Forensic approaches to monitoring and individually identifying New Zealand vertebrate pests

Abstract

The brushtail possum (Trichosurus vulpecula) is New Zealand’s number-one vertebrate pest in terms of its ecological damage to native flora and fauna. One of the basic knowledges needed to plan, perform and evaluate wildlife management inputs and outcomes is population abundance. Although the most common method used in New Zealand for possum abundance monitoring is the trap-catch method, the bait interference technique has advantageous properties, including its minimal risk to non-target fauna, low operational cost, and ease of use. Similar to the trap-catch method, animal abundance is inferred by an index of proportion of baits interfered with. However, non-toxic bait can be interfered with by many individual animals, and likewise a single individual can interfere with multiple baits, which is an intrinsic drawback to the method, confounding the results due to an unknown probability of multiple interference of bait.

This study assessed the issue of visits to a single bait by multiple species and individuals and visits to multiple baits by a single individual by using forensic science approaches in developing a novel method to identify the responsible species and individual animals from the bitemarks that animals leave as interference.

The issue of identifying multiple species biting on bait was assessed by applying the bait to captive animals of known species and defining the species from its bitemark’s characteristics. Possum and rodent bitemarks were defined by measuring single tooth-mark widths where possums were 2.87 ± 0.51 mm, as opposed to 0.94 ± 0.28 mm for combined rat species and 0.46 ± 0.07 mm for mouse. Paired t-tests showed significant differences (p<0.05) between possum and rodent tooth mark widths. Qualitative description of striation- and impression-type bitemarks also identified characteristic features of possums and rodents.

In order to assess the prevalence of multiple visits, a novel method to identify individual animals was developed by applying the forensic firearm and toolmark examination principle to microscopic features observed on the bitemarks. Trials using known simulated bitemarks, blind test of random bitemarks, known ‘live’ bitemarks from captive animals, and finally, a random bitemark selection from known individuals which reached 89% precision and 99% accuracy. This demonstrated that bitemarks can be used under controlled laboratory conditions to reliably identify individual animals.

Field application of the above methods were conducted for 10 nights in paired blocks of 10 m interval 12 × 12 grids, separated by 100 m. One block was treated with a food lure to follow the most recent pest monitoring protocol. The number of interfered WaxTags® was significantly higher (p<0.05) with food lure up to the third night, and showed no difference afterward. By applying the novel method to the field, previously unknown information was obtained: The mean and range of nightly activities recorded by the 20 identified individuals were 2.5 tags (range 1–33) interfered with, 128.1 m (0–311) travelled distance and 0.11 ha (0–2.8) in covered area. Up to three individuals bit one tag in one night, and WaxTags® were repeatedly bitten for up to six nights at the same location. This study’s novel individual identification method has added another tool in wildlife management techniques.