Hawke’s Bay study shows predator control contributes to farmland biodiversity

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Predator control for conservation purposes tends to focus on reserves, sanctuaries and remnants of native habitat rather than open pastures. When researchers publish articles on pastural predator control, they’re generally focused on possums and TB eradication. But landscape scale predator control on multi-tenure farmland is increasingly playing a part in Predator Free 2050 goals and Hawke’s Bay Regional Council is leading the way.

In a research paper just released online and due to be published in the New Zealand Journal of Ecology early next year, they outline the work they’ve been doing and what Regional Council staff and their Landcare Research colleagues have learnt so far.

Someone checking a trap in a field
Hawke’s Bay Regional Council staff-member, Pouri Rakete-Stones notes trapping results from a prototype podiTRAP. Image credit: Supplied

“Although landscape-scale predator control may be desirable, financial and logistical challenges often prevent it. The tools and techniques used to control predators at localised scales may be prohibitively expensive at the landscape scale. Managing wildlife across different land tenures can also be challenging, both logistically and socially. Access to private property may not always be feasible, and landholders may vary in their attitudes towards proposed management activities. Practical and affordable methods are needed to reduce the impacts of invasive predators across multi-tenure, pastoral landscapes.”

In this study, part of Hawke’s Bay’s ‘Cape to City Project’, invasive predators were controlled over 6000 ha of farmland with fragments of native bush adjacent to an 800-ha conservation reserve where intensive predator control had been in place since 1996.

“By removing invasive predators from a pastoral landscape with fragments of native forest, we aimed to facilitate recovery of native fauna. We predicted that predator control would lead to increased abundance and distribution of native prey species. For example, many of New Zealand’s lizard and invertebrate taxa have declined due to the impacts of mammalian predators.”

The study took place on 4 neighbouring pastoral properties: Opouahi, Rangiora, Toronui and Rimu Stations.

“These sheep and cattle stations are mainly covered by introduced pasture grass with fragments of native beech forest. Beech forest fragments range in size from about 10 to 100 ha. Adjoining the study area to the north is Boundary Stream Mainland Island, an area of mixed broadleaf and podocarp forest managed by the Department of Conservation (DOC). Invasive predators have been controlled in Boundary Stream since 1996 as part of DOC’s Mainland Island programme. There was no recent history of predator control on the adjacent pastoral properties.”

Predator control was carried out on Opouahi and Rangiora Stations while Toronui and Rimu Stations were non-treatment areas. In November 2011, Hawkes Bay Regional Council deployed 680 kill trap across an area of 6000 ha. These included 550 DOC-250 traps for mustelids, and 130 Timms traps for cats.

“Traps were spaced 100 m apart in bush fragments or 200 m apart on cleared farmland, based on the assumption that predators were more likely to be found in bush fragments. Traps were baited with various combinations of fresh rabbit meat, a rabbit-based paste (Erayz®, Connovation Ltd) or a synthetic, rat-scented lure (Goodnature).”

During the study, the way the trapping was carried out evolved to be more cost-effective.

“Traps were initially checked every 3 weeks at an annual cost of $5.53 per hectare, however, from November 2014, they were checked four times a year (January, April, June and November), which cost $2.30 per hectare. The DOC-250 traps were left in place for the duration of the study. The Timms traps were left in place for the first year, after which cat control was conducted in two annual pulses (May and August each year).”

A variety of methods were used to control cats. The pulsed cat control was carried out using a combination of two types of live traps – cage (Havahart Trap) and leg-hold (Victor #11/2 soft-catch). Kill traps were also used -Timms and Possum Master – along with opportunistic shooting.

“Live traps were checked daily and captured predators were euthanased. Cat control targeted areas of high rabbit activity as rabbit abundance is a strong predictor of cat abundance.”

Both predator numbers and native species were monitored before at the various sites.

“Predator populations were monitored using large tracking tunnels, which also detected native lizards. Invertebrates were monitored using artificial shelters (weta houses). Occupancy modelling showed that site use by cats and hedgehogs was significantly lower in the predator-removal area than in the non-treatment area. Site use by mustelids also appeared to be lower in the treatment area, although sample sizes were too small to allow firm conclusions. Site use by invasive rats was higher in the treatment area, while that of house mice showed no difference between treatments.”

Some native fauna benefited, but not all species.

“There was evidence of positive responses of some native biodiversity, with site use by native lizards increasing significantly in the treatment area, but not in the non-treatment area. Counts of native cockroaches were higher in the treatment area, but other invertebrates were detected in similar numbers in both areas. Our results show that low-cost predator control in a pastoral landscape can reduce invasive predator populations, with apparent benefits for some, but not all, native fauna.”

Labour costs can be prohibitive on such large, landscape-scale projects, but the Hawke’s Bay team found some innovative ways to make trap-checking less labour-intensive and thus lower costs.

“The spatial coverage of our trapping effort was made possible by placing traps in accessible locations where they could be checked rapidly by staff on an ATV. This design maximised the number of traps that could be checked in a day, thereby increasing the area that could be trapped with the available budget. Our network of kill traps also used mechanical signals that allowed the trapper to see whether a trap had been triggered without dismounting the ATV, saving time and reducing labour costs. Recent developments in wireless sensor networks may further reduce costs of trapping by alerting managers when a trap is triggered.”

The full research article is published online by the New Zealand Journal of Ecology and is freely available.

Predator control on farmland for biodiversity conservation: a case study from Hawke’s Bay, New Zealand (2019 – In Press.