Pest fencing or pest trapping?
New Zealanders’ interest in ecological restoration has never been greater. There are now more than 60 sanctuaries around the country where introduced mammal pests are being controlled to protect native flora and fauna.
Whether they are managed by the Department of Conservation (DOC) or community organisations, the sanctuaries face the same challenge: to achieve the best possible predator control with limited funding. A key question is whether the most cost-effective primary method of pest control is through fencing or trapping.
Multi-disciplinary approach
To answer that question we combined Landcare Research pest ecology and management expertise and modelling capability, with DOC’s expertise and data from two large predator management programmes. At Macraes Flat (Otago) DOC is using trapping and exclusion fences to protect the last stronghold for endangered grand and Otago skinks, while at Burwood Bush (Southland) leaky fences combined with trapping inside and immediately outside the fences is being used at the centre for captive breeding of endangered takahē.
We considered the costs of three options – mammal exclusion fences, semi-permeable fences and trapping – over a 50 year period and factored in the effectiveness of each option for reducing mammal predator numbers.
Using typical baseline costs and predator control figures we calculated that an expensive exclusion fence, designed to keep out all mammalian pests, is the most cost-effective option for areas smaller than about 1 ha; that a lower-cost, semi-permeable fence which will knowingly ‘leak’ some pests is the best option for 1-219 ha; and that trapping (based on 0.2 traps per hectare and a 1,500m buffer area to reduce predator reinvasion) was the best option for areas above 219 ha.
The researchers noted the outcomes would be different if a leaky fence could be made even cheaper. For example, reducing the cost from $120m to $100m-1 nearly doubled its range as the cheapest method from 0.5-50 ha to 0.5-78 ha, and extended its cost-effective range from 0.75-219 ha to 0.5-364 ha.
Emerging trapping technology, such as self-resetting traps or long-life lures, would improve the trapping proposition, as would a reduction in trap maintenance costs. If costs could be trimmed from $300 to $100 per trap per year (e.g. using long-life lures), trapping would become the most cost-effective method for areas greater than about 15 ha.
As expected the model showed that costs per hectare gradually increased with decreasing protected area, escalating dramatically for areas below 20 ha. The cost of protecting 100 ha, for example was $885-$1,709 per hectare per year, compared with $3,233-$5,853 per hectare for 10 ha. Interestingly if a sanctuary manager has an annual budget of $200,000 they could protect 132 ha with a high-quality exclusion fence, 272 ha with a leaky fence, and 807 ha with trapping.
The modelling supports the current thinking that cheaper leaky fences should be considered for small- to medium-sized protected areas, especially if the goal is broad biodiversity improvements. However indigenous species that are highly sensitive to predation can only ever be protected on the mainland by exclusion fences.
This research benefited from close working relationships between Landcare Research and DOC, with two team members from each organisation.
Simple economic guide
The research findings have been peer-reviewed and published, and presented at the biosecurity and pest control NETS (National Education and Training Seminar) Conference, the 2013 Sanctuaries of New Zealand Workshop, and the 2013 New Zealand Ecological Society Conference.
This research provides a simple economic guide for agencies and communities contemplating the obvious options for sustained pest control.