A living ark
The fence
The success of Karori Sanctuary depends on the ability to exclude mammalian pests and domestic pets permanently from the Karori Reservoir Valley. Karori Sanctuary's fence is the most effective protective barrier against key mammalian predators in New Zealand, but requires modification to successfully exclude mice.
Why a fence?
Walking Karori Sanctuary's fenceline
Because of the urban location of Karori Sanctuary, concern about the future effects of ongoing poison operations on the ecology of the Sanctuary 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 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 Karori Sanctuary so it was decided to design a new predator proof fence.
An extensive programme of animal trialling 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.
| Species | Reason tested | Components trialed |
| Possum | Superior climbing ability | Top hat, wall (height) |
| Cat | Highest jumper | Wall (height) |
| Norway rat | Most active burrower | Skirt |
| Stoat | Best jumper of all the smaller animals. Behaviour generally unknown. | Wall (height and mesh), skirt (hat) |
| Mouse | Small size | Wall (mesh) |
A cat tests the fence during trials
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 kilometres long, completely enclosing the Karori Reservoir Valley.
Gate entry
Public entrance to the Sanctuary is at a single, heavily monitored entrance at Waiapu Road.
To assist with the pest free status of the Sanctuary, all bags carried by visitors and staff must be emptied and repacked before entering the Sanctuary 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
Further notes on the predator fence (pdf file 136KB).
