The Fence

Possum (stuffed) on the fence

The success of the Karori Wildlife Sanctuary depends on the ability to exclude mammalian pests and domestic pets permanently from the Karori Reservoir Valley. The Karori Wildlife Sanctuary fence is the most effective protective barrier against key mammalian predators in New Zealand, but requires modification to successfully exclude mice.

Why a fence?

Because of the urban location of the Karori Wildlife Sanctuary, concern about the future effects of ongoing poison operations on the ecology of the valley and the need to protect threatened birds from domestic pets, it was decided that a fence was the only viable option to secure the site against reinvasion of mammalian pests.

The fence wanders near the suburb of Karori

The fence is designed to exclude 14 species of mammalian pests that have been found in and near the Sanctuary, and also to prevent entry by domestic pets.

It has been built with durable, robust materials, to withstand the Wellington climate, the daily wear and tear of an urban environment and to be easy to maintain. The design does not rely on electrification.

Design trials

A group of 22 conservation managers, scientists and engineers got together in 1993 to study a variety of fence designs. None proved suitable for the needs of the Karori Wildlife Sanctuary so it was decided to design a new predator proof fence.

Key Species for Trialing

An extensive programme of animal trialing began in 1994. The trials tested a range of animal capabilities such as jumping, climbing, digging and their ability to pass through different size gaps.

Trialing the top hat against cats

These trials resulted in several prototype designs that were then tested against nearly 200 animals (including rats, mice, cats, stoats, possums, weasels and ferrets).

Finally we looked at issues like materials, installation costs, wind and visual effects. The trials resulted in four distinctive designs.

The chosen design is the simplest, most robust and easiest to install.
There are three parts to the design:

• curved top hat
• wire mesh wall
• underground skirt

		Materials The fence is made of a tight wire weave mesh with a minimum gap of 6x50mm which was thought to exclude mice. At the base a woven mesh skirt extends outwards below the ground for 400mm. The skirt has proved to be an effective barrier to all burrowing animals. To stop jumping animals the fence only needs to be 1.8 metres high. However the fence has been built alongside a popular recreation track so for safety reasons the height has been raised to 2.2 metres. All materials used in construction of the fence are extremely strong to prevent vandalism and should last 20 years before needing replacement. Construction Detailed design work was completed in 1998 and construction of the fence was completed in August 1999. It is formed on the inside of a 3 metre wide track and is 8.6 kilometers long, completely enclosing the Karori Reservoir Valley.
Prototype fences (click image for larger view)
Evolution of the hat (click image for larger view)

Gate entry

Main gate entry

Public entrance to the Sanctuary
is at a single, heavily monitored entrance at Waiapu Road.

To maintain the pest free status of the Sanctuary, all bags carried by visitors and staff must be emptied and repacked before entering the valley in the same way that visitors to Little Barrier or Kapiti Island Sanctuaries do.

Monitoring

However, there is always the possibility of mechanical failure, breaches of the fence or subsidence allowing re-invasion, so ongoing monitoring of the fence, ground and vegetation, as well as having strategies to detect and control re-invasions, are a permanent and very important part of the management of the Sanctuary.

More information about the predator fence

Download Stephen Fuller's notes on the predator fence (pdf file 136KB).
For further information, contact Keith Calder.