Long-term automated monitoring of the distribution of small carnivores

1. What they did

Small carnivores such as stoats are hard to monitor: they are rare, wary, wide-ranging and often avoid traps, and short surveys can miss animals that live in the area. The authors tested the Scentinel, a new automated “smart” tracking tunnel. Each time an animal enters, a weighbridge in the floor weighs it, an infrared camera photographs it, and a data logger records the date and time. Animals above a set weight are given a small dose of bait, and a lure outside the tunnel draws animals in.

The trial had two stages in New Zealand’s North Island:

• Checking identifications (February to April 2005). On farmland near Waotu, 24 Scentinels ran continuously for about 11 weeks. The animals in the photographs were checked against footprints left on tracking papers inside the tunnels.
• Comparing methods (May to June 2005). In native forest at Te Umukaraka Bush, they set three lines of 10 standard footprint tracking tunnels 1 km apart, following the Department of Conservation (DOC) protocol, with two Scentinels on each line. The tracking tunnels were run for three short sessions (one night for rats with peanut butter, then three nights for stoats with rabbit meat), while the Scentinels ran continuously for five weeks.

They also made a rough calculation comparing the yearly cost of the two methods.

2. What they found

Identifications on farmland

• The Scentinels recorded 1559 visitors in 1718 trap-nights, and 98% were identified to species from photographs and weights. Most were hedgehogs (871 visits), rats (283), ferrets (198) and cats (98), with some possums and rabbits.
• The tracking papers confirmed the camera identifications. The footprints alone identified fewer visitors (86%), partly because the papers were changed only weekly and prints became confused. They did detect 28 mouse visits, which the cameras were set to ignore.
• There were 300 false events, mostly from minor mechanical faults that were fixed as the design improved, or from cattle or possums.
• The photographs showed some unexpected behaviour: some rats seemed to avoid stepping on the ink pads, and some animals visited in groups.

Comparison in forest

• Four of the six Scentinels worked without error. Two were knocked over by large animals, causing most of the 107 false events. Nearly all visitors (99%) were identified: 656 rat visits, 15 possum visits and 9 stoat visits.
• The tracking tunnels recorded rats and mice on all three lines, with no clear difference between lines. Rain spoiled the first session.
• The Scentinels gave a very different picture: 88% of rat visits were on just one line. The rats’ weights suggested many different animals, so rats may have been patchily spread rather than even.
• A Scentinel first recorded a stoat on 31 May. The tracking tunnels did not record a stoat until 16 June. Weights suggested at least one female (147 g) and one or more males. The eight visits by a male, on two lines 1 km apart, looked so similar that they may all have been the same animal, although the DOC protocol would count them as two.
• Seven of the nine stoat visits were on the same line where rats were most common.

Costs

Scentinels would cost about four times more to set up than tracking tunnels, but by the authors’ calculation a survey with Scentinels would need 12 staff-days a year instead of 36. They estimated that Scentinels could give more detailed data for less than half the yearly cost.

3. What they concluded

• The Scentinel performed reliably in the field, and its cameras documented the presence and behaviour of individual mustelids and rodents.
• Tracking tunnels are cheap to make but take much labour to use; they can sample many sites over a short period. Scentinels cost more to make but less to run; they sample fewer sites over a long period and give more detail, including earlier detection of stoats.
• Where volunteers keep labour costs low, tracking tunnels may still be preferred. Where labour costs are high and early detection of invaders is critical, Scentinels may be the better choice.
• Short monitoring periods can miss wide-ranging carnivores that visit any one place only now and then. The patchy rat records and the stoat records raise questions about sampling design that need further testing.

4. How tracking was or was not used

Footprint tracking was used in two ways. In the first stage, footprints on tracking papers inside the Scentinels were used to check the camera identifications. In the second stage, standard footprint tracking tunnels with ink pads and paper were the comparison method. The papers were taken to the laboratory and the footprints identified by comparison with a published identification guide (Ratz 1997). Tracking rates were scored as the percentage of tunnels on a line marked by each species. No tracks and signs were read in the field and no trailing was reported.

5. Tracker or observer certification

Not reported. The paper does not say who identified the footprints on the tracking papers or how experienced they were.

6. Other technologies, skills or innovations

• The Scentinel: a tunnel with a programmable weighbridge, a fish-eye infrared camera, a data logger and two aerosol bait cans. Weight thresholds decide whether a visitor is ignored, photographed, or photographed and given bait. Data are stored and downloaded on command, so the devices did not need visiting until the end of a survey.
• An egg-based bait that stays fresh in sealed cans, a daily “taster” lure outside the tunnel, ferret anal gland lure and peanut butter.
• Body weight used to help identify species and the sex of stoats.
• A newer model (Mk 6) includes an optional automated Passive Integrated Transponder (PIT) tag reader for recognising marked individuals.

7. Implications for conservation (from the paper)

• Automated devices can record detailed monitoring data on small carnivores in remote areas over long periods, without interference or bait loss from common non-target species.
• Detecting rare animals early matters in places such as New Zealand’s kiwi sanctuaries, where stoats must be kept at low numbers, and in Australia, where small threatened marsupials are expected to recover after fox control.
• Labour, not equipment, is the main cost of regular monitoring.