Individual study: The efficacy of fox Vulpes vulpes exclusion fence designs for threatened species protection at the Arid Recovery Project Reserve, South Australia
Moseby K.E. & Read J.L. (2006) The efficacy of feral cat, fox and rabbit exclusion fence designs for threatened species protection. Biological Conservation, 127, 429-437
Many government and private conservation organizations on mainland Australia rely on exclusion fences for the protection and reintroduction of threatened species. Predation by red foxes Vulpes vulpes on native species such as the eastern barred bandicoot Perameles gunnii, mala Lagorchestes hirsutus, burrowing bettong Bettongia lesueur, greater bilby Macrotis lagotis and western barred bandicoot Perameles bougainville can be a major driver of population declines. Fence testing helps to maximize the effectiveness of fence exclosure design, however, most types of exclusion fence have not been trialed in Australia. Experimental pen trials were conducted to test the efficacy and cost effectiveness of fox exclusion fences at the Arid Recovery conservation project reserve.
Study site: The Arid Recovery conservation project has removed red foxes Vulpes vulpes, European rabbits Oryctolagus cuniculus (see: www.conservationevidence.com/ViewSummary.aspx?ID=10421) and feral cats Felis catus (www.conservationevidence.com/ViewSummary.aspx?ID=10419) from a 60 km² fenced reserve. Fox densities around the reserve at the time of the fence trials were approximately 0.6-0.9 individuals per km². The habitat is lowland shrubland/chenopod (dominated by saltbush Atriplex spp. and bluebush Maireana spp.). Two trials were set up, one (Trial 1) in a 119 x 25 m test area, and the second (Trial 2) on a much larger, reserve-wide scale.
Trial 1: Trial 1 was conducted from February to December 2004.
Fox capture & release - Foxes were caught using soft rubber leg-hold traps set around the reserve perimeter and upon capture, quickly released into the pens. Only uninjured foxes were used in the trial. Released animals were observed until their escape or retreat into the shelter of vegetation cover. Dents, rips and fur caught on the netting were used, as well as direct observation, to identify the exact escape points.
Fence design - A 119 x 25 m rectangular pen was constructed using pine posts 1.8 m high (from ground level) spaced at 10 m intervals, and at each corner of the pen. Corrugated iron was nailed around the fence posts to inhibit foxes from scaling them. Seven selvage wires were attached to these posts at 30 cm intervals. Wire mesh netting (40 mm mesh diameter) x 1.4 (wire thickness) x 900 mm (high) was clipped to them from ground level to 90 cm height using ring fasteners. A net foot apron (40 x 1.4 x 300 mm) was clipped to the ground level selvage wire, stretched into the exclosure to 30 cm, and buried. A non-rigid overhang was then created by clipping wire netting (50 x 1.4 x 1200 mm) to the fence at a height of 90 cm, then vertically weaving 90 cm lengths of fencing wire through the top 60 cm and bending it to form a complete arch. Extra netting was added to reinforce the corners. (This was the fifth of a total of nine designs used in fence trials - see also Cases 221 and 223).
Within the pen, two perpendicular fences (designated 'Design 8' and 'Design 9') were fitted around a central section of dense vegetation. Foxes were released in the open outer sections and encouraged to breach the central fences to gain greater cover which was present on the opposite side of the fence.
Design 8 used 30 x 1.4 x 1200 mm netting, with 30 cm buried as a foot apron and 90 cm erected vertically. The top of the netting was reinforced with 75 mm of barbed wire, plus additional 10 cm lengths of PVC conduit strung 15 cm above. These freely rotating conduit lengths served to prevent foxes from gaining a purchase to the upper wire if climbing or jumping onto the top of the fence.
Design 9 was a shorter version (115 cm tall) of design 5 (see original paper). Steel droppers, 1.65 m high were spaced at 10 m intervals with five selvage wires strung at heights of 0, 30, 65, 90, and 115 cm. The netting was 30 x 1.4 x 1200 mm, consisting of a 30 cm horizontal footing and 90 cm vertical netting. The top netting (50 x 1.4 x 900 mm), included 30 cm of vertical netting and a 60 cm ‘floppy’ overhang.
Trial 2: The reserve scale trials were conducted between 1997-2005. The same design as the 119 x 25 m rectangular pen was incorporated into the 14 km² external fence of the reserve. However, two electric wires at heights of 1.3 and 1.6 m were added to this fence (which was designated 'Design 7'). A minor alteration was needed to add extra support for the electric wires by repositioning them adjacent to selvage wires at 1.2 and 1.5 m. The last fox was removed from the exclosure in February 1999, and incursions into the reserve were monitored between July 1999 and January 2005.
In January 2005, another part of the reserve, Red Lake, was fenced. The fencing followed design 9 but with the addition of steel droppers spaced every 7 m to give extra rigidity to the ‘floppy’ overhang.
Effectiveness of fence designs:
Design 7 - As of July 2005, no fox has been seen within the 14 km² external fence of the reserve for six years. A total of 130 foxes have been caught in leghold traps set outside the perimeter between 1999 and 2004. This suggests that foxes come into regular contact with the fence but are unable to scale or dig under it.
Design 8 - this fence design proved ineffective, as two male foxes escaped within two minutes of release. One pushed itself through a 75 mm gap between the barbed wire and the top of the netting. The other climbed over the rollers at the corner of the fence.
Design 9 - 27 foxes were tested in design 9. Biting and ramming were the most attempted ways of escape. However, no fox escaped, whether by digging under, chewing through or jumping on top of the fence.
Best fence features: The rounded arc 60 cm ‘floppy’ overhang successfully contained and excluded foxes in the trials. The lower height fence (Design 9) of 1.15 m was a suitable barrier because the overhang curved back, making it difficult for the animals to jump over it. The ‘floppy’ nature of the overhang was not a factor in its success. However, floppy netting was easier and cheaper to install than a rigid structure.
Steel posts (recycled bore casings were used to minimise costs) were more effective than wooden ones, though large amounts of netting around the posts facilitated climbing. Foxes were able to climb up the wooden posts. A modification such as a wider overhang may be necessary when planning corners.
The non-electrified ‘floppy’ overhang (Design 5) proved just as effective as the electrified fence (Design 7) in the in situ field trials.
The 30 cm foot apron was effective at preventing foxes from digging at the base of the fence.
The 30 mm (mesh diameter) hexagonal netting, which accounted for 57% of the cost of Design 9, proved unnecessary because the 40 mm (mesh diameter) netting was effective at barring foxes. Material costs using 30 mm diameter wire mesh was AUD$8,814/km, this was reduced to AUD$6,939/km using 40 mm diameter wire mesh.
Conclusions: In these trials, steel post, 40 mm diameter non-electrified fences with a 60 cm ‘floppy’ overhang proved to be the cheapest and most effective fox exclusion fence design. Where netting joints do not overlap, it is recommended that maximum spacing of clips to join net sections is 10 cm to prevent foxes from pushing through gaps and escaping. Electric fences that have no overhang above them will be ineffective at deterring foxes. The foxes were able to jump over the 30 cm high electrified wires and between wires placed more than 8 cm from the netting.
Note: If using or referring to this published study, please read and quote the original paper.