Action: Install mammal crossing points along fences on farmland
Key messagesRead our guidance on Key messages before continuing
- Four studies evaluated the effects on mammals of installing mammal crossing points along fences on farmland. Two studies were in Namibia and one each was in the USA and the UK.
KEY COMMUNITY RESPONSE (0 STUDIES)
POPULATION RESPONSE (0 STUDIES)
BEHAVIOUR (4 STUDIES)
- Use (4 studies): A study in the USA found that pronghorn antelopes crossed a modified cattle grid which prevented escape of domestic sheep and cows. A controlled, before-and-after study in Namibia found installing swing gates through game fencing reduced the digging of holes by animals under the fence, whilst preventing large predator entry. A study in the UK found that a vertical-sided ditch under an electric fence allowed access by otters. A before-and-after study in Namibia found that tyres installed as crossings through fences were used by wild mammals and reduced fence maintenance requirements.
Fences erected to retain domestic livestock or, in some cases, exclude wild herbivores or carnivores may also act as a barrier to non-target species. Crossings may be installed to retain habitat connectivity for specific species. Crossing designs vary between different situations depending on the nature of the original fence and the species being targeted for continued access.
For wildlife-permeable fencing (as opposed to specific crossing points) see Use livestock fences that are permeable to wildlife.
Supporting evidence from individual studies
A study in 1965 of grassland at a site in Wyoming, USA (Mapston et al. 1970) found that a modified pass based on a cattle grid design enabled passage by pronghorn antelopes Antilocapra americana whilst preventing escape of domestic sheep and cows. A total of 100 antelope were observed jumping across the grills, during five separate crossing events. Antelopes crossed grills at fence corners more than they crossed those along straight fences. A range of designs were trailed, the optimal being a 6-foot-long grill in a 5.5-foot-wide fence opening. The grill consisted of 13 bars at 6 inch-intervals. These were mounted on 10-inch-high timbers with earth ramps running up to both ends.
A controlled, before-and-after study in 2001–2002 on a game and livestock farm in Otjiwarongo district, Namibia (Schumann et al. 2006) found that installing swing gates along animal routes in game fencing reduced the digging of holes by animals under the fence, whilst preventing large predator entry. Fewer holes were dug under a fence section with gates installed on animal routes (12.2 holes/survey) than on sections with evenly spaced gates (20.2 holes/survey) or no gates (19.1 holes/survey). Before gate installation, there was no significant difference in hole numbers between sections (animal route gates: 20.0 holes/survey; evenly spaced gates: 25.7 holes/survey; no gates: 21.7 holes/survey). Warthogs Phacochoerus aethiopicus were the most frequent gate users. Jackals Canis mesomelas, cheetahs Acinonyx jubatus and leopards Panthera pardus passed through holes but not the gates. A game fence (4,800 m long) was divided into three equal sections. One had six gates on established animal routes, one had eight evenly spaced gates and one had no gates. Swing gates comprised a metal frame (45 × 30 cm) covered with galvanised fencing (75-mm mesh). Holes were surveyed and filled at 3–15-day intervals, from August 2001 to April 2002. Animals were identified by signs and heat sensitive cameras.
A study in 2005 at a wetland reserve in Cambridgeshire, UK (Gulickx et al. 2007) found that a vertical-sided ditch under an electric fence allowed access to the site by otters Lutra lutra. Several otter spraints were found within the fenced area. Some were at the edge of the ditch under the fence, indicating probable otter use of that route. No evidence of red foxes Vulpes vulpes using the route was identified. The ditch, 1 m deep and 3 m wide, flowed under the boundary of the fenced reserve. Ditch sides were supported by wooden boards, to maintain the banks as vertical, so that entry could only be achieved by swimming. The fence, 1.3 m high and 2 km long, was electrified year-round. It was installed in 2005 to deter entry by foxes, for the purpose of reducing predation on nesting birds.
A before-and-after study in 2010 on a farm in Namibia (Weise et al. 2014) found that tyres installed as passageways through fences facilitated movements of wild mammals, especially carnivores, and reduced fence maintenance requirements. During 96 days, 11 mammal species, including nine carnivores, used one crossing. The most frequently recorded species were black-backed jackal Canis mesomelas (44 occasions), porcupine Hystrix africaeaustralis (21 occasions) and cheetah Acinonyx jubatus (nine occasions, seven different animals). Fewer fence holes needed mending after tyre installation (13.6 holes/day) than before (31.3 holes/day). Forty-nine discarded car tyres (37 cm radius opening) were installed at ground level into a 19.1-km-long, 2.4-m-high fence. Tyre locations, 35–907 m apart, were prioritised to areas of high warthog Phacochoerus africanus digging activity. One tyre was monitored with a camera trap for 96 days from August–December 2010. Holes needing maintenance were counted for 10 days before and 10 days after tyre installation.
- Mapston R.D., Zobell R.S., Winter K.B. & Dooley W.D. (1970) A pass for antelope in sheep-tight fences. Journal of Range Management, 23, 457-459
- Schumann M., Schumann B., Dickman A., Watson L.H. & Marker L. (2006) Assessing the use of swing gates in game fences as a potential non-lethal predator exclusion technique. South African Journal of Wildlife Research, 36, 173-181
- Gulickx M.M.C., Beecroft R.C. & Green A.C. (2007) Creation of a 'water pathway' for otters Lutra lutra, under an electric fence at Kingfishers Bridge, Cambridgeshire, England. Conservation Evidence, 4, 28-29
- Weise F.J., Wessels Q., Munro S. & Solberg M. (2014) Using artificial passageways to facilitate the movement of wildlife on Namibian farmland. South African Journal of Wildlife Research, 44, 161-166