricts Sphagnum recovery in naturally-regenerating peatlands, andmay impact on some peatlands where restoration work is occurring (Tolsma 2008b). Tolsma (2008b) arguesthat in the Victorian Alps, many areas of burnt peatland, particularly in the East Alps Unit, are unlikely tocompletely recover while wild horses remain in the area (Tolsma 2008b).Peatland communities are likely to face further strain as climate change further alters their functioning(Dawson 2009b). At the same time it is suggested that the importance of the functional role that AlpineSphagnum peatlands play in water release and flow downstream will increase (DEWHA 2009a).1 'Area impacted' is the estimated proportion of the peatland system that was compacted, trampled or pugged by wildhorses.19
The Ecology of Wild Horses and their Environmental Impact in the Victorian Alps May 2013Photo 6: Wild horse trampling of a bog area at The Playgrounds (source: Parks Victoria 2010)3.5 Impacts on waterways (streams and stream-banks)Wild horse activity can lead to the degradation of waterways and a reduction of water quality in streams,water holes and springs due to the trampling of stream banks and fouling of water holes (Csurhes et al. 2009).Wetlands, waterways and their adjacent riparian ecosystems generally have very high diversity andproductivity (Dawson 2009b), but are typically fragile and susceptible to damage by wild horses (and otherlarge hoofed animals) from selective grazing, trampling, pugging, wallowing and crossing (Costin 1954;Whinam & Comfort 1996; Williams et al. 1997; Evans 1998; Whinam & Chilcott 2002; McDougall 2007; NPWS2007; Prober & Thiele 2007; Dawson 2010). Streambanks are easily churned up and broken down by wildhorse hooves sinking into wet soils (Dyring 1990). Grazing by large hard-hooved herbivores can affectstreamside vegetation, stream channel morphology, shape and quality of the water column and the structureof the soil portion of the streambank, which has flow on effects for aquatic and terrestrial wildlife (Kauffmann& Krueger 1984; Rogers 1994; Beever & Brussard 2000).In the Victorian Alps, grazing and trampling within the vicinity of streams has been found to result in bankslumping and breakdown, vegetation disturbance and increased soil compaction (see Photos 7, 8 and 9). Thiscan lead to a decrease in infiltration, increased run-off as well as further drainage channel development,lateral erosion and stream incision (Dyring 1990).The Cowombat Flat and Native Cat Flat monitoring program has revealed significant changes to streamfunction and structure as a result of wild horse exclosure fencing. The unfenced stream areas had more bareground, were more entrenched and had distinct, open water channels, whereas areas within the exclosureshad indistinct channels, a more or less complete vegetation cover across the channel and no or little visibleopen water (Prober & Thiele 2007, Wild & Poll 2012). There was a significant increase in stream depth overtime in unfenced plots at Native Cat Flat while stream depth within the exclosures remained stable, suggestingaccelerated erosion in horse occupied areas (Wild & Poll 2012). Stream condition was better and consistentlyimproved over time in the horse exclosure plots which exhibited less pugging, stream slumping (collapse of thestream bank) and bare ground than unfenced areas (Wild & Poll 2012). The extra vegetation and lack of bareground have led to slower water flow along the stream in the horse exclosures (Prober & Thiele 2007, Wild &Poll 2012). In contrast the increased bare ground and lack of vegetation in unfenced areas is contributing tofaster flows and accelerated erosion of the stream channel, particularly at native Cat Flat where steep bankswith undercutting were observed (Wild & Poll 2012). Subsequently Prober and Thiele (2007) and Wild and Poll(2012) argue that at a catchment level, wild horse trampling and grazing may be impacting significantly onstream and catchment hydrology and water quality.20
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