There are a number of feral animals found in the South West region, which have a devastating impact not only on the environment but also on agricultural production. The species which appear to cause the most harm and are relatively common include feral cats, foxes, feral pigs and rabbits. Some other species which are found in the South West but are not as common include wild horses, deer, wild dogs, goats, bird and fish species. It is estimated that $620 million dollars per year is spent on feral animal control in Western Australia alone.
Foxes and Feral Cats
Foxes and feral cats appear to be the major reason for declines in a number of the fauna species found in the South West and as a result the Department of Parks and Wildlife have been running a program for a number of years called ‘Western Shield’, which baits huge areas of the south west for foxes and cats. As a result of this program, a number of species have been removed from Australia’s threatened species list. For more information on the Western Shield program please visit http://www.dpaw.wa.gov.au/management/pests-diseases/westernshield.
In addition, the Red Card for Rabbits and Foxes is a community based feral animal control program that operates across the agricultural regions of Western Australia. Each year local community groups, sporting clubs, local governments and individual land holders come together to run a co-ordinated control program across their local patch during the autumn months.
The theory behind the program is that the best results can be achieved by co-ordinating control activities at the same time across the landscape. In that way areas where feral animals are removed are not reinfested by animals moving in from neighbouring territories.
The largest part of the Red Card program is the community baiting programs. Each year local groups co-ordinate their baiting so that the maximum impact can be achieved. Baiting is carried out in autumn as this is generally lambing time in the south west and many foxes are on the move searching for food following the long summer. For more information including upcoming control dates, contact details and previous years records visit: http://www.redcard.net.au/.
In biological terms, feral pigs have been relatively recently introduced to the south west of WA and appear to be expanding their range (Masters 1979; Woolnough et al. 2004; 2005). The area of suitable habitat for feral pigs in the region is much larger than the currently known distribution of feral pigs and it is logical to expect that feral pigs will continue to expand into the surrounding habitat where control measures are unable to prevent this and where the required movements from essential resources are not too great (Choquenot and Ruscoe 2003; Cowled et al 2009). The precise area of occupancy, abundance and distribution of feral pigs in the south west of WA is unknown due to difficulties in estimating population size and inconsistent approaches to recording feral pig presence and abundance (Bain and Kinnear, 2015).
The National Threat Abatement Plan for feral pigs identifies predation, physical modification of habitat, competition and disease transmission as key threatening processes that pigs contribute to in the Australian environment (DoE 2005). The plan states that the ecological parameters most likely to be affected by feral pigs are species composition, succession, and nutrient and water cycles.
The Feral pig control strategy for south west Western Australia: 2015 – 2020 outlines the impacts of feral pigs on the south west environment
- Direct predation on bird chicks, reptiles, reptile and bird eggs, frogs, soil organisms, earthworms and other invertebrates (McIlroy, 1990; Mitchell, 1993; Roberts et al. 1996; Fordham et al. 2006; Mitchell 2008), underground fungi, fruit, seeds, roots, tubers, bulbs and plant foliage (Choquenot et al. 1996; Laurance and Harrington 1997; Melzer et al. 2009).
- Physical removal of plants and altered floristics, vegetation structure and habitat quality as a result of digging, wallowing or feeding behaviours (Bratton 1977; Wood and Barrett 1979).
- Reduced regeneration of plants due to decreased seedling recruitment, survival and alteration of soil structure (Choquenot et al. 1996; Hone 2002; Adams 2014).
- Increased invasion and spread of weeds as a result of disturbed vegetation and movement of soil and weed material (Li et al. 2013; Lynes and Campbell 2000 Tierney and Cushman 2006).
- Increased soil friability due to digging activities and an associated reduction in the presence of invertebrates, microbes and bulb-producing plants (Hone 2002; Mohr et al.2005).
- Altered soil and nutrient properties as a result of digging. For example, accelerated leaching of calcium, copper, magnesium, potassium, phosphorus, and zinc from the soil (Singer et al. 1984); negative impacts on soil building processes (Ford and Grace 1998); and accelerated rates of soil erosion (Sierra 2001). Feral pigs are estimated to turn over approximately 12.89 million tonnes of soil every 12 months in the northern jarrah forest, a figure which has been equated to a commercial mining operation (Adams 2014).
- Increased oxidation and acidification processes within organic, peat based wetland systems as a result of digging and wallowing, particularly following fire when the soil is more exposed and accessible (Burnside et al. 2012).
- Reduced quality of surface and ground water due to erosion, siltation and pollution through digging and wallowing (Statham and Middleton 1987; Choquenot at al. 1996; Fordham et al. 2006; Mitchell 2008).
- Creation of habitat suitable for disease vectors, particularly mosquitoes that breed in the shallow wallows where surface water is present (Choquenot at al. 1996).
- Actual vectoring of exotic diseases and pathogens such as Phytophthora cinnamomi (dieback) and Batrachochytrium dendrobatidis (frog Chytrid fungus) in hooves or digestive tract (Kliejunas and Ko 1976; Masters 1979; Li et al. 2014). The spread of dieback has also been associated with soil disturbance and and reduction of litter cover by pigs (Brown 1976). In addition, physical damage of plants can provide entry points for infection and increase susceptibility of flora populations to disease, parasitic and fungal attacks.
- Increased access for other introduced predators, such as feral cats and foxes, as a result of disturbed vegetation.
- Competition with native species for food, water and shelter (Taft 1999).
161 species of nationally listed threatened flora and fauna have been identified in the National Threat Abatement Advice (2013) as being adversely affected by feral pigs. 16 of these are found in south west WA and 12 of these are endemic to the region.The TAA (2013) only includes species that are federally listed under the EPBC Act. There is also a large range of taxa listed as threatened or specially protected on State lists that are likely to be adversely affected by feral pigs. For example of the 442 state listed threatened and priority fauna species (Parks and Wildlife 2014), 55 are associated with sensitive riparian and granite habitats, are known to be dependent on a specific vegetation structure, or lay their eggs in the soil. Similarly, many of the 2319 state listed threatened and priority flora species (Florabase 2014) and associated ecological communities are likely to be at risk from feral pigs, particularly where they occur in habitats favoured by pigs. Feral pigs cause their greatest damage in environmentally sensitive areas and other natural ecosystems of conservation concern. In some cases, areas affected by feral pigs are showing no sign of recovery more than six years after pigs have been removed (e.g. Bain et al. 2015). Impacts of feral pigs are often more severe following fire or some other disturbance that opens up the understorey or exposes the soil (Burnside et al 2012). Feral pigs may also have significant impacts on biodiversity through less direct effects such as impacts on non-target species from control measures such as dogging and poisoning.
Feral pigs cause approximately $106.5 million in agricultural damage in Australia each year (McLeod 2004; DoE 2005). The main impacts that feral pigs have on agricultural values include:
- Predation on newborn lambs (Pavlov et al. 1981; Mitchell and Baloch 2007)
- Reduction in yields of grain, fruit and vegetable crops by consuming or trampling plants or up-rooting vegetation (Tisdell 1982; Choquenot et al. 1996; Mitchell and Baloch 2007; Gong et al. 2009).
- Removal of plantation seedlings and predation on roots of recently planted trees (Lipscomb 1989; Campbell and Long 2009).
- Physical damage to fences and water sources, and fouling of dams and waterholes through wallowing and defecation (Tisdell 1982; Mitchell and Baloch 2007).
- Competition with livestock for pasture (Choquenot et al. 1996; Bomford and Hart 2002; Mitchell and Baloch 2007).
- Potential spread of infections and diseases to domestic livestock, including leptospirosis (Leptospira interrogans), brucellosis (Brucella sp.), melioidosis (Burkholderia pseudomallei), tuberculosis (Mycobacterium tuberculosis) and sparganosis (Spirometra sp.) (Choquenot et al. 1996; Heymann 2008). These bacteria and parasites have been directly linked to feral pigs in QLD and SA however the incidence of infection and disease is relatively low (Heymann 2008).
Water Resources Feral pigs can pose a significant risk to water resources, especially within PDWSAs. The main impacts that pigs have on water resources include:
- Degradation of surface water quality through wallowing and foraging (DoW 2009).
- Introduction of infectious waterborne pathogens into the water supply that may infect humans, such as protozoan parasites such as Giardia, Cryptosporidium, Balantidium coli and Entamoeba histolytica (DoW 2009). In some cases, these zoonotic pathogens can be life threatening (Hampton et al. 2006).
- Destruction and erosion of riparian vegetation, which provides important natural protection against contamination, erosion and turbidity from increased siltation (DoW 2009).
- Interference with chlorination and filtration treatment processes due to added particulates in the water, which can shield micro-organisms from effective disinfection and promote bacteria growth (WHO 2004).
In addition, feral pig control methods that require people to stay in PDWSA for long periods, can increase the risk of contamination to the drinking water source through direct introduction of contaminants into the water body from humans, dogs and vehicles; damage of protective vegetation and soil erosion by vehicles; or accidental fires. These risks can be managed for legitimate pig control activities by having policies and work instructions that actively mitigate the risks. Feral pig carcasses in the catchment, particularly those close to reservoirs, feeder streams or production bores also present a pathogen risk to drinking water quality (DoW 2009), as do native animal carcasses. Feral pig control activities can only occur within PDWSAs with the approval of the land manager.
Economy and Human Health
Feral pigs can act as hosts or vectors of several endemic and exotic diseases and parasites that affect humans and domestic animals. The major bacteria and parasites of concern are Q fever (Coxiella burnetii), leptospirosis (Leptospira interrogans), brucellosis (Brucella sp.), melioidosis (Burkholderia pseudomallei), tuberculosis (Mycobacterium tuberculosis) and sparganosis (Spirometra sp.). Of these, Q fever is the most commonly recorded disease presenting in humans following interaction with the body fluid and urine of feral pigs. However, the potential remains for the involvement of feral pigs in the transmission of a range of diseases due to their widespread distribution, their ability to act as hosts for a range of bacteria and parasites and their occurrence in habitats where these organisms are likely to be present. The involvement of feral pigs in an exotic disease outbreak could delay disease detection; increase the rate and extent of disease spread; make disease eradication measures expensive, time-consuming or impossible; and have severe repercussions for Australia’s livestock industries (Choquenot et al. 1996).
The cost of feral pigs to the agricultural industry in Australia has been valued in excess of $106 million each year (McLeod 2004; DoE 2005). These values do not include the potential economic costs arising from the loss of endangered and threatened species, damage to National Parks, water quality impacts and the transmission of diseases. The costs are likely to be substantially higher and if left unmanaged, feral pigs will affect local tourism, conservation, agriculture, water resources and human health.
In addition to the potential negative impacts of feral pigs on economy and human health, the increase in recreational hunting in some areas may equate to an economic benefit associated with equipment sales and marketing products.
Available control techniques for feral pigs are usually incapable of removing all individuals in a single event and require integration with other techniques, repeated applications of the technique or a sequence of techniques to maximise effectiveness. The advantages and disadvantages of each available control technique are summarised in the table below, and a full discussion of each technique follows in Sections 5.1 – 5.10 of the Feral pig control strategy for south west Western Australia: 2015 – 2020.
Feral Pig Control Options
The Feral pig control strategy for south west Western Australia: 2015 – 2020 represents a framework for feral pig management in the south west of Western Australia and is consistent with the requirements of the National Feral Pig Threat Abatement Plan (DoE 2005). Objectives and strategies outlined in Section 12 are relevant to the geographical area represented in Figure 3 however the strategies are broad enough that they can be applied more widely should groups in areas outside of this region be interested in adopting a similar approach.
Groups involved in feral pig control in the south west of Western Australia include government and non-government agencies, private landholders and community groups with a diverse range of values, perspectives and responsibilities relevant to the threat of feral pigs in this landscape. Section 12 attempts to facilitate a more integrated, cooperative and targeted approach by these multiple stakeholders to achieve the outcomes desired for feral pig management in this region.
Feral Animals and Climate Change
It is thought that climate change will benefit some invasive animals and cause others to decline. However, because disturbance and stress to native species and ecological communities often benefit invasive species, and because climate change will bring new invasive threats, the overall threat from invasive species is likely to increase in the future.
Other impacts that feral animals are attributed to include; damage to infrastructure such as fences, fouling of water sources, spread of disease, damage to native vegetation, competition with native animals for food and resources, soil erosion and spread of weeds.
Department of Agriculture and Food
The Department of Agriculture and Food, Western Australia manages pests through policy development, risk assessment, research and development, provision of technical advice and information, implementation of regulation, emergency response, property inspections, industry liaison, and the planning and coordination of significant species control/eradication programs. Please see http://www.agric.wa.gov.au for further information.
- Invasive Animals and Climate Change Fact sheet – Invasive Species Council
- Bain, K., and Kinnear, K. (2015). Feral pig control strategy for south west Western Australia: 2015-2020. Plan prepared for South West Catchment Council, Western Australia.