Predator-proof fences are critical for eco-sanctuaries but without monitoring behind the fence a predator incursion may go unnoticed with catastrophic consequences.
We recently read a distressing article in a back issue of Wilderness magazine about a stoat intrusion at the predator fenced Orokonui Ecosanctuary in Otago. The stoat was detected after the sanctuary's population of South Island saddleback mysteriously disappeared.
Unfortunately a mated female stoat is able to delay the onset of her pregnancy until conditions to raise her young are optimal so it only requires one intrusion to result in the establishment of a small population. You can download a copy of the article here.
This story reinforces the need to maintain a method of monitoring or surveillance appropriate to the design of your project. In a predator exclusion area such as a fenced sanctuary or an island, where predators have been eradicated, a surveillance system is necessary to allow the detection of an intrusion as quickly as possible.
Intuitively this surveillance system would focus on the perimeter of the area. However it must also extend into the core of the area as an arriving predator is not site attached and can quickly move away from the boundary and beyond any surveillance system limited just to the boundary.
In a predator controlled area, a monitoring system is necessary to allow the periodic assessment of predator abundance. The results of this can either inform when predator control should be carried out, or allow a judgement of how effectively control is reducing predator numbers.
The characteristics of surveillance and monitoring systems differ as they are essentially measuring different things. Without predators a surveillance system must be able to unambiguously detect the presence and identity of an invading predator.
Rodents, possums and mustelids all leave distinctive signs so the recording of bite marks and footprints are possible. Long lived surveillance points that don't require regular servicing means their effectiveness doesn't reduce due to baits or lures degrading between surveillance checks.
In contrast, predator monitoring systems need to measure changes in the abundance of predators. However, measuring the actual abundance of predator populations is rather difficult so monitoring techniques instead focus on measuring an index of predator density. This could be a trapping index, usually expressed as captures per 100 trap nights, or it could be a tracking index.
The key to designing an effective monitoring system is to ensure it is easily repeatable and it is representative of the habitats that exist within the control area. Ideally the monitoring system should use a technique that is different to the predator control and if the control area has 15% of shrubland, then 15% of the monitoring points should be within this habitat type.
It must be remembered that an index of predator abundance has some particular characteristics. While it provides a measure of the change of predator abundance between monitoring episodes at a control site it is unreliable for comparing predator abundance between different control sites.
Monitoring and surveillance provides insight into what is going on with predator populations. Without it predator control is like trying to solve a puzzle in a black box and risks wasting precious time and resources as the unfortunate experience at Orokonui amply demonstrates.
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