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The Ecology of Wild Horses and their Environmental Impact in the Victorian Alps
May 2013
Background Paper 1 of 3
The Ecology of Wild Horses and their Environmental Impact in the Victorian
Alps
Wild horse exclusion plot, Native Cat Flat 2004 (source: Parks Victoria).
Parks Victoria
May 2013
This paper was written by: Joanna Axford1, Michelle Dawson2 and Daniel Brown3
1
2
3
Formerly Parks Victoria, Bright
Eco Logical Australia
Parks Victoria, Bright
Acknowledgements: Arn Tolsma and Nick Clemann (Arthur Rylah Institute for Environmental Research,
DEPI) provided content and reviewed this paper. Charlie Pascoe, Dave Foster and Mike Dower (Parks Victoria)
provided information on wild horse impacts in the Alpine National Park, and Malcolm Kennedy (formerly Parks
Victoria) reviewed this paper. Joanne Lenehan, PhD candidate (University of New England), provided
unpublished results of her study into wild horse impacts in Guy Fawkes River National Park. Alison Matthews
(Charles Sturt University), Associate Professor J. Gilkerson (Equine Infectious Disease Laboratory, University of
Melbourne) and H. Crabb (Principal Veterinary Officer-Intensive Farming Systems, DEPI Victoria) were
consulted on various sections of this paper.
The Ecology of Wild Horses and their Environmental Impact in the Victorian Alps
Acronym List
ABA:
Australian Brumby Alliance
ABMA:
Alpine Brumby Management Association
AALC:
Australian Alps Liaison Committee
AANPs:
Australian Alps National Parks
ANP:
Alpine National Park
BAW:
Bureau of Animal Welfare
COP:
Code of Practice
DEPI:
Department of Environment and Primary Industries
EPBC:
Environment Protection and Biodiversity Conservation Act 1999
FFG:
Flora and Fauna Guarantee Act 1988
KNP:
Kosciuszko National Park
RSPCA:
Royal Society for the Prevention of Cruelty to Animals
SOP:
Standard Operating Procedure
VBA:
Victorian Brumby Association
May 2013
The Ecology of Wild Horses and their Environmental Impact in the Victorian Alps
May 2013
Table of Contents
Introduction to Wild Horse Background Papers ..................................................................................... 1
1.
Introduction .................................................................................................................................... 2
2.
Wild horse ecology in the Victorian Alps ........................................................................................ 2
3.1
3.2
3.3
3.4
3.5
3.6
3.
Wild horse distribution ........................................................................................................... 2
Wild horse population trends ................................................................................................. 3
Wild horse demography.......................................................................................................... 6
Wild horse social organisation and movement ...................................................................... 6
Wild horse habitat and diet preferences ................................................................................ 7
Wild horse mortality factors ................................................................................................... 8
Wild horse environmental impacts ................................................................................................. 9
3.1
3.2
3.3
3.4
3.5
3.6
Environmental impacts of wild horses.................................................................................... 9
Impacts on soil and substrate ............................................................................................... 11
Impacts on vegetation .......................................................................................................... 14
Impacts on peatlands ............................................................................................................ 18
Impacts on waterways (streams and stream-banks) ............................................................ 20
Impacts on fauna .................................................................................................................. 22
4.
Wild horse biosecurity issues........................................................................................................ 25
5.
Gaps in knowledge ........................................................................................................................ 26
References ............................................................................................................................................ 27
Appendix 1: Officially listed plant ecological communities at risk of severe damage from wild horse
activity ................................................................................................................................................... 36
Appendix 2: FFG-listed and EPBC-listed plant species potentially at risk from wild horse activity in the
eastern Victorian Alps ........................................................................................................................... 37
Appendix 3: Officially listed or threatened fauna species potentially at risk from feral horse activity in
the eastern Victorian Alps..................................................................................................................... 39
The Ecology of Wild Horses and their Environmental Impact in the Victorian Alps
May 2013
Introduction to Wild Horse Background Papers
Horses (Equus caballus) living in unmanaged, wild populations in Australia are generally known by three terms;
feral horses, wild horses and brumbies. Any introduced domestic animal that lives in unmanaged, self
sustaining, wild populations is by definition a feral animal. However, some people are uncomfortable with the
term ‘feral’ being associated with horses and prefer the terms ‘wild horse’ or ‘brumby’. ‘Brumby’ is a colloquial
term often used in Australian folklore; however some people believe the term elicits a romanticised view of
horses and detracts from their environmental impacts. In this series of papers the term ‘wild horse’ will be
used, as it is a generally accepted term and clearly refers to un-domesticated horses living in the wild.
Horses were introduced to Australia by early European settlers and Australia now has the highest population
of wild horses in the world, with more than 300 000 (Dobbie et al. 1993). Wild horses are a pest species in
Australia, that is, an “animal that has, or has the potential to have, an adverse economic, environmental or
social/cultural impact” (Natural Resource Management Ministerial Council 2007).
Wild horses occur across the Australian Alps and have been identified as a high priority threat to natural
values of the region (Coyne 2001). The “degradation and loss of habitat caused by feral horses” is listed as a
potentially threatening process under Victoria’s Flora and Fauna Guarantee Act (1988).
In Victoria, wild horses occur within the Victorian Alps, with a smaller population present in the Barmah Forest
(Wright et al. 2006). This series of three Background Papers will focus on wild horses in the Victorian Alps
(Alpine National Park (ANP) and surrounding State forests).
The Background Papers investigate the ecology, environmental impacts, human dimensions and management
and control of wild horses in the Victorian Alps. They are arranged in the following order:
Background Paper 1: The Ecology of Wild Horses and their Environmental Impact in the Victorian Alps. The first
section of this paper considers the ecological dimensions of wild horses in the Victorian Alps including their
distribution, population trends, demography, habitat, diet and mortality factors. The environmental impacts of
wild horses within the region are then considered including their impacts on: soil and substrate, vegetation,
peatlands, waterways and fauna. Biosecurity issues are also discussed.
Background Paper 2: The Human Dimensions of Wild Horse Management in the Victorian Alps. This paper
provides the social context for wild horse management in the Victorian Alps. A brief history of wild horses in
the region and the major stakeholder groups involved is outlined. The socio-economic and cultural heritage
values of wild horses are then explored followed by a discussion on public perceptions about wild horses and
their management in the Victorian Alps. Research from national and international investigations into
perceptions towards wild horses is drawn upon to help unravel the complexity of this value-laden issue.
Background Paper 3: Wild Horse Management and Control Methods. This paper considers the management of
wild horses in the Victorian Alps and considers control methods for managing wild horses. An overview of how
wild horses have been managed in the Victorian Alps and the legislation and policy framework for wild horse
management is provided. The paper explores welfare issues and costs associated with wild horse control,
levels of control and control options.
This series of papers was prepared based on available literature and research, and, through consultation with
experts where possible. The papers provide a foundation for discussion concerning the future management of
wild horses within the Victorian Alps.
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The Ecology of Wild Horses and their Environmental Impact in the Victorian Alps
May 2013
1. Introduction
In Victoria wild horses (Equus caballus) occur in the Victorian Alps with a smaller population present in the
Barmah Forest, on the Murray River (Menkhorst 1995; Wright et al. 2006). Wild horses are considered a
significant environmental threat to the Victorian Alps (including the Alpine National Park (ANP) and adjacent
State forests). The “degradation and loss of habitat caused by feral horses” is listed as a potentially threatening
process under Victoria’s Flora and Fauna Guarantee Act (1988). This paper will explore the ecological
dimensions of wild horses in the Victorian Alps and the extent to which they are affecting the natural values of
the region.
Several studies and reviews have been conducted on the ecology and environmental impacts of wild horses in
various parts of the Australian Alps, including: Dyring 1990; Thiele & Prober 1999a, 1999b; Walter 2002, 2003;
Walter & Hone 2003; Montague-Drake 2005; Prober & Thiele 2007; Nimmo & Miller 2007; Laake et al. 2008;
Dawson 2005, 2009a, 2009b; and Venn et al. 2009. To help unravel the significance of wild horses in the
Victorian Alps, information in this paper is also drawn from national and international literature as well as
anecdotal information on wild horses in the Victorian Alps.
2. Wild horse ecology in the Victorian Alps
3.1 Wild horse distribution
Wild horses in the Australian Alps are relatively isolated from populations elsewhere in Australia. The largest
populations in Australia occur in dry and tropical environments, mainly in the Northern Territory and
Queensland, but also Western Australia and South Australia (Dobbie et al. 1993). In much of Australia, drought
limits the distribution of wild horses (Dobbie et al. 1993). However, this is not evident in the Australian Alps
which occupy wetter bioregions with more consistent rainfall (Hobbs & McIntyre 2005). The distribution of
wild horses in Australia has also been strongly influenced by human intervention including farming
infrastructure such as fencing (McKnight 1976).
Wild horses were initially introduced into the Australian Alps by European settlers in the 1830s. Graziers
managed the distribution (and numbers) of wild horses to varying degrees from the mid-1800s up until cattle
grazing ceased early this century (Walter 2002; Foster 2004). Non-human influences on wild horse distribution
in the Alps include preferred habitat, geographical barriers and natural events such as severe snow storms, fire
and drought (Walter 2002). Between 1990 (Dyring 1990) and 2002 (Walter 2002) wild horse distribution
appeared to be relatively stable in the ANP, however, in the past ten years wild horse populations have
expanded their range in a number of locations, including spreading to new locations on the Bogong High Plains
(Dawson 2009). Dawson (2009) also suggests that there is suitable habitat for wild horses in Australian Alps
that is currently not occupied. Furthermore, it is predicted that with climate change, areas at higher elevation
will become more suitable for wild horses (Dunlop & Brown 2008; Green & Pickering 2002).
The largest population of wild horses in the Victorian Alps is in the eastern Alps (east of Omeo) and is
connected to a population in Kosciuszko National Park, NSW to the north (Figures 1 and 2). It extends south to
the Nunniong Plains, as far west as Mt Pinnibar and Buenba Creek, and to Deddick and Amboyne east of the
Snowy River. The second, smaller population is on the southern Bogong High Plains, between Falls Creek and
Mount Hotham, and in the headwaters of the Cobungra, Bundara and Victoria Rivers (Ethos NRM 2012). The
Bogong High Plains/Cobungra population is isolated from the Eastern Alps population by around 30km and
considered to have a lower density of horses than the east Alps population (Ethos NRM 2012) (figure 2). Both
populations, while situated predominantly within the ANP, extend into adjacent State forests and reserves
and, probably, private land.
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The Ecology of Wild Horses and their Environmental Impact in the Victorian Alps
May 2013
Records of small groups of horses have occurred at other disjunct locations across the Victorian Alps from time
to time, however these isolated records are considered not represent extant populations.
In spring 2009 a group of 18 wild horses was discovered in the headwaters of the Moroka River, near Mt
Wellington in the Alpine National Park and the adjacent Carey State Forest. This group of horses is suspected
to have been illegally introduced, but was gradually trapped and removed by Parks Victoria and DEPI over the
subsequent summer and autumn. In early winter 2010, another new group was discovered in State forest at
Connors Plain, north-west of Licola. This group was also trapped and removed. Monitoring continues in both
areas to ensure that all the horses have been captured.
Mount Buffalo
National Park
Alpine National Park
Alpine National Park
Alpine National Park
Snowy River
National Park
Figure 1: Estimated wild horse distribution in the Victorian Alps, showing the two disjunct populations. This distribution
map is based on a report by Ethos NRM (2012) that used previous horse records and interviews with a broad range of
stakeholders (with knowledge and experience of wild horses in the Victorian Alps) to estimate the distribution of wild
horses in the Victorian Alps. NB: This map is based on anecdotal qualitative information and provides only a broad guide to
wild horse distribution.
3.2 Wild horse population trends
The most reliable estimates of wild horse population size have come from aerial surveys across the Australian
Alps National Parks (AANPs) conducted in 2001, 2003 and 2009 (Walter & Hone 2003; Dawson 2009). These
surveys were developed with the aim of providing a repeatable and robust method for monitoring wild horse
population size in parts of the AANPs (not including adjacent areas such as State forests and private land)
(Walter & Hone 2003). Surveys are conducted from a helicopter at a fixed-height and speed along east-west
transects at two kilometre intervals and analysed using line-transect techniques (Walter & Hone 2003). This
method was designed to minimise several potential sources of bias such as decreasing detectability of horses
with distance from the aircraft (Walter & Hone 2003), and counting horses more than once (see Linklater &
Cameron 2002).
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The Ecology of Wild Horses and their Environmental Impact in the Victorian Alps
May 2013
Figure 2: Estimated distribution and relative density of wild horses in the Victorian Alps. This map is based on a report by
Ethos NRM (2012) that used previous horse records and interviews with a broad range of stakeholders (with knowledge
and experience of wild horses in the Victorian Alps) to estimate the distribution and density of wild horses in the Victorian
Alps. NB: This map is based on anecdotal qualitative information and provides only a broad guide to wild horse distribution
and density.
Wild horse populations can increase at a maximum rate of 21-37% per annum, depending on local
environmental conditions (Eberhardt 1987; Garrott et al. 1991a; Linklater et al. 2004; Grange et al. 2009;
Scorolli 2010). Populations do not sustain these levels of growth indefinitely. Population growth slows, stops or
becomes negative when horses are removed from the population by people (e.g. Garrott & Taylor 1990); or
when populations become limited by resources such as food or water (Dobbie et al. 1993; Grange et al. 2009;
Scorolli & Lopez 2010). The wild horse populations in the Australian Alps were estimated to be increasing at
21% per annum between 2003 and 2009 after broad-scale, intense bushfires in the summer of 2002/3 reduced
populations by approximately 54 percent (Dawson 2009). In a study of three small wild horse populations in
the Australian Alps (one in Victoria and two in New South Wales) prior to the 2002/3 bushfires, Dawson and
Hone (2012) observed that the populations were either stable or increasing at a rate of up to nine percent per
annum. More stable rates were observed in long established unmanaged populations (including Cowombat
Flat) and a higher rate of increase in an area where human intervention had recently been reduced (Dawson
and Hone 2012).
Trends in the size of the wild horse population in parts of the ANP have been derived using aerial survey
results since 2001 (Figure 3). The population appears to have fluctuated between 2001 and 2009, primarily as
a result of the severe and extensive bushfires in 2002/3 (Dawson 2009). The estimates show a rapid growth
phase between 2003 and 2009 (with an annual growth rate of 21%) consistent with recovery post-fire. This
rapid population increase has occurred despite the removal of more than 700 horses by Parks Victoria during
this period. The size of the population in the surveyed portion of the ANP has been in the order of 1000 to
3500 horses between 1990 and 2009, with the highest estimates in the most recent 2009 survey (3442 horses
± 874 SE). However, these estimates are based on aerial surveys that cover less than half the area known to be
occupied by wild horses (ELA 2012) and there is no indication that the population has stopped growing since
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The Ecology of Wild Horses and their Environmental Impact in the Victorian Alps
May 2013
2009. Thus, the true size of the wild horse population in the Victorian Alps is expected to be significantly
higher.
Population modelling has been undertaken to estimate the wild horse population size for the entire Victorian
Alps. This modelling involves using estimated wild horse density and distribution mapping to extrapolate the
population estimates derived from aerial surveys across the entire area occupied by wild horses. Wild horse
population growth is estimated using a logistic growth model that calculates population growth towards an
estimated carrying capacity (derived from field studies), which varies in response to environmental variability
(rainfall) (ELA 2012). This modelling suggests that the entire Victorian Alps wild horse population is now
approximately 9718 horses (SE: 8151 – 10 896) (ELA 2012) (figure 3). This represents an average wild horse
2
density of 3.4 per km across their current distribution in the east Victorian Alps. The highest documented
-2
densities of wild horses in the Australian Alps is 6.4 km at Cowombat Flat, Victoria, at a time when the
population was stable (Dawson & Hone 2012).
12000
Estimated population size (aerial survey area only)
Estimated population size (extrapolated for whole Victorian Alps)
Horses removed
8000
6000
4000
Bushfires
88
1011
95
121
1853442 193
150
9718
2159
7087
2293
2000
4902
Number of Horses
10000
122
145
0
2001
2002
2003
2004
2005
2006
2007
Year
2008
2009
2010
2011
2012
Figure 3: Estimates of wild horse population size ± SE derived from aerial surveys of the surveyed portion of the ANP
(Dawson 2009) (blue columns) and extrapolated for the whole area occupied by wild horses in the Victorian Alps (ELA
2012) (green columns). The 2012 estimate is derived from modeling wild horse population growth from 2009 towards
estimated carrying capacity (ELA 2012). The number of horses removed from the Victorian Alps through Parks Victoria
management programs since 2005 is also included (red columns).
The Bogong High Plains wild horse population remained relatively static at around 80 - 100 horses between
2005 and 2009 (Dawson and Miller 2008), despite the removal of 102 horses over the same period. However,
aerial surveys indicate that the population has decreased in the equivalent survey area between 2009 and
2012, from around 90 to 50 horses (figure 4). Seventy six horses were removed during this same period.
5
The Ecology of Wild Horses and their Environmental Impact in the Victorian Alps
140
May 2013
Estimated Horse Population
Horses Removed
120
Number of Horses
100
80
55
60
37
40
38
31
20
10
11
6
92
7
95
88
51
0
2005
2006
2007
2008
2009
2010
2011
2012
Year
Figure 4: Estimates of wild horse population size ± SE for the Bogong High Plains population only. The number of horses
removed from these areas by Parks Victoria since 2005 is also included (red columns). Population estimates are derived
from aerial surveys (Dawson and Miller 2008).
3.3 Wild horse demography
Wild horses have an annual breeding season, producing one young at a time (Grange et al. 2009). The first
young are usually produced when females are three years of age. When wild horse density is low and food is
abundant they occasionally reproduce at two years of age (Berger 1986; Duncan 1992). During a three year
study from 1999 to 2002, the youngest mare observed with a foal in the Australian Alps was three years old
(Dawson & Hone 2012). Foaling rates increase up to the age of five and mares continue to have high foaling
rates until the onset of senescence at 15-18 years of age (Garrott & Taylor 1990; Garrott et al. 1991b; Duncan
1992; Linklater et al. 2004; Grange et al. 2009). Foals are usually born in the summer months when food
availability is at its highest (after an 11 month gestation) but can be born at any time of year (Dawson
unpublished data). Foaling rates observed for wild horses in three separate populations in the Australian Alps
between 1999 and 2002 were lower than those reported in other environments with 42-62% of adult females
observed with foals (Dawson & Hone 2012).
Between 1999 and 2002 survival rates of adult wild horses in the Australian Alps were generally high (91% per
annum) with little annual variation, while survival rates in the first three years of life are lower and more
variable (63-75% per annum) (Dawson & Hone 2012). This is similar to wild horse populations from around the
world (Garrott & Taylor 1990; Linklater et al. 2004; Grange et al. 2009; Scorolli & Lopez Cazorla 2010). There is
no data on the lifespan of wild horses in the Australian Alps, however studies of wild horses in Maryland,
United States of America (USA) found wild horses lived as long as 20 years (Kirkpatrick & Turner 2008).
3.4 Wild horse social organisation and movement
Wild horses live in small social units as harem or bachelor groups. Harem groups consist of a dominant male,
multiple females and their off-spring (Menkhorst 1995). Bachelors are non-dominant males that have been
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The Ecology of Wild Horses and their Environmental Impact in the Victorian Alps
May 2013
forced out of their harems; they generally occur alone or in groups of two or three (Dobbie et al. 1993). The
average group size (harem and bachelor groups) from three sites across the Australian Alps was 5.65 (± 0.51
SE) (Walter & Hone 2003), while Drying (1990) found that harem group sizes in the Alpine region ranged from
two to 11 individuals, with groups typically consisting of one stallion, two mares and one foal. Wild horses
rarely have periods of social isolation during their lifetime except for bachelors in their pre-harem formation
stage (van Dierendonck 2006). Harem groups tend to be stable breeding units and generally favour permanent
locations around reliable water sources (Dobbie et al. 1993). Within harem groups, adult mares form a longterm stable nucleus, while the breeding stallion is regularly replaced (van Dierendonck 2006). Bachelor groups
are more mobile and unstable (Dobbie et al. 1993).
Groups of wild horses are loyal to undefended home ranges with central core use areas (Linklater 2000). The
home ranges of groups overlap entirely with other groups and home range size increases with group size
(Linklater 2000). Home ranges of wild horses in the Australian Alps have not been studied but there is
information available from other environments. Home range sizes vary within a region and between regions. In
Queensland and central Australia wild horses were estimated to have a home range of approximately 100km²
and 70km² respectively (Dobbie et al. 1993). In contrast, wild horses at Kiamanawa in New Zealand, which has
2
a similar climate to the Australian Alps, had home ranges 0.96 to 17.7 km (Linklater 2000). Given the climatic
similarities, home range of horses in the Victorian Alps are likely to be similar to Kaimanawa.
As prey animals in their native environment, a horse’s primary defence mechanism is rapid flight away from
the threat of danger. It is therefore advantageous that they identify potential predators as quickly as possible
(van Dierendonck 2006). This is aided by their monocular and binocular vision, which enables them to have an
extensive view of their surrounds (Dobbie et al. 1993). With well developed hearing any movement is readily
detected (Dobbie et al. 1993).
3.5 Wild horse habitat and diet preferences
Wild horses occupy a range of habitats across Australia and the world. While they are best adapted to open
grassy plains they will also use rugged country (Norris & Low 2005). Wild horses are present from the highest
to the lowest elevations in the Australian Alps (Walter 2002). Some groups may migrate to lower elevations in
winter but many horses maintain a high elevation (1600 metres) home range throughout winter (Dawson,
unpublished data). Drying (1990) found that wild horses in Kosciuszko National Park (at a site in NSW six
kilometres from the Victorian border) made extensive use of heaths and grasslands for feeding, whilst
avoiding the forests at all times of the year; this preference for open areas was broad-based with no
discrimination between feeding and other activities. The preference for grasslands over forest is universal for
wild horses (Pratt et al. 1986; Keiper & Berger 1982; Berger 1986; Linklater et al. 2000). The only exception
has been observed in the middle of the day in summer when wild horses seek refuge from the heat and
horseflies (Tabanidae) in forested areas (Duncan 1983; Dyring 1990; Keiper & Berger 1982; Berger 1986).
Wild horses spend most (55-65%) of their time feeding (Duncan 1980). They are generalist grazers with a
strong preference for the greatest concentrations of high quality food (green plant matter); when green plant
matter becomes sparse, the horses’ tactic is to search out areas with the greatest concentrations of perennial
herbaceous plants, green or dead (Duncan 1983). Their diet mainly includes herbaceous plants (grasses, reeds,
sedges and forbs), but they will also eat roots, bark, buds and fruit (Csurhes et al. 2009). Due to their forwardcut front incisors they are able to graze close to the ground (Dobbie et al. 1993). Horses have a different
digestive system to most ungulates (hoofed mammals), which enables them to consume large quantities of
low quality food and survive on a lower quality diet than cattle (Janis 2007). Cattle require time to chew their
cud so cannot consume such large quantities but tend to select higher quality food (Janis 2007).
Horses must drink at least once a day in summer and at least every second day during winter (Norris & Low
2005). If food is plentiful horses will graze near water sources (Dobbie et al. 1993).
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3.6 Wild horse mortality factors
With few predators, unmanaged wild horse populations may increase in size and distribution until they
approach carrying-capacity; at this point the population size stabilises as a result of a decrease in birth rates
and survival rates caused by limited food availability (Grange et al. 2009; Scorolli & Lopez Cazorla 2010). For
example, in the Camargue in France, Grange et al. (2009) found that a decrease in available food resources, as
a result of increasing density, caused a loss of body condition and the survival of foals and adult females
decreased with increasing density.
In sub-alpine and montane environments of the Australian Alps, there is evidence that between 1999 and
2002, the growth of three wild horse populations was limited by food availability (Dawson & Hone 2012). The
density of horses at the three sites was higher than the average determined from aerial surveys (see above).
The Cowombat wild horse population had the highest density (6.4 km²), adult horses had the poorest
condition, recruitment (from birth to 3-years-old) was low, pasture biomass was low and population growth
was zero. In general however, unlike the Camargue example above, wild horses in the Australian Alps are not
contained and do not currently occupy their entire potential range (Dawson 2009). Therefore there is scope
for the wild horse population to spread.
It is not clear whether wild horse populations across the Australian Alps would reach ‘carrying capacity’
because the ability to reach equilibrium density depends on the variability of environmental conditions. In
stochastic environments characterised by a high degree of unpredictable environmental variance (e.g. rainfall),
an equilibrium is not reached (McLeod 1997). There are several examples of environmental events, in
particular drought, snow and fire, which have lead to dramatic declines in wild horse populations (thus
preventing the population from reaching carrying capacity). The eastern side of the AANPs (e.g. Buchan River,
Lower Snowy, Suggan Buggan) lies in a rain-shadow and has limited available water. The drought in 1982-83
was reported to have led to a dramatic decline of most wild horses in this area (Walter 2002), similar patterns
are observed in central Australia (Dobbie et al. 1993). Drought can affect horses through thirst, starvation and
ingestion of poisonous plants (Dobbie et al. 1993).
At higher elevations wild horse mortality occurs as a result of severe snow events or long periods of snow
cover in the alpine area (Walter 2002). In winter wild horses at higher elevations, such as those on the Bogong
High Plains, have to dig through snow to access food for many weeks of the year which leads to a loss of body
condition in the horses (Dawson unpublished data). In some cases severe snow events have resulted in
mortality and in one historic event in the Brindabella’s (Australian Capital Territory (ACT)) an entire population
was wiped out (Walter 2002).
The 2003 fires had a substantial impact on the wild horse population of the Australian Alps, with a sharp
decrease in the wild horse population size following the fires (Figure 2). After the fires, Walter (2003) indicated
that when wild horse numbers were low, that there was great potential for the population to increase
dramatically due to the increased availability of high quality food and reduced population pressure. This was
demonstrated by the results of the 2009 aerial survey which showed that the population had increased by
224% since 2003 (Dawson 2009a). In a slightly different context, Catling (1991) predicted that wild horses
would be advantaged by frequent low intensity fires due to a simplification in forest structure.
Additional wild horse mortality factors that should be considered include wild dogs and parasitism. There have
been reports of wild dogs chasing foals in the Victorian Alps (Walter 2002). Parasitism and disease may also be
causes of mortality and/or reduced health. However, neither of these factors has been formally investigated or
quantified.
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The Ecology of Wild Horses and their Environmental Impact in the Victorian Alps
May 2013
3. Wild horse environmental impacts
3.1 Environmental impacts of wild horses
Australia's ecosystems have evolved without the grazing pressure and the physical impact of heavy, hardhoofed animals (Carr & Turner 1959a; 1959b Ashton & Williams 1989; Green et al. 2006). Wild horse activity in
the Victorian Alps represents a type and intensity of impact to which native ecosystems and their components
are not adapted. The environmental impacts of wild horses are recognised by Victoria’s environment
legislation with “degradation and loss of habitat caused by feral horses” listed as a threatening process under
the Flora and Fauna Guarantee Act (1988).
The social and behavioural habits of wild horses as well as their physical characteristics (i.e. hard hooves, large
size, dietary preferences and general requirements), impact on the environment directly and indirectly.
Trampling and grazing are the most researched and known agents of change associated with wild horses (Loydi
& Zalba 2009). However, other negative actions include: consumption of native plants, bark chewing,
compaction of soils, pugging (trampling of wet soils leaving a dense mat of deep footprints), track formation,
wallowing (rolling), and the redistribution of nutrients and plant seeds via dung and urine. The impacts of
these actions are summarised in Table 1.
Table 1: Summary of wild horse environmental impacts
Element
Soil & substrate
Peatlands (also
known as bogs or
mossbeds)
Waterways
(streams &
streambanks)
Vegetation (&
communities)
Impacts
•
Exposed soil surface;
•
soil loss & erosion;
•
down-slope sedimentation;
•
soil pugging, drying & compaction;
•
loss of soil structural composition
especially on wet soils; &
•
creation of nutrient hotspots
(especially nitrogen).
•
Drying out of bogs & potential
draining of entire bog systems;
•
creation of bare pavements;
•
incision & soil erosion;
•
silt deposition downstream;
•
dominance of unpalatable species; &
•
loss of habitat for threatened
species.
•
Degradation of stream function;
•
incision & channelling;
•
soil compaction leading to decreased
infiltration;
•
increased downslope sedimentation;
•
increased nutrient loads;
•
lateral erosion;
•
streambank disturbance & slumping;
•
fouling of waterholes; &
•
changes in water flow & drainage
patterns.
•
Removal of native vegetation cover;
•
dispersal of weed seeds;
•
changes to the vegetation structure
& species composition of the ground
stratum;
•
native tree mortality;
•
increase vulnerability of threatened
vegetation; &
•
increased nutrient loads.
Australian Alps research
Dyring 1990
Whinam et al. 1994
General research
Berman & Jarman 1998
Beever & Herrick 2006
De Stoppelaire et al. 2004
Rogers 1991, 1994
Turner 1987
Loydi & Zalba 2009
Dyring 1990
Whinam & Chilcott 2002
Whinam et al. 2003
Tolsma 2008a, 2008b
Rogers 1991, 1994
Prober & Thiele 2007
Dyring 1990
Whinam & Comfort 1996
Whinam & Chilcott 2002
Prober & Theile 2007
Wild & Poll 2012
Beever & Brussard 2000
Rogers 1991, 1994
Dyring 1990
Whinam & Chilcott 2002
Whinam & Comfort 1996
Whinam et al. 1994
Prober & Theile 2007
Walter 2002
McKay 2001
Leigh et al. 1991
Thomas 2010
Wild & Poll 2012
Beever & Brussard 2000
Beever et al. 2003
Beever et al. 2008
Rogers 1991, 1994
Turner 1987
Loydi & Zalba 2009
De Stoppelaire et al. 2004
Bridle & Kirkpatrick 1999
Cambell & Gibson 2001
Schott 2002
9
The Ecology of Wild Horses and their Environmental Impact in the Victorian Alps
Native fauna
•
•
•
•
•
Competition for resources;
altered food availability;
habitat modification & loss;
increased vulnerability of threatened
species; &
competitive exclusion.
Clemann et al. 2001
Clemann 2002
Brown et al. 2007
May 2013
Berman & Jarman 1998
Lenehan 2010
Zalba & Cossani 2009
Beever & Herrick 2006
Beever et al. 2008
Nano et al 2003
The degree of wild horse related degradation in the Victorian Alps will depend on:
•
•
•
•
•
•
•
•
wild horse density;
scale of activity (extent);
topography – including slope;
elevation;
climate;
recent weather;
local drainage;
access to suitable areas;
•
•
•
•
•
•
timing (i.e. seasonality of grazing);
the resilience of the vegetation community;
soil type (i.e. fineness and/or wetness);
the frequency and intensity of use;
effects of other sympatric species; and
the longer-term disturbance history of the site (Whinam et
al. 1994; Beever et al. 2003; Beever et al. 2008).
These factors vary throughout the Victorian Alps but some generalisations can be made.
Concerns about the environmental impacts of wild horses in the Australian Alps were first raised in the 1950s
(Costin 1954). While there is extensive evidence of wild horse impacts in the Victorian Alps, relatively few
studies have been undertaken to quantify these impacts. Research includes:
•
•
•
Dyring (1990) conducted research on the effects of wild horses on sub-alpine and montane
environments in Australia. Wild horse impacts on soils, vegetation and streams were quantified in
four small catchments in the southern Snowy Mountains. Seasonal habitat usage and the abundance
of wild horses were also investigated. Wild horses were found to either initiate or perpetuate changes
in sub-alpine and montane environments. Rates of environmental change could not however be
investigated in the short time-frame of the study.
In 1999 an experimental program was established in the East Alps Unit of the ANP to determine the
effects of wild horse activity on grasslands and stream margins (Prober & Thiele 2007; Wild and Poll
2012). Exclosure plots, that prevent horses from accessing a defined area but allow access for other
animals, were established at Cowombat Flat and Native Cat Flat. Detailed vegetation monitoring of
these exclosure plots, as well as monitoring of stream bank condition, disturbance and erosion was
undertaken in 1999, 2005 and 2012 (Prober & Thiele 2007; Wild and Poll 2012). The results show that
changes to stream structure and function as a result of wild horses are clear and substantial, with
significantly more incision and damage in the wild horse occupied area (outside the wild horse
exclosures). Exclosure from horses has led to clear increases in vegetation height and increased litter
cover (Wild and Poll 2012). The effect of wild horses on vegetation structure and composition was
less consistent; however there was a trend for the recovery of dense swards of sedges and grasses
associated with the competitive exclusion of some lower stature species inside horse exclosures (Wild
and Poll 2012). In contrast, horse occupied areas tended to be characterised by low herbfield turfs,
likely to be maintained by the preferential grazing of grasses and sedges by horses (ibid).
In 2008 Arn Tolsma from the Arthur Rylah Institute assessed the status and needs of 105 individual
mossbeds (also commonly termed peatlands or bogs) in the Victorian Alps (Tolsma 2008a; 2008b).
This work supplemented broad-scale post-fire assessment of mossbeds that have been conducted by
the Arthur Rylah Institute since 2004. The aims included: to assess the current state of sub-alpine
mossbed communities, estimate potential threats to mossbeds and determine restoration and other
management needs. Tolsma found that most systems show signs of contraction over a decadal scale,
and few systems could be considered in relatively good condition. Evidence of wild horse activity
10
The Ecology of Wild Horses and their Environmental Impact in the Victorian Alps
•
May 2013
(tracks, compaction, trampling, pugging and stream bank slumping) was observed in 67% (70 of 104)
of the mossbeds assessed that year. Tolsma argued that on-going activity by wild horses is of great
concern in the east Alps of Victoria. In the East Alps Unit of the ANP, 97% (63 of 65) of peatland
systems were found to be impacted (i.e. compacted, trampled or pugged) by wild horses (Tolsma
2008b).
In 2011 the AALC commenced a project that aims to quantify the impacts of wild horses on upland
streams and wetlands in the Australian Alps. The results of this study will be available in autumn 2013
and will provide further evidence of the impacts of wild horses on sensitive environments.
A number of protected areas regionally, nationally and internationally recognise that wild horses have a
negative impact on the environment and have developed strategies to address these impacts (Table 2).
Table 2: Examples of wild horse management plans and strategies and their ecological rationale
Region
Wild horse strategy
Rationale for the plan (ecological)
AANPs
Kosciuszko National Park Horse
Management Plan.
The National Parks & Wildlife Service has legislative responsibility to
protect native habitats & wildlife within its reserves & a responsibility
to minimise the impact of introduced species, including wild horses.
The 2006 Plan of Management called for a wild horse management
plan & the exclusion of wild horses from key areas. (NSW NPWS 2008).
The plan aims to minimise the negative impact of wild horses including
grazing on sensitive vegetation, trampling of streambanks, trail
formation & erosion. These can lead to draining of entire bog systems,
loss of habitat for threatened species & silt deposition downstream.
(ACT Government 2007).
The National Parks & Wildlife Service has legislative responsibility to
protect native habitats & wildlife within its reserves & a responsibility
to minimise the impact of introduced species, including wild horses
(NSW NPWS 2006a). Wild horses are an introduced species that have
adverse impacts on Australian ecosystems with particularly severe
consequences for native fauna & flora (English 2001).
Wild horses have been identified as posing a threat to the conservation
values of the park & water quality. (NSW NPWS 2006b).
Queensland Parks & Wildlife Service has a legal obligation to conserve
& protect the natural values of Canarvon NP & control threatening
processes caused by pest species including wild horses. Destructive
impacts by wild horses have led to: the deterioration of aquatic
ecosystems & serious landscape dysfunction (losses in biomass,
accelerated erosion, soil compaction, altered species composition &
vegetation structure & altered fire ecology). (Weaver 2007).
Horses have been shown to adversely affect nationally significant
ecological values. There is a need to eliminate the impacts of horses on
important conservation values. (DOC 2006).
Namadgi National Park Feral horse
Management Plan.
National
Guy Fawkes River National Park:
Horse Management Plan.
Feral horse Management Plan for
Oxley Wild Rivers National Park
Protecting the natural & cultural
values of Carnavon National Park:
A plan to manage wild horses &
other pest animals
New
Zealand
Kaimanawa Wild Horses Plan.
In addition to these wild horse plans and strategies, a series of workshops on wild horse impact and
management in the Australian Alps (see: Walters & Hallam 1993; O’Brien & Solomon 2004) and a national
workshop (see: Dawson et al. 2006) have demonstrated widespread concern from scientists and practitioners
about the impacts of wild horses in alpine and sub-alpine environments.
The impacts that wild horses have on the soils and substrate, peatlands , waterways, vegetation and fauna of
the Victorian Alps is considered below in greater detail.
3.2 Impacts on soil and substrate
It is generally accepted that alpine areas are more susceptible to damage by hard hooved animals such as wild
horses than most other environments, due to their wet fragile soils and slow vegetation growth rates (Whinam
et al. 1994). Wild horse trampling and grazing can lead to major changes to the soil, including: pugging, drying,
11
The Ecology of Wild Horses and their Environmental Impact in the Victorian Alps
May 2013
compaction and erosion (Berman & Jarman 1988; Dyring 1990; Beever & Herrick 2006; De Stoppelaire et al.
2004).
Some of the immediate effects of wild horses include the creation of tracks and bare patches due to trampling,
wallowing and horse camps (see Photo 1, 2 and 3). Trampling and wallowing have been found to cause
localised damage by reducing organic matter and exposing and compacting the soil surface (Dyring 1990).
Track networks are formed by the movement of wild horses. Dyring (1990) found that wild horses produce
extensive track networks in the Australian Alps. Continual trampling by wild horses can increase soil
compaction and therefore reduce aeration and pore space of soils and subsequently decrease water
infiltration and moisture content of soils. In addition, trampling and wallowing reduce plant cover and diversity
(Dyring 1990). The loss of vegetation cover means a reduction in shading for soils and less organic matter
inputs, resulting in greater erosion and a reduced ability of the soil to retain moisture (Beever & Herrick 2006).
Beever and Herrick (2006) found in western Great Basin sites (USA), three to 15 times lower penetration
resistance (a measure of soil compaction) in the soil surfaces of sites without wild horses (compared to those
with wild horses).
Photo 1: Trampled area at Cowombat Flat (source: Arn Tolsma 2008).
Photo 2: Wild horse camp, Davies Plain (Source: Arn Tolsma 2008)
12
The Ecology of Wild Horses and their Environmental Impact in the Victorian Alps
May 2013
Photo 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 (averaging
between 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. Compaction
was found to be most severe on dry soils, where 20-50 passes by wild horses resulted in significantly
compacted soils. Compaction was found not to increase substantially with subsequent passes. Therefore an
average group of four wild horses using a new track twice daily for less than a week will result in significant
compaction. Wild horses were found to have similar usage of sub-alpine and montane areas, with no
difference 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 has
substantial immediate as well as delayed effects on the soils of shrubland, herbfield and bolster heath
communities, however effects on dry grassland soils were less evident. Wild horse exclosure experiments in
the eastern Victorian Alps have shown that continued grazing and trampling by wild horses has maintained low
herbfield turfs, where the soil surface is more susceptible to trampling impacts than the grasslands found in
the 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 damage
than dry soils. This is supported by Rogers (1994) who demonstrated that dry areas are more resistant to
fracturing than wet areas, which were more easily broken up by trampling. The loss of soil structural
composition 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 downslope
sedimentation, water ponding, and opportunities for the establishment of weeds (Rogers 1994). The gradient
of the slope has been found in other studies to directly correspond to the level of erosion. The steeper the
slope 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 increase
the exposure of soils to the elements (such as wind, rain and needle ice), leads to the removal of vegetation
and 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). Wild
horses caused considerable erosion in a sandy environment in the USA: over a five to seven year period fenced
plots (excluding horses) were on average 0.63 m higher than unfenced plots in sand dunes habitats (De
Stoppelaire et al. 2004).
Pugging of soil (trampling of wet soils leaving a dense mat of deep footprints), in particular around wetlands
and waterways, can change soil nutrient status, and increase water turbidity and sediment loads in adjacent
waterways (O’Connor 2005). Dyring (1990) found that wild horses in the Australian Alps can create nutrient
13
The Ecology of Wild Horses and their Environmental Impact in the Victorian Alps
May 2013
hotspots due to the high levels of nutrients (especially nitrogen) in their dung and urine. Manure in dung piles
increases the availability of soil nutrients, especially nitrogen and phosphorous, thus creating microhabitats
suitable for weed invasion (Loydi & Zalba 2009).
3.3 Impacts on vegetation
The 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 vegetation
is a reduction in vegetation cover and height, however they may also alter plant species composition, richness
and diversity, and contribute to weed invasion (Turner 1987; Dyring 1990; Rogers 1991; Beever & Brussard
2000; De Stoppelaire et al. 2004; Loydi & Zalba 2008; Wild and Poll 2012). Trampling by wild horses also alters
vegetation, 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 wallowing
Vertebrate 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 introduced
herbivores 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) in
their 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 of
natives herbivores and rabbits alone (Bridle & Kirkpatrick 1999).
McKay (2001) conducted a survey of the impacts of horse-riding and walking on the alpine vegetation of
Mount Bogong. Trampling was found to reduce the height, cover and abundance of both shrub and ground
flora within track areas and to result in greater exposure of bare ground. Beever et al. (2008) argue that the
loss 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 that
horse trampling and grazing has resulted in the removal of vegetation and increased bare ground in horse
occupied areas (Wild & Poll 2012). In contrast, fenced horse exclosure plots showed a trend for increasing
vegetation cover and decreasing bare ground as they recovered from past horse disturbance over a thirteen
year period (ibid).
It is argued that grazing may have an impact on the reproductive success of some flora species by impacting on
the dispersal opportunities of wind-dispersed species as well as their ability to attract pollinators; both of
these functions are affected by flower height (Bridle & Kirkpatrick 1999). For example, on the Assateague
Barrier Island (USA), a small herbaceous annual, Amaranthus pumilus was once abundant and is now limited to
a 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. In
Australia, clipping experiments on alpine herbs have shown that flowering success may be retarded if plants
are 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 structure
Grazing by introduced herbivores can alter the appearance, productivity and composition of vegetation
communities (Dyring 1990; Hobbs & Hyeneke 1992). This may be due to reduced regeneration/recruitment as
a direct result of selective grazing or the physical impacts of trampling and erosion. Wild horses selectively
graze palatable species and have the potential to change the composition of threatened vegetation
14
The Ecology of Wild Horses and their Environmental Impact in the Victorian Alps
May 2013
communities (Dyring 1990). Redistribution of nutrients through dung can also contribute to the changes in
vegetation 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 the
persistence of less vigorous species (Hobbs & Hyeneke 1992).
Dyring (1990) found trampled sites (and areas adjacent to track systems) had lower native plant diversity and a
higher abundance of exotic species. Plants found on wild horse tracks were characteristically non-woody
prostrate fast-growing annuals and grasses, with hemicryptophitic life-forms (species with renewal buds near
the surface), which tolerated trampling better than upright plants (Dyring 1990). Similarly, horse occupied sites
at Cowombat Flat and Native Cat Flat exhibited a predominance of hemicryptophitic species in contrast to
horse exclosure plots, which were dominated by grasses and sedges (Wild & Poll 2012). Grasslands are more
resilient to the impact of wild horses than other communities where ferns, mosses and shrubs are important
components of the vegetation (Whinam et al. 1994; Venn et al. 2009). Whinam and Chilott (1999) found that
shrubs and shrubland communities were more vulnerable to trampling than other life-forms or vegetation
types in central Tasmanian alpine vegetation.
As well as affecting vegetation community composition, wild horse activity can affect the physical structure of
vegetation communities. Exclosure experiments have shown that vegetation cover and height is far greater in
horse 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, the
following 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 exclusion
of 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 of
horses on vegetation height, plots inside horse exclosures were often characterised by dense swards of sedges
and grasses associated with the competitive exclusion of some lower stature species (Wild & Poll 2012). In
contrast, horse occupied areas tended to be characterised by low herbfield turfs, likely to be maintained by
the preferential grazing of grasses and sedges by horses (ibid). Wild and Poll (2012) suggest that the
resurgence of dense sedges and grasses and competitive exclusion of some lower stature species may indicate
restoration 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 species
richness as well as alter the species composition and structure of the vegetation (by increasing the
predominance 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 series
of monitoring and exclosure plots in the Kaimanawa Mountains, New Zealand, Rogers (1991; 1994), found that
wild horses severely disrupted the composition of the native vegetation. In the grazed plots, species biomass
and 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 2013
Photo 4: Cowombat Flat wild horse exclusion plots from the air (source: Geoff Robinson).
3.3.3 Threatened vegetation
In the Victorian Alps, wild horses are considered to be one of the major threats to alpine ecosystems (Tolsma
2008b). Wild horses are considered to be a serious threat to at least five plant communities listed in the
Victorian 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 FFG
Act. 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 listed
alpine 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 by
trampling, or their regeneration disrupted (McDougall 1982; McDougall & Walsh 2007). Similarly, the Alpine
Snowpatch Community, situated on steep sheltered slopes, is subject to constant irrigation during the thaw
which 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 in
the 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 Biodiversity
Conservation Act 1999 (EPBC) and/or the Victorian FFG Act 1988, and that are potentially at risk from wild
horses, are presented in Appendix 2.
In order to mitigate the threat that wild horses pose to threatened flora species in the Victorian Alps, Parks
Victoria has established three wild horse exclusion fences around particularly threatened and sensitive
16
The Ecology of Wild Horses and their Environmental Impact in the Victorian Alps
May 2013
communities. In 2002 two wild horse exclusion fences were established on Davies Plain around sub-alpine
bogs known to contain several threatened plant species and the threatened Alpine Water Skink. In 2010 an
exclusion 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 their
habitat 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 encouragement
Wild horses can facilitate weed invasion through dispersal and the creation of a favourable environment for
weeds 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). Therefore
wild horses have the potential to disperse weeds both long and short distances and can subsequently
contribute to the establishment of weed species across several spatial scales (Nimmo & Miller 2007). Weaver
and Adams (1996) argue that within their home range, horses are a potentially significant vector in the
dispersal of a range of weed species.
While wild horses are less likely than domestic horses (i.e. recreational riding horses or horses illegally released
into the region) to introduce new weeds into the Victorian Alps, there is potential for this to occur, especially
considering their increasing range and the potential for wild horses to occur across tenures (i.e. farms, state
forests and national park). Many species of seed are transported in the dung of wild horses. Dung can be a
source of viable seed taxa not otherwise found in a community (Campbell & Gibson 2001). Campbell and
Gibson (2001) found that horses pass large numbers of seeds through their digestive tract generally within
48hours of consumption (but sometimes longer), and many of these seeds remain viable (Weaver & Adams
1996). 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 native
vegetation along horse-riding tracks in three national parks in Victoria (including the ANP). Twenty-nine
species of weeds were found to be dispersed via horse manure.
Wild horse disturbance (i.e. dung, soil disturbance and pugging) can provide favourable environmental
conditions for the germination and colonisation of weed species (Dyring 1990; Rogers 1991; Loydi & Zalba
2009). Wild horse dung can result in significant changes in vegetation and can introduce and encourage some
invasive weed species that could eventually colonise more pristine areas (Loydi & Zalba 2009). Nutrients in
dung can favour weed establishment, with weeds establishing more vigorously in areas both trampled and
subject to deposition of dung. In a study of the potential for horse dung to act as an invasion window in
montane pampas grasslands, Loydi and Zalba (2009) found that the cover of introduced species was higher in
dung piles than in control plots. Dyring (1990) speculated that the redistribution of nutrients through uneven
17
The Ecology of Wild Horses and their Environmental Impact in the Victorian Alps
May 2013
dung deposits and selective grazing would lead to exotic plants such as White Clover (Trifolium repens) and
Cats Ears (Hypochoeris radicata), which readily colonise dung and disturbed areas, becoming established in
disturbed grassland and healthlands.
Wild horse disturbance was found to enhance the spread of smothering weeds in New Zealand’s Kaimanawa
Mountains (Rogers 1991; 1994). In the Australian Alps, exotic species were found to colonise the disturbed
area 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 Yorkshire
fog grass (Holcus lanatus, an invasive grass species native to Europe) and red fescue grass (Festuca rubra, an
exotic 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 2010
unpublished report) Thomas (2010) made a series of assessments regarding wild horses and their relationship
with 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 Track
appeared to be associated with wild horses;
there was evidence of wild horses grazing on Brown top bent grass (Agrostis capillaris), which may be
aiding the spread of this weed; and
wild horses appear to avoid Yorkshire fog grass (Holcus Ianatus), providing this species with a
competitive advantage.
3.3.5 Native tree mortality
Wild horses have been found to affect native tree mortality in Australia. The chewing and stripping of bark by
horses can reduce regeneration of seedlings and may result in the death of mature trees. A significant negative
relationship was found between bark chewing damage and overall tree health in Guy Fawkes River National
Park (Schott 2002). Impact assessments identified bark-chewing damage was most severe along drainage-lines
and in close proximity with other water sources that are heavily used by wild horses. Schott (2002) found that
wild 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 the
south-east corner of the mainland (Tolsma 2008b). “Alpine Sphagnum bogs and associated fens” are listed as
an endangered ecological community under federal legislation (Environmental Protection and Biodiversity
Conservation Act 1999). Alpine bogs are also protected by Victorian legislation, where they are listed as a
threatened community under the Flora and Fauna Guarantee Act (1988). These ecosystems are of national
importance 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 regulation
of 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 horses
are known to have a detrimental impact on Sphagnum growth and integrity, with the resulting vegetation loss
ultimately leading to degradation of this ecological community (Dyring 1990; TSSC 2009). Wild horses are
therefore a major threat to Alpine Sphagnum peatland communities (Whinam & Chilcott 2002; Tolsma 2008b;
TSSC 2009; DEWHA 2009a).
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The Ecology of Wild Horses and their Environmental Impact in the Victorian Alps
May 2013
Bogs and peatlands are highly sensitive to damage by trampling and grazing. Preferential grazing of palatable
herbs and grasses in Sphagnum bogs, combined with browsing of new growth of shrubs can lead to increased
dominance by unpalatable species (Whinam & Chilcott 2002). Sphagnum is easily crushed and broken up by
wild horses trampling the delicate vegetation, selectively grazing and wallowing in pools and waterways (TSSC
2009). Trampling may lead to the drying out of bogs, the loss of Sphagnum cover as well as changes to other
vegetation, 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 trampling
and grazing, irrespective of local variations in horse density and habitat abundance (Rogers 1991). The
vulnerability of bogs in New Zealand was also found to increase with wetness (Rogers 1991). Whinam and
Chilcott (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 the
status of peatland systems in the Victorian Alps was investigated by Tolsma (2008a; 2008b). In the East Alps
Unit of the ANP, 97% (n=63) of peatland systems were found to be impacted (i.e. compacted, trampled or
pugged) by wild horses (Tolsma 2008b). Only two peatlands assessed by Tolsma (2008b) in this unit of the ANP
did not show any signs of current wild horse activity: a fenced peatland at Davies Plain and an isolated system
on the south-east edge of the Unit. Whinam and Chilcott (2002) argue that wild horses are one of the greatest
threats to the survival of peatlands.
Channelling is a form of erosion and water diversion which can result from wild horse activity. Trampling and
wallowing cause channels to form in the disturbed Sphagnum. Channelling leads to changes in water flow, as
water exits the disturbed bog more rapidly. This action may completely alter the drainage pattern and result in
drier conditions (Whinam & Chilcott 2002; TSSC 2009). The formation of channels is detrimental to the fens
associated 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 (primarily
in the 2003 fires) Tolsma (2008b) found that wild horse activity in burnt sub-alpine to alpine peatlands in the
Victorian Alps varied from minor trampling at peatland margins to extensive pugging across almost the entire
burnt peatland surface. On average, 25% of all peatland surface area assessed in the East Alps Unit of the ANP
1
and 16% of peatland area assessed in State forest to the south was impacted by trampling (Tolsma 2008a)
(see Photo 6). Trampling by wild horses restricts Sphagnum recovery in naturally-regenerating peatlands, and
may impact on some peatlands where restoration work is occurring (Tolsma 2008b). Tolsma (2008b) argues
that in the Victorian Alps, many areas of burnt peatland, particularly in the East Alps Unit, are unlikely to
completely 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 Alpine
Sphagnum 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 wild
horses.
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The Ecology of Wild Horses and their Environmental Impact in the Victorian Alps
May 2013
Photo 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 and
productivity (Dawson 2009b), but are typically fragile and susceptible to damage by wild horses (and other
large 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; NPWS
2007; Prober & Thiele 2007; Dawson 2010). Streambanks are easily churned up and broken down by wild
horse hooves sinking into wet soils (Dyring 1990). Grazing by large hard-hooved herbivores can affect
streamside vegetation, stream channel morphology, shape and quality of the water column and the structure
of 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 bank
slumping and breakdown, vegetation disturbance and increased soil compaction (see Photos 7, 8 and 9). This
can 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 stream
function and structure as a result of wild horse exclosure fencing. The unfenced stream areas had more bare
ground, were more entrenched and had distinct, open water channels, whereas areas within the exclosures
had indistinct channels, a more or less complete vegetation cover across the channel and no or little visible
open water (Prober & Thiele 2007, Wild & Poll 2012). There was a significant increase in stream depth over
time in unfenced plots at Native Cat Flat while stream depth within the exclosures remained stable, suggesting
accelerated erosion in horse occupied areas (Wild & Poll 2012). Stream condition was better and consistently
improved over time in the horse exclosure plots which exhibited less pugging, stream slumping (collapse of the
stream bank) and bare ground than unfenced areas (Wild & Poll 2012). The extra vegetation and lack of bare
ground 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 to
faster flows and accelerated erosion of the stream channel, particularly at native Cat Flat where steep banks
with 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 on
stream and catchment hydrology and water quality.
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The Ecology of Wild Horses and their Environmental Impact in the Victorian Alps
May 2013
In 2011 the AALC commenced a project that aims to quantify the impacts of wild horses on upland streams
and wetlands in the Australian Alps. The results of this study will be available in autumn 2013 and will provide
further evidence of the impacts of wild horses on sensitive stream and wetland environments.
Photo 7: Wild horse pugging of waterway resulting in bank slumping, Murray Headwaters (source: Parks Victoria 2006).
Photo 8: Wild horse damage to Murray River water source (source: Parks Victoria 2006)
Photo 9: Wild horse trampling at the head of Moscow Creek, Cobberas Range, ANP (source: Parks Victoria 2006).
21
The Ecology of Wild Horses and their Environmental Impact in the Victorian Alps
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3.6 Impacts on fauna
Wild horse populations can alter the composition of bird, fish, crustacean, small mammal, reptile and ant
communities (Nimmo & Miller 2007) and have been identified as a major threat to a range of native wildlife
species in the Victorian Alps. The 2001 AALC Natural Heritage Working Group Workshop of field staff and
scientists, ranked wild horses as one of the top five priority fauna threats to the Australian Alps (Coyne 2001).
Wild horses can affect native fauna through causing habitat modification or loss, and also through competition
for resources (i.e. food, water, shelter and space) (Olsen & Low 2006; Nimmo & Miller 2007).
3.6.1 Competition and altered food availability
Wild horses compete for resources with native herbivores, especially macropods, including: grey kangaroos
(Macropus giganteus), red-necked wallabies (M.rufogriseus) and swamp wallabies (Wallabia bicolour). Various
studies have demonstrated that high densities of wild horses correspond with reduced densities of macropods.
Berman and Jarman (1988) found few signs of macropods in the Northern Territory in areas heavily grazed by
wild horses. A study of wild horse impacts in Guy Fawkes River National Park by Lenehan (2010) provides
evidence of wild horses displacing kangaroos from prime feeding habitats. Dung transects conducted in the
national park showed a strong negative relationship between wild horse dung and macropod dung. As wild
horses were removed from the transect area (as part of NSW National Parks and Wildlife Service (NPWS) horse
capture program, where 250 wild horses were removed) there was a significant progressive decline in horse
dung and increase in macropod dung. Indicating that macropods were responding positively to decreases in
horse abundance (Lenehan 2010).
Although it has not been investigated to date, it is possible that wild horses also compete with other native
mammals such as wombats (Vombatus ursinus). Wombats also feed primarily on grasses (such as Poa and
other grass species and some shrubs) and like wild horses, dig below the snow to reach low vegetation during
winter (Matthews 2010).
Wild horse activity may constrain resources for many species and may increase the vulnerability of threatened
species (Nano et al. 2003). A reduction in the density of wild horses in Finke Gorge National Park and
surrounding area correlated with a striking increase in black-footed rock wallabies (Petrogale lateralis) and
small fauna populations (Matthews et al. 2001). In 1990 no fresh black-footed rock wallaby dung was
recorded, however after approximately 6000 wild horses were removed (and the population kept at low
numbers), the amount of fresh dung recorded steadily increased over the following ten years. It is suggested
that the wild horse population may also have suppressed the endangered stick-nest rat (Zyzomys
pedunculatus) in the West MacDonald ranges (Nano et al. 2003). Despite numerous surveys the species was
not recorded between 1960 and 1996 and it was concluded the species was extinct. However in 1996 it was
rediscovered at numerous sites, a finding that corresponded with the removal of 30 000 wild horses (Nano et
al. 2003). The major food plants identified in the diet of the stick-nest rat were also palatable to wild horses
and other introduced ungulates (ibid.).
3.6.2 Habitat modification
Wild horse grazing can result in a more open shrubland and increased area of bare ground, which in turn
causes changes for associated wildlife (Zalba & Cozzani 2009). The removal of vegetation through grazing,
trampling, wallowing and rubbing can result in a loss of shrub and ground vegetation connectivity. The habitat
value of a site for animal species that require shrubs for shade, food or protection from predators and other
threats may be reduced by the loss of this connectivity (Beever et al. 2008).
Simplification of habitat by wild horses can lead to a reduction in foraging, nesting and roosting opportunities
for native animals, such as ground dwelling, foraging and roosting birds (O’Connor 2005). Zalba and Cozzani
(2009) investigated the impact of wild horses on different bird communities in the Pampas grasslands in
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The Ecology of Wild Horses and their Environmental Impact in the Victorian Alps
May 2013
Argentina. The presence of wild horses was associated with an increase in the rate of egg predation, which
varied from 12.5% within exclosures to 70% in grazed areas. This was directly related to increased visibility of
the nests and subsequent exposure to predators, nest trampling and reduced nesting sites. In Victoria’s
Barmah Forest wild horses are listed as a threat to some ground nesting birds including the Australasian
Bittern and Little Bittern (SAC 1997a; 1997b). Due to a lack of research it is unknown if wild horses pose a
threat to any birds in the Victorian Alps.
Wild horses may potentially cause the collapse of wildlife burrows (Theile & Prober 1999a). In the Victorian
Alps many medium and small mammals, such as the bush rat (Rattus fuscipes), swamp rat (R. lutreolus) and
broad-tooothed rat (Mastacomys fuscus), use burrows that may be susceptible to trampling damage. Wombat
burrows that, for example are dug in open ground with little protective cover over the burrow may be
susceptible to damage by wild horses. In the USA, exclosure plots around springs exhibited more smallmammal burrow entrances than plots at horse-grazed springs (Beever & Brussard 2000).
The affect that wild horses have on invertebrate eco-engineers such as ants in the Victorian Alps is currently
unknown. Beever and Herrick (2006) found a greater abundance of ant mounds in sites where wild horses had
been removed over ten years ago. This may have a substantial influence on ecosystem health and the survival
of other species.
Extensive damage to the structural complexity of ground-level vegetation and habitat caused by wild horses
threatens the reptile and amphibian species of the Victorian Alps. Structural complexity of habitat is a critical
requirement for many reptile species and communities (Clemann et al. 2001). Litter cover, ground cover and
the degree of soil compaction are all important elements for reptiles (e.g. Brown 2001; Jellinek et al. 2004).
Similarly, most amphibian species in the Australian Alps rely on the structural complexity formed by vegetation
along the margins of streams and ponds, especially for male calling and female egg laying sites (Osborne 1990;
Green & Osborne 1994). Grazing and trampling by ungulates such as wild horses in the alpine region affects
habitat complexity through: direct removal of vegetation; destruction of structural components in the ground
layer; and, soil compaction (Clemann et al. 2001). For example, the alpine water skink (Eulamprus kosciuskoi)
(photo 10), alpine she-oak skink (Cyclodomorphus praealtus) and alpine bog skink (Pseudemoia cryodroma) are
threatened by wild horse impacts on peatlands, wet heath and riparian areas, which provide vital habitat for
these species (Clemann et al. 2001; Clemann 2001; Meredith et al. 2003; Steane et al. 2005).
As discussed, peatland habitats are extremely vulnerable to wild horse activity. Horse damage to peatlands has
immediate effects on habitat structure, and leads to hydrological consequences such as deteriorating water
quality and loss of landscape structure. Damage to habitat caused by wild horses is specifically identified as a
threat to the federally endangered alpine she-oak skink, which occurs in close proximity to bogs, streams and
wet heaths. Recommended actions to mitigate this threat include the development and implementation of a
management plan for the control of wild horses (DEWHA 2009b).
Historically, the threatened alpine tree frog (Litoria verreauxii alpine) was widespread and abundant across
most of the high country of mainland south-eastern Australia (Clemann & Gillespie 2004). The species has
since retracted from much of its previous habitat. Populations of the alpine tree frog are known to be subject
to a number of threatening processes, including the trampling of habitat by wild horses (Clemann & Gillespie
2004). The threat of further population declines is likely and remains a cause for concern (Brown et al. 2007).
Small mammal communities may be affected by grazing ungulates such as wild horses indirectly through
alteration in vegetation structure, water quality and other site characteristics (Giuliano & Homyack 2004).
Removal of ungulates (mostly cattle) grazing from streams and associated riparian zones led to an increase in
mammalian species richness and abundance within two years in a study in south west Pennsylvania (USA)
(Giuliano & Homyack 2004). A similar result was shown in the central Kimberley across several habitats after
23
The Ecology of Wild Horses and their Environmental Impact in the Victorian Alps
May 2013
the removal of cattle (Legge et al. 2007). In a study of horse impacts using exclosures in the USA, qualitative
differences were observed in small mammal activity (Beever & Brussard 2000).
There are a limited number of small mammal species in the Australian Alps, and complexity of vegetation
structure is associated with habitat quality for these species (Carron 1985). The habitat of small mammals in
Alpine regions is extremely sensitive and threatened by climate change (Green et al. 2008). A species of
particular concern is the broad-toothed rat Mastacomys fuscus (Green & Osborne 2003 and references within)
whose habitat is shrinking and is likely to face a further declines with climate change (Green et al. 2008). The
broad-toothed rat inhabits areas of moderate-to-dense grass and/or sedge cover (Green et al. 2008). This
environment is favoured by wild horses for grazing and the habitat can be broken-up through wild horse
activity. O’Brien et al. (2008) found populations of broad-toothed rats that existed in isolated patches were
likely to suffer local extinction due to low levels of success emigrating through unsuitable habitat to reach
suitable habitat.
3.6.3 Threatened fauna in the Victorian Alps
Herpetofauna (reptiles and frogs) are central components of the vertebrate fauna of the Victorian Alps and
several threatened species are endemic to alpine areas (Clemann et al. 2001). A large proportion of this group
are officially listed as threatened at the federal and state level (Appendix 3). Herpetofauna in this region are
generally dependent on bog, grass and wet heath habitats (Clemann et al. 2001). As previously mentioned,
most alpine herpetofauna are intimately linked with habitat structure, particularly close to ground level
(Clemann et al. 2001). Wild horses are believed to be one of the processes directly or indirectly threatening
these species (Clemann et al. 2001). For example, the alpine water skink (Eulamprus kosciuskoi) (photo 10) is
listed as critically endangered in Victoria and is only found in the high altitude areas of south-eastern Australia
(Steane et al. 2005). Steane et al. (2005) found the preferred habitat of alpine water skinks to be mossbeds in
good condition (i.e. high cover of live Sphagnum and low cover of bare-ground, rock and non-mossbed
vegetation). Any threat to Sphagnum cover, such as that posed by wild horse grazing and trampling, severely
threatens the habitat of the alpine water skink (Steane et al. 2005). Horses have impacted much habitat that
was previously occupied by the alpine water skink on Forlorn Hope Plain in the Buchan headwaters.
Consequently, to protect threatened alpine water skink populations from trampling by wild horses, wild horse
exclusion fences at Davies Plain around two sub-alpine bogs have been constructed (see section 3.3.3).
Photo 10: The alpine water skink (Eulamprus kosciuskoi), an endangered species whose habitat is threatened
by wild horses (Source: Nick Clemann).
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The Ecology of Wild Horses and their Environmental Impact in the Victorian Alps
May 2013
4. Wild horse biosecurity issues
Wild horses are a prominent reservoir for exotic and endemic diseases and parasites that can affect native
wildlife, domestic horses, stock and humans (Bradshaw et al. 2007). The spread of pests and disease by feral
animals 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 and
transmitting disease. Diseases of concern are either those that have the potential to establish or exist already
within Australia. Diseases are highly variable, for example they may be equine specific or they may affect
several species, transmissibility varies, and symptoms range from minor ailments to mortality. The wild horse
population in the Australian Alps poses a potential risk to other species that may co-habit with them such as
native 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 the
disease 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 of
vertebrate 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 in
places such as Grey Mare in Kosciusko National Park and keeping them fungus free is a critical biodiversity
priority.
Wild horses can play a potential role in a disease outbreak of a newly imported disease (Biosecurity Australia
2009). There are many steps involved in the outbreak of a new disease. Wild horses must first be exposed to
the disease, once exposure occurs, a range of outbreak scenarios are possible from no spread to widespread
establishment. Outbreak scenarios are dependent on the epidemiology of each disease agent. The role that
wild horses play in a potential disease outbreak scenario for a newly imported disease was considered in a risk
assessment by Biosecurity Australia (2009). In general, wild horses were thought to have a limited ability to
affect domestic horse populations because of their remoteness. Equine Influenza received the highest risk
assessment (moderate). There was an outbreak of Equine Influenza in 2007, which led to prohibitions on horse
movements in Victoria to control the spread of the disease, however, it was eliminated (Animal Health
Australia 2009).
There is little known about the incidence and prevalence of many existing diseases of domestic horses in
Australia, and much less is known about the frequency of disease in wild horses. There are a range of zoonotic
diseases (diseases that horses can share with humans and other livestock) which are present in Australia and
may 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 affect
native species.
Hendra virus is a relatively new and emerging disease, first reported in 1994 in Queensland (Equine Species
Working Group n.d). Subsequently there is not a great deal known about the disease, however, horses are
likely to be dead-end hosts. It causes severe respiratory illness in horses and is frequently fatal. Humans can
contract the disease if they have direct contact with blood or saliva of an infected horse (Equine Species
Working Group n.d).
Murray Valley Encephalitis is a virus spread by mosquitoes that can infect many animals, including horses and
humans (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 widespread
outbreak 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 2013
Humans, domestic animals and wildlife are susceptible to the bacterial disease Leptospirosis and infected
animals may become sick or show no symptoms at all (Equine Species Working Group n.d). Salmonellosis is a
bacterial disease that can infect horses and humans and is of concern because some strains exhibit resistance
to 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 rabies
in Australia; however, there have been no reports of horses being affected by this disease. Tetanus can be a
fatal disease and its spores are widely distributed, usually in soil, dust and manure; horses and humans are the
most susceptible of all of the animal species to tetanus (Equine Species Working Group n.d). Ringworm is a
fungal skin disease. Numerous species can transmit ringworm to humans (Equine Species Working Group n.d).
5. Gaps in knowledge
There has been a limited amount of research into wild horse biology, behaviour and ecological impacts in the
Alpine region to-date. An important area for investigation that will aid in guiding effective management is the
relationship between wild horse density and damage (Dawson et al. 2006). Quantification of the relationship
between density, impact and population control has been identified as a priority research area for the
management of wild horses by the AALC. The AALC has initiated a project that aims to address this gap by
developing a protocol for determining horse density at the local scale and investigating the influence of horse
density on impacts. Site-specific research using experimental trials to determine local levels of resistance to
impact 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 are
likely to significantly under-estimate the total population size. There are large areas known to be occupied by
wild horses that have been excluded from previous aerial surveys, including Byadbo and adjoining areas of the
Alpine National Park and adjacent State Forests (Dawson 2009). There are plans to cover a larger portion of
the area occupied by wild horse in future aerial population surveys in order to obtain a more accurate estimate
of the total Victorian Alps wild horse population size. Wild horses do not currently occupy all suitable habitat
within the Victorian Alps. Mapping potential horse habitat that could potentially be colonised by wild horses is
required 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 have
not been investigated at a broad spatial scale. The effects of chronic disturbance over broader areas is
subsequently 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 been
considered. These studies are important; however they may not be representative of the Victorian Alps system
more 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 through
time as the environment varies (e.g. drought, fire, climate change etc). The environmental impact of wild
horses may be exacerbated in drought years, as they tend to concentrate around watering points (Dawson
2010). Impacts are also likely to increase substantially after fire (such as those that occurred in Victoria in 2003
and 2006/07) when Alpine environments such as peatlands are particularly susceptible. Climate change is
likely to influence the magnitude of the impacts of wild horses in future years. Alpine species and communities
are amongst the most vulnerable to the effects of climate change and therefore their resilience to climate
change requires that they be protected and maintained in as healthy a condition as possible (DSE 2010). This
pressure combined with the impact of wild horses requires further investigation.
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May 2013
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The Ecology of Wild Horses and their Environmental Impact in the Victorian Alps
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Appendix 1: Officially listed plant ecological communities at risk of
severe damage from wild horse activity
Floristic Community or
Ecological Vegetation Class
Alpine Bog Community
EPBC
Endangered*
FFG
Listed
Fen (Bog Pool) Community
Endangered*
Listed
Alpine Snowpatch
Community
Listed
Caltha introloba Herbland
Community
Montane Swamp Complex
Community
Listed
Listed
Main consequences
Disruption of plant regeneration, selective grazing
leading to compositional changes, weed invasion,
severe trampling, soil loss, and loss of hydrological
function.
Disruption of plant regeneration, selective grazing
leading to compositional changes, weed invasion,
severe trampling, and loss of hydrological function.
Severe trampling, soil loss, displacement of
vegetation, weed invasion, selective grazing
leading to compositional changes.
Soil loss, displacement of vegetation, weed
invasion
Severe trampling, soil loss, displacement of
vegetation, weed invasion, smothering by dung
piles, selective grazing leading to compositional
changes.
* Alpine Bog and Fen Pool communities are combined in the EPBC listing
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The Ecology of Wild Horses and their Environmental Impact in the Victorian Alps
May 2013
Appendix 2: FFG-listed and EPBC-listed plant species potentially at
risk from wild horse activity in the eastern Victorian Alps AROT = Australian
Rare or Threatened status, VROT = Victorian Rare or Threatened status. e = endangered (Victoria), r =- rare
(Victoria), v = vulnerable (Victoria), V = Vulnerable (EPBC, Australia).
Species
Bogong Eyebright (Euphrasia
eichleri)
Snow Pratia (Lobelia gelida)
Austral Toad-flax (Thesium
australe)
Tasmanian Bladderwort
(Utricularia monanthos)
Allied Bent-grass (Deyeuxia
affinis)
Cushion Rush (Juncus
antarcticus)
Rock Poa (Poa saxicola)
Marsh Leek-orchid
(Prasophyllum niphopedium)
Mountain Daisy (Brachyscome
sp. 3)
Slender Parrot-pea (Almaleea
capitata)
Wire-head Sedge (Carex
cephalotes)
Bogong Apple-moss
(Bartramia subsymmetrica)
Austral Moonwort
(Botrychium australe)
Rough Eyebright (Euphrasia
scabra)
Felted Buttercup (Ranunculus
muelleri)
Olive Mallee (Eucalyptus
elaeophloia)
Star Sedge (Carex echinata)
Snow Wort (Abrotenella
nivigena)
Alpine Marsh Marigold
(Psychrophila introloba)
Carpet Sedge (Carex jackiana)
Turquoise Coprosma
(Coprosma moorei)
Dwarf Coprosma (Coprosma
pumila)
Snow Pennywort (Diplaspis
nivis)
Sky Lily (Herpolirion novaezelandiae)
Tuft-rush (Oreobolus
oxycarpus subsp. oxycarpus)
Alpine Tuft-rush (Oreobolus
pumilo subsp. pumilio)
Parantennaria (Parantennaria
uniceps)
Snow Aciphyll (Aciphylla
glacialis)
Listed
FFG,
EPBC
FFG,
EPBC
FFG,
EPBC
FFG
AROT,
VROT
V, v
V, v
V, v
v
Main consequences
May be selectively grazed - more common in ungrazed open heath
& grassland.
Trampling & loss of habitat - ephemeral pools in wet alpine
grasslands & creek margins
Trampling of habitat - grasslands & grassy woodlands. Considered
to be palatable to stock.
Trampling & loss of habitat - Caltha introloba Herbland Community
& stream edges
Trampling & loss of habitat - Caltha introloba Herbland Community
FFG
e
FFG
v
FFG
v
FFG
e
FFG
v
Trampling & loss of habitat - Caltha introloba Herbland
Community, Alpine Snowpatch Community & bog margins
Trampling & loss of habitat - Alpine Snowpatch Community, open
heath & grassland. Palatable, & rare in grazed areas.
Trampling & loss of habitat - alpine wet heathlands & bogs.
Observed to be directly impacted.
Trampling & loss of habitat - alpine wet grasslands & herbfields
FFG
v
Trampling of habitat - sub-alpine heathlands & stream fringes
FFG
v
Trampling & loss of habitat - alpine herbfields
FFG
e
Trampling & loss of habitat - wet grasslands & bog margins
FFG
v
FFG
e
Trampling & loss of habitat - subalpine grassland & margins of bogs
& streams. Intolerant of disturbance.
Trampling & loss of habitat - grassy edges of wetlands
v
r
Trampling & loss of habitat - alpine herbfields, wet grasslands &
bog margins. Observed to be directly impacted.
Trampling of habitat - sub-alpine woodlands. Observed to be
directly impacted.
Trampling & loss of habitat - alpine wet heathlands & bogs
Trampling & loss of habitat - alpine wet heathlands, bogs & moist
herbfields
Trampling & loss of habitat - Caltha introloba Herbland Community
r
r
Trampling & loss of habitat - moist alpine herbfields
Trampling of habitat - alpine wet heathlands, bogs & grasslands
e
Trampling of habitat - alpine woodlands, heathlands & grasslands
r
r
Trampling of habitat - alpine wet heathlands, bogs, grasslands &
herbfields
Trampling & loss of habitat - alpine wet heathlands, bogs &
herbfields
Trampling & loss of habitat - alpine wet heathlands & bogs
r
Trampling & loss of habitat - alpine wet heathlands & bogs
v
Trampling & loss of habitat - Caltha introloba Herbland Community
r
Selective grazing in open heath & grassland. Highly palatable &
most common in ungrazed areas.
v
v
v
r
37
The Ecology of Wild Horses and their Environmental Impact in the Victorian Alps
Mountain Aciphyll (Aciphylla
simplicifolia)
Native Wintercress (Barbarea
grayi)
Grassy Moonwort
(Botrychium lunaria)
Baw Baw Daisy (Brachyscome
obovata)
Marsh Daisy (Brachyscome
radicans)
Spreading Bitter-cress
(Cardamine astoniae)
Star Sedge (Carex echinata)
r
Raleigh Sedge (Carex raleighii)
r
Marsh Tree-moss (Climacium
dendroides)
Creeping Coprosma
(Coprosma perpusilla)
White Billy-buttons
(Craspedia alba)
Tufted Hair-grass
(Deschampsia caespitosa)
Skirted Bent-grass (Deyeuxia
talariata)
Tailed Eyebright (Euphrasia
caudata)
Slender Gingidia (Gingidia
harveyana)
Alpine Holy-grass (Hierochloe
submutica)
Tussock Woodrush (Luzula
alpestris)
Slender Woodrush (Luzula
atrata)
Spreading Clubmoss
(Lycopodium scariosum)
Hump Moss (Meesia muelleri)
Branched Caraway
(Oreomyrrhis brevipes)
Wedge Oschatzia (Oschatzia
cuneifolia)
Matted Rice-flower (Pimelea
biflora)
Hard-head Bush-pea
(Pultenaea capitellata)
Southern Sheep-grass
(Rytidosperma australe)
May 2013
v
Selectively grazed in grasslands & bog margins. Highly palatable,
most common in ungrazed areas.
Trampling & loss of habitat - bogs & stream margins. Highly
palatable to stock.
Trampling of grassy habitat. Intolerant of disturbance.
r
Trampling & loss of habitat - alpine wet heathlands & bogs
r
Trampling & loss of habitat - alpine wet heathlands & bogs.
v
v
Trampling of habitat - wet grasslands and bog margins. Palatable
to stock.
Trampling & loss of habitat - alpine wet heathlands & bogs.
Palatable to stock.
Trampling & loss of habitat - alpine wet heathlands & bogs.
Palatable to stock.
Physical damage to habitat - within streams
r
Trampling & loss of habitat - alpine wet heathlands & bogs.
v
v
Trampling & damage to habitat - Caltha herbfields & valley
bottoms. Palatable to stock.
Trampling & loss of habitat - Caltha introloba Herbland Community
& fens
Trampling & loss of habitat - alpine wet heathlands & bogs.
r
Trampling & loss of habitat - Alpine Sphagnum bogs
v
v
Trampling & loss of habitat - stream margins. Highly palatable,
hence selectively grazed.
Trampling & loss of habitat - alpine wet heathlands & bogs.
r
Trampling & loss of habitat - grassy edges of wetlands
v
Trampling & loss of habitat - damp grasslands
r
Trampling & loss of habitat - alpine wet heathlands & bogs.
r
v
Trampling & loss of habitat - boggy grasslands
Selective grazing in rocky habitat. Palatable, and now extremely
rare in grazed areas.
Trampling & loss of habitat - alpine wet heathlands and bogs.
v
v
r
r
r
r
Possible selective grazing in open heath & grassland. Rare in
grazed areas.
Trampling & loss of habitat - margins of alpine wet heathlands
e
Trampling & loss of habitat - Caltha introloba Herbland Community
38
The Ecology of Wild Horses and their Environmental Impact in the Victorian Alps
May 2013
Appendix 3: Officially listed or threatened fauna species potentially at
risk from feral horse activity in the eastern Victorian Alps
Species
Alpine Water Skink
(Eulamprus kosciuskoi)
Alpine Bog Skink
(Pseudomoia cryodroma)
EPBC
Alpine She-oak Skink
(Cyclodomorphus
praealtus)
Guthega Skink (Liopholis
guthega)
Mountain Skink (Liopholis
Montana)
Alpine Tree Frog (Litoria
verreauxii alpina)
FFG
Listed
2
Listed
DSE
Critically
Endangered
Endangered
Endangered
Listed
Endangered
Under
consideration
Listed
Critically
Endangered
Data Deficient
Vulnerable
Listed
Critically
Endangered
Main consequences
Loss and degradation of habitat - alpine bog
and alpine fen (bog pool) communities
Loss and degradation of habitat - alpine bog
and alpine fen (bog pool) communities,
woodlands and heathlands
Loss and degradation of habitat - alpine
tussock grasslands, alpine low heathlands
Loss and degradation of habitat - alpine
heathlands
Loss and degradation of habitat - alpine
woodlands
Loss and degradation of habitat - alpine and
subalpine wetlands, riparian zones and
ephemeral pools.
39