Chapter 4
Fences or Ferals? Benefits and Costs of Conservation Fencing in Australia Chris R. Dickman
Introduction The arrival of white settlers on Australia’s east coast in 1788 set in train a series of events that had very large effects on the nation’s first people and on the native flora and fauna. The grand sweep of these events and their repercussions have been documented at some length (e.g. Clark 1981) but, for some elements of the Australian vertebrate fauna, arguably the most important change wrought by the new settlers was the introduction of new species. Domestic animals were imported to provide meat, dairy produce, leather and other materials, companionship, and a sense of the familiar in a new environment (Rolls 1969). Some were introduced for sporting and hunting purposes or, like house cats Felis catus, deliberately to replace the native fauna; others such as black rats Rattus rattus and house mice Mus musculus were carried as stowaways on the early ships (Low 1999; Long 2003). From the midnineteenth century acclimatisation societies systematically imported many new species of birds and mammals to Australia. At the same time state governments began developing policies such as the Marsupial Destruction Acts to extirpate native species that were considered to interfere with the interests of the new settlers (Hrdina 1997; Dickman 2007). Although these attitudes no longer prevail, the activities of the early European settlers facilitated the spread of many new species in the Australian environment. Two introduced carnivores – the European red fox Vulpes vulpes and the domestic house cat – have spread widely, and are now of particular concern owing to their damaging effects on a wide range of native Australian vertebrates (Salo et al. 2007).
C.R. Dickman (*) Institute of Wildlife Research, School of Biological Sciences, University of Sydney, Sydney, NSW 2006, Australia e-mail:
[email protected] M.J. Somers and M.W. Hayward (eds.), Fencing for Conservation: Restriction of Evolutionary Potential or a Riposte to Threatening Processes?, DOI 10.1007/978-1-4614-0902-1_4, © Springer Science+Business Media, LLC 2012
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The first cats were brought to Australia in the late eighteenth century, but appear to have remained close to villages and townships in the early years of settlement. They spread into less disturbed areas from points of introduction along the coast between 1824 and 1886 (Gaynor 2000; Abbott 2002), and now occur in all terrestrial habitats in all parts of the continent as well as on over 40 islands offshore (Dickman 1992). Cat populations have been categorised as domestic if their resource requirements are wholly and intentionally met by humans, and feral if the population is self-sustaining and not dependent on access to human-derived resources (Moodie 1995). A third category, stray or semi-feral, includes cats that use remote human resources such as those at rubbish tips (Denny et al. 2002). Although cats may move between categories within their lifetimes or between generations, most attention has focused on the impacts of feral cats on native fauna (Denny and Dickman 2010). The major impact of feral cats is almost certainly that of direct predation, with historical evidence suggesting that this predator may have driven seven species of small native mammals to extinction and contemporary experimental evidence suggesting that cats can severely deplete local populations of extant small mammals and birds (Dickman 1996a, b; Risbey et al. 2000; de Tores and Marlow 2012). Cats may also affect native vertebrates by acting as vectors for diseases such as toxoplasmosis and sparganosis (Moodie 1995) and are suspected to have further deleterious impacts on ecologically equivalent native species such as quolls Dasyurus spp. via competition (Glen and Dickman 2005, 2008). Predation by feral cats has been listed by the Australian government as a key threatening process under the Environment Protection and Biodiversity Conservation (EPBC) Act 1999. The red fox was introduced to Australia on several occasions in the midnineteenth century, but became established in the early 1870s near Melbourne following the earlier spread of the European rabbit Oryctolagus cuniculus (Rolls 1969; Abbott 2011). Foxes spread rapidly – up to 160 km a year – in southern coastal and sub-coastal areas, probably with human assistance; the rate of spread was slower in arid regions and in the tropical north where human settlement was sparse (Saunders et al. 1995). Foxes now occupy the southern three quarters of Australia. They appear transiently in some of the more arid regions after heavy rainfalls have stimulated eruptions of small mammals (Letnic and Dickman 2010), and are moving slowly into tropical areas in both Queensland and Western Australia (Long 1988; Saunders et al. 2010). A wealth of historical, circumstantial and experimental evidence confirms that foxes have strongly deleterious effects on a wide range of native Australian species, with depredation affecting mammals, birds and reptiles weighing 35–5,500 g most severely (Burbidge and McKenzie 1989; Kinnear et al. 2010; Saunders et al. 2010). Foxes may exert further effects via transmission of diseases such as sarcoptic mange and sparganosis, competition and an array of indirect effects (Saunders et al. 2010), but the relative importance of these nonpredatory interactions remains unclear. Foxes are also a recognised pest for sheep farmers, with rates of predation on lambs ranging from 1 to 30% (Lugton 1993; Saunders et al. 2010). As for feral cats, predation by the European red fox has been listed as a key threatening process under the EPBC Act 1999.
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Fences or Ferals? Benefits and Costs of Conservation Fencing in Australia
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Several methods can be employed to reduce the detrimental effects of feral cats and foxes. Shooting and trapping are often used but are costly, labour-intensive and potentially effective only in small areas. In agricultural landscapes the impacts of foxes are sometimes controlled by drives, den fumigation, or the deployment of guard animals in the paddocks (Saunders and McLeod 2007). However, the most commonly used control method in both conservation reserves and agricultural areas is the setting of baits laced with the poison sodium fluoroacetate, or “1080” (Saunders et al. 2010). In New South Wales about a million baits are laid each year over several hundred thousand hectares (Saunders and McLeod 2007), while equally large or larger areas are baited annually in Victoria and Western Australia (Armstrong 2004; Department of Sustainability and Environment 2005). The consequent reductions in numbers of predators, mostly foxes, allow the persistence of a wide range of native vertebrates (Kinnear et al. 2002; Dexter and Murray 2009; Mahon 2009). Despite these benefits, there are general concerns that 1080 may place some nontarget species at risk of poisoning, that its mode of action is not humane (Marks et al. 2009), and that in the long term selection for 1080-resistance will occur (Twigg et al. 2002). In addition to these concerns, feral cats seldom take 1080-baits and their control over large areas remains an almost intractable problem (Risbey et al. 1997; Short et al. 1997; but cf. Algar et al. 2007). Current research on new toxins such as para-aminopropiophenone (PAPP) and new methods of bait delivery give some hope for the future, but will need to meet concerns about possible effects on non-target species before new baits can be deployed at a broad scale (Denny and Dickman 2010). Several authors have proposed that populations of foxes and cats could be reduced over large areas by dingoes Canis lupus dingo, but to be effective this would require cessation of the active culling and suppression programs of the dingo by the pastoral community (Johnson et al. 2007; Dickman et al. 2009; Letnic et al. 2009). Biological control of both predator species has also been suggested, but research on this possibility has been unsuccessful so far (Strive et al. 2007; Denny and Dickman 2010). Against this background, an enterprising but contentious approach has been developed in recent years to provide complete and potentially long term protection for native fauna against the depredations of both feral cats and foxes. This is the conservation fence. Fences have long been used to retain livestock or provide protection against pests or wildlife intruders (Hayward and Kerley 2009), but fences designed to secure large and viable populations of high-value native species are a more recent concept. In Australia the first such fence appears to have been completed by John Wamsley at Warrawong in South Australia in 1975 (De Alessi 2003/04). Such fences have considerable advantages over other methods of fauna protection, but also are very expensive and have been criticised as being little more than captive breeding areas (Pickard 2007). In this chapter I describe different kinds of conservation fences that are used in Australia, their rationale, effectiveness and costs, and conclude by discussing ways in which fence designs could be extended and improved. Because of the disproportionately negative effects of feral cats and foxes on native biota, I focus primarily on fences that are intended to exclude these predators. I also focus on built structures.
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The importance of natural barriers such as water bodies and peninsulas in limiting or excluding feral cats and foxes has been explored elsewhere (Dickman 1992; Short et al. 1994; Burbidge and Manly 2002) and, with the exception of buffers created by the deployment of 1080 baits (Armstrong 2004), there is little evidence that metaphorical barriers (sensu Hayward and Kerley 2009) are effective for these predators.
Kinds of Conservation Fence Australia has the distinction of having built some of the earliest and longest fences that may have had some positive outcomes for conservation, even if the beneficial consequences were inadvertent. In the 1880s, a fence was constructed to limit incursions of dingoes into the sheep-grazing lands of south-eastern Australia; at 3,374 km, it is second in length only to the Great Wall of China (Breckwoldt 1988). Initially this fence would have helped to reduce predation on kangaroos and emus (Caughley et al. 1980), although it is also an effective barrier to their movement (Letnic 2007). Soon after work began on the dingo fence, thousands of kilometres of rabbit-proof fences were constructed in eastern, central and western regions of Australia to limit the spread of this pest species (Broomhall 1991). This almost certainly delayed the destructive effects of rabbit grazing and excavation in some districts, and perhaps also slowed the local rate of increase of fox numbers (Newsome et al. 1997). Despite the great effort involved in the construction and maintenance of fences for rabbits and dingoes, however, neither was designed to exclude feral cats or foxes and there is little evidence that either of these predators was deterred by them. Virtually all fences constructed in the last 25–30 years to exclude feral cats and foxes have used wire mesh or netting with a buried apron running along the base of the fence on both sides, with or without an overhanging top, and sometimes with wires carrying charge sufficient to shock and repel animals attempting to cross (Fig. 4.1). Once construction is complete, foxes, feral cats and sometimes other invasive species are removed from the area to be protected; introduced predators may even leave of their own accord once prey such as rabbits have been removed. In most cases the fence surrounds an irregularly shaped area and constitutes the entire barrier against predators that might otherwise move in from outside. However, innovative designs at Heirisson Prong and Peron Peninsula in Western Australia and at Venus Bay in South Australia have used geography to advantage, with physical fences erected across the narrow necks of peninsulas preventing land-based incursion of predators and natural seawater barriers preventing ingress elsewhere (Short et al. 1994; Morris et al. 2004; Department for Environment and Heritage 2006). Current conservation fence designs in Australia use ideas developed in Europe and the United States (e.g. Forster 1975; Lokemoen et al. 1982), but with many refinements that improve their reliability for local conditions. In the first instance, a barrier fence that is intended to keep out particular species must be designed with the physical characteristics and abilities of those species in mind. These include the sizes of the animals and their ability to jump, dig, climb and bite through given fence materials.
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0.35 m 3 electrical hot wires insulated off fence posts (continuous around perimeter)
2.5 m
INSIDE
Rabbit proof wire netting (dotted line)
Buried netting (aprons) on both sides of fence
0.85 m Concrete pad footing
0.25 m
Fig. 4.1 Side view of a predator-proof fence. This example was developed by the Western Australian Department of Environment and Conservation to protect a small mammal breeding enclosure near Narrogin, Western Australia. At 2.5 m this design is higher than usual and has a rigid rather than floppy top to the fence. See text for a discussion of the many different fence designs that have been trialled and of the designs that are in most current use (redrawn from Saunders and McLeod 2007)
Feral cats weigh 3–7 kg and have field-measured head widths that vary from 5.5 to 9.0 cm (Denny 2005); strength of jaw closure is moderate but not sufficient to cut standard gauge fence wire. Standing jumps of 1.4 m have been observed and, provided that they can obtain good purchase on a substrate, cats can use their footpads and sharp claws to climb high above ground (Coman and McCutchan 1994). Cats are reluctant diggers, but can excavate short burrows in soft substrates. Foxes weigh 4–8 kg, zygomatic width (»head width) ranges from 7.0 to 9.1 cm (Lloyd 1980) and the strength of jaw closure is sufficient to tear through some types of woven wire fences (Coman and McCutchan 1994). Foxes are generally considered to be poor climbers, but have been observed scaling netting and chain mesh fences over 2 m high and to be capable of leaping fences in excess of 1.3 m (Coman and McCutchan 1994; Moseby and Read 2006). They are proficient diggers. With these attributes, Coman and McCutchan (1994) recommended that barrier fences for these predators should be at least 1.4 m high, have wire netting or chain mesh aprons to prevent digging, have mechanical or electrical barriers such as netting overhangs and/or pulsed offset wires to dissuade climbing, mesh or netting in
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at least the lower half of the fence and be checked and maintained regularly. However, these authors and Long and Robley (2004) also noted that even the best predator-proof fences usually just curtail predator ingress rather than completely prevent it, and recommended that experimental testing be carried out to optimise fence design. Two such trials have since been published. The first, by Moseby and Read (2006), placed feral cats and foxes overnight into pens constructed of wire netting and recorded how animals behaved and made their escape. Weak points in the fence design were then improved incrementally until animals were no longer observed to escape. Wire netting fences 115 or 180 cm high with a foot apron and a 60 cm-wide internal overhang of floppy wire proved to be equally effective at containing captive predators, although the authors cautioned that the efficacy of the lower fences needed further testing. Field tests of a much longer exclusion fence, 180 cm in height, confirmed the efficacy of the apron and overhang design over periods of up to 6 years (Moseby and Read 2006). This study revealed further that metal fence posts greatly reduced animals’ ability to climb and confirmed that electric wires had a deterrent effect only when a substantial physical barrier was already in place. In the second experimental study, Robley et al. (2007) placed individual feral cats or foxes in pens that were bisected by an experimental fence, and left animals there for 3 days and nights to observe whether they could breach the fence. Six different fence designs were used, with enticements of open space and structured refuge on the other side to encourage animals to attempt fence crossings. With the exception that both predators were found to breach fences that were 1.2 m high, the authors’ conclusions were remarkably similar to those of Moseby and Read (2006). Thus, they concluded that effective predator-proof fences should be 1.8 m high with an apron of hard wire mesh to prevent foxes chewing through and a recurved overhang at least 60 cm wide at the top to deter animals from climbing over the fence from underneath (Robley et al. 2007). In contrast to other studies (e.g. Poole and McKillop 2002), but like Moseby and Read (2006), Robley et al. (2007) found that electric wires could add slightly to the effectiveness of an appropriate physical fence but could not be justified in terms of the extra capital and maintenance costs they entailed.
Rationale The major objective of predator-proof fences is, of course, conservation. Despite this, it is possible to distinguish several different motivations for fence construction. These are listed in Table 4.1, together with examples of fenced areas that were developed with different conservation imperatives in mind. In the first instance fences may be constructed to conserve threatened species in situ. Although this would seem to be an obvious motivation for the construction of fences, it has been invoked relatively infrequently and even then only when populations that are critical to a species’ survival have been identified. In the case of the
Box gum grassy woodland Mallee woodland, spinifex, scrub
Mallee woodland, spinifex, scrub
Regenerating forest Hummock grassland Hummock grassland Semi-arid shrubland and woodland Grassy woodland Chenopod shrubland, woodland, sand dunes Coastal heath, grassland, woodland
Riparian woodland Woodland, open grassland Woodland, open grassland Coastal heath, open low shrubland
Open low shrubland, dense closed shrubland, some hummock grassland Swan coastal plain
Reintroduction of threatened species Mulligans Flat Woodland Sanctuary, ACT Scotia Sanctuary, stage 1, NSW
Scotia Sanctuary, stage 2, NSW
Eraring Power Station, Newcastle, NSW Uluru-Kata Tjuta National Park, NT Watarrka National Park, NT Currawinya National Park, Qld Richard Underwood Nature Refuge, Qld Arid Recovery, SA
Banrock Station, SA Woodlands Historic Park, Vic Hamilton Community Parklands, Vic Heirisson Prong, WA
Peron Peninsula, WA
Karakamia Sanctuary, WA
Venus Bay, SA
Grassy woodland Swan coastal plain Swan coastal plain
In situ protection of threatened species Epping Forest National Park, Qld Twin Swamps Nature Reserve, WA Ellen Brook Nature Reserve, WA
275
105,000
1,600 300 100 1,200
1,460
160 170 100 2,500 105 6,000
4,000
484 4,000
2,500 150 34
(continued)
Myrmecobius fasciatus, Isoodon obesulus, Pseudocheirus occidentalis, Bettongia penicillata, Macropus eugenii, Setonix brachyurus
Climacteris picumnus (further species are to be reintroduced) Manorina melanocephalus, Leporillus conditor, Myrmecobius fasciatus, Macrotis lagotis, Bettongia lesueur, B. penicillata, Onychogalea fraenata, Lagorchestes hirsutus (extra-limital) Myrmecobius fasciatus, Bettongia penicillata, Onychogalea fraenata (further species are to be reintroduced) Phascolarctos cinereus Lagorchestes hirsutus Lagorchestes hirsutus Macrotis lagotis Lasiorhinus krefftii Aspidites ramsayi, Myrmecobius fasciatus, Macrotis lagotis, Perameles bougainville, Bettongia lesueur, Leporillus conditor Burhinus grallarius, Macrotis lagotis, Bettongia penicillata, Leporillus conditor Macrotis lagotis (at least two more species are to be reintroduced) Perameles gunnii Perameles gunnii Perameles bougainville, Bettongia lesueur, Leporillus conditor, Pseudomys fieldi Leipoa ocellata, Isoodon obesulus, Macrotis lagotis, Bettongia penicillata, Lagostrophus fasciatus, Lagorchestes hirsutus
Lasiorhinus krefftii Pseudemydura umbrina Pseudemydura umbrina
Table 4.1 Examples of predator-proof fences in Australia, showing the main conservation rationale for construction, location, biome, area enclosed and key species protected Location Biome Area (ha) Key species protected
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Mixed forest, woodland and swamp
Isoodon obesulus, Trichosurus vulpecula, Macropus irma, Setonix brachyurus
Dasyurus viverrinus, Perameles gunnii, Petrogale penicillata
Waterfowl Waterfowl, local fauna native to the area
Ornithorhynchus anatinus, Isoodon obesulus, Bettongia penicillata, Macropus eugenii Myrmecobius fasciatus, Macrotis lagotis, Bettongia lesueur, B. penicillata Burhinus grallarius, Leipoa ocellata, Isoodon obesulus, Bettongia penicillata Dasyurus viverrinus, Petaurus norfolcensis, Bettongia gaimardi Bettongia penicillata Isoodon obesulus, Bettongia penicillata (at least two more species are to be reintroduced)
Wide range of local native fauna Unspecified large kangaroos and wallabies
Macrotis lagotis, Trichosurus vulpecula (nine more species are to be reintroduced) Phascogale calura, Isoodon obesulus, Trichosurus vulpecula, Bettongia penicillata
Key species protected
Sites protecting
