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 20134. Wild horse biosecurity issuesWild horses are a prominent reservoir for exotic and endemic diseases and parasites that can affect nativewildlife, domestic horses, stock and humans (Bradshaw et al. 2007). The spread of pests and disease by feralanimals is considered by the federal government of Australia to be a major biosecurity risk (DAFF 2010).Wild horses in the Victorian Alps have the potential to be a biosecurity risk through harbouring andtransmitting disease. Diseases of concern are either those that have the potential to establish or exist alreadywithin Australia. Diseases are highly variable, for example they may be equine specific or they may affectseveral species, transmissibility varies, and symptoms range from minor ailments to mortality. The wild horsepopulation in the Australian Alps poses a potential risk to other species that may co-habit with them such asnative species, pigs, deer and cattle.Wild horses have the potential to transport the Amphibian Chytrid fungus (Batrachochytrium dendrobatidis)between waterbodies and between catchments. This pathogenic and highly transmissible fungus, results in thedisease chytridiomycosis, that has caused the decline or extinction of up to 200 species of frogs (Skerratt et al.2007). Skerratt et al. (2007, p.125) refer to the impact of this fungus as the, “most spectacular loss ofvertebrate biodiversity due to disease in recorded history”. As well as being a potential vector of this disease,horses may introduce novel strains of the fungus to new areas. Some fungus free frog populations remain inplaces such as Grey Mare in Kosciusko National Park and keeping them fungus free is a critical biodiversitypriority.Wild horses can play a potential role in a disease outbreak of a newly imported disease (Biosecurity Australia2009). There are many steps involved in the outbreak of a new disease. Wild horses must first be exposed tothe disease, once exposure occurs, a range of outbreak scenarios are possible from no spread to widespreadestablishment. Outbreak scenarios are dependent on the epidemiology of each disease agent. The role thatwild horses play in a potential disease outbreak scenario for a newly imported disease was considered in a riskassessment by Biosecurity Australia (2009). In general, wild horses were thought to have a limited ability toaffect domestic horse populations because of their remoteness. Equine Influenza received the highest riskassessment (moderate). There was an outbreak of Equine Influenza in 2007, which led to prohibitions on horsemovements in Victoria to control the spread of the disease, however, it was eliminated (Animal HealthAustralia 2009).There is little known about the incidence and prevalence of many existing diseases of domestic horses inAustralia, and much less is known about the frequency of disease in wild horses. There are a range of zoonoticdiseases (diseases that horses can share with humans and other livestock) which are present in Australia andmay be of concern including anthrax, Hendra virus disease, Murray Valley Encephalitis, leptospirosis,Australian bat lyssavirus, Salmonellosis, Tetanus and ringworm. It is not known how these diseases may affectnative species.Hendra virus is a relatively new and emerging disease, first reported in 1994 in Queensland (Equine SpeciesWorking Group n.d). Subsequently there is not a great deal known about the disease, however, horses arelikely to be dead-end hosts. It causes severe respiratory illness in horses and is frequently fatal. Humans cancontract the disease if they have direct contact with blood or saliva of an infected horse (Equine SpeciesWorking Group n.d).Murray Valley Encephalitis is a virus spread by mosquitoes that can infect many animals, including horses andhumans (DPI 2012). The virus causes swelling of the brain and is potentially fatal for humans (DPI 2012).Outbreaks in south eastern Australia are generally associated with wet conditions. In 2011 a widespreadoutbreak infected domestic horses in north and north western Victoria (DPI 2012).25
The Ecology of Wild Horses and their Environmental Impact in the Victorian Alps May 2013Humans, domestic animals and wildlife are susceptible to the bacterial disease Leptospirosis and infectedanimals may become sick or show no symptoms at all (Equine Species Working Group n.d). Salmonellosis is abacterial disease that can infect horses and humans and is of concern because some strains exhibit resistanceto multiple antibiotics (Equine Species Working Group n.d). Salmonella may impact upon many species(including native wildlife) particularly if they are under stress. Australian bat lyssavirus is the only type of rabiesin Australia; however, there have been no reports of horses being affected by this disease. Tetanus can be afatal disease and its spores are widely distributed, usually in soil, dust and manure; horses and humans are themost susceptible of all of the animal species to tetanus (Equine Species Working Group n.d). Ringworm is afungal skin disease. Numerous species can transmit ringworm to humans (Equine Species Working Group n.d).5. Gaps in knowledgeThere has been a limited amount of research into wild horse biology, behaviour and ecological impacts in theAlpine region to-date. An important area for investigation that will aid in guiding effective management is therelationship between wild horse density and damage (Dawson et al. 2006). Quantification of the relationshipbetween density, impact and population control has been identified as a priority research area for themanagement of wild horses by the AALC. The AALC has initiated a project that aims to address this gap bydeveloping a protocol for determining horse density at the local scale and investigating the influence of horsedensity on impacts. Site-specific research using experimental trials to determine local levels of resistance toimpact would also provide important information (Venn et al. 2009).Previous aerial surveys of wild horse populations have not covered the whole area occupied by horses and arelikely to significantly under-estimate the total population size. There are large areas known to be occupied bywild horses that have been excluded from previous aerial surveys, including Byadbo and adjoining areas of theAlpine National Park and adjacent State Forests (Dawson 2009). There are plans to cover a larger portion ofthe area occupied by wild horse in future aerial population surveys in order to obtain a more accurate estimateof the total Victorian Alps wild horse population size. Wild horses do not currently occupy all suitable habitatwithin the Victorian Alps. Mapping potential horse habitat that could potentially be colonised by wild horses isrequired and work has commenced to address this gap (Dawson 2009).The effects of wild horse grazing on vegetation have been investigated at small spatial scales; however havenot been investigated at a broad spatial scale. The effects of chronic disturbance over broader areas issubsequently not well understood within the Victorian Alps, greater Alpine area, Australia or internationally(see Beever et al. 2003; Beever et al. 2008). To-date only studies at a small single spatial scale have beenconsidered. These studies are important; however they may not be representative of the Victorian Alps systemmore broadly and may not represent the conditions prevalent on a landscape scale (Nimmo & Miller 2007)Finally there is a need to further investigate how the environmental impact of wild horse’s changes throughtime as the environment varies (e.g. drought, fire, climate change etc). The environmental impact of wildhorses may be exacerbated in drought years, as they tend to concentrate around watering points (Dawson2010). Impacts are also likely to increase substantially after fire (such as those that occurred in Victoria in 2003and 2006/07) when Alpine environments such as peatlands are particularly susceptible. Climate change islikely to influence the magnitude of the impacts of wild horses in future years. Alpine species and communitiesare amongst the most vulnerable to the effects of climate change and therefore their resilience to climatechange requires that they be protected and maintained in as healthy a condition as possible (DSE 2010). Thispressure combined with the impact of wild horses requires further investigation.26
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