Morgan Coleman

A field test of two methods for density estimation

Abstract Density of wildlife populations is a key variable for management, yet reliable estimation is elusive. We tested one established method (trapping webs and distance analysis) and one novel method (inverse prediction from capture–recapture data) on a population of brushtail possums (Trichosurus vulpecula) whose density also could be determined by exhaustive removal. The study area was approximately 315 ha of coastal plantation forest surrounded on 3 sides by sand and water. We placed 4 lines of 9 cage traps at 20-m spacing in a square to form a “hollow grid.” We set 5 hollow grids, each comprising 36 traps, for 5 days; we tagged and released possums. We later set 5 trapping webs of 50 traps each at the same sites; we caught possums and removed them over 4 days. Wide-area removal used a combination of acute poisoning and leghold trapping. The estimate of density by inverse prediction (1.88/ha, SE = 0.26) was consistent with the removal estimate (2.27/ha), whereas estimates from trapping webs were positively biased (6.5 to 8.0/ha, depending on method of analysis). The inverse prediction method frees capture–recapture from the straitjacket of conventional grids and should allow accurate landscape-scale estimation of density once the requisite trapping effort is identified.

The effect of poisoned and notional vaccinated buffers on possum (Trichosurus vulpecula) movements: minimising the risk of bovine tuberculosis spread from forest to farmland

Context. Vaccination of brushtail possums (Trichosurus vulpecula) has been proposed as a cost-effective alternative to lethal control for preventing potentially bovine tuberculosis (Tb)-infected possums from crossing forested buffer zones that abut farmland. Aim. Evaluation of these two management option requires an estimation of the buffer width required to reduce the risk of disease spread to an acceptable level. Methods. The movements of two groups of adult and subadult possums were monitored for up to 12 months in the Kaimanawa Range, North Island of New Zealand, using GPS technology. One group was in untreated forest immediately adjacent to a recently poisoned forest buffer, and the second group was 2 km further into untreated forest, which mimicked a vaccinated buffer with no reduction in possum abundance. Key results. Close to the poisoned buffer, where the initial population density was 0.49–1.45 ha–1, adult possum home ranges averaged 10.2 ha in the summer immediately after control and 9.5 ha in the following winter. Two kilometres into the untreated forest, where the density was >3 ha–1, the corresponding values were only 3.5 ha and 2.8 ha. Over the first 6 months of monitoring, a ~350-m wide poisoned buffer would have contained 95% of movements by adult possums, as well as movements by most individuals, but a ~150-m wide vaccinated buffer would have been as effective. Equivalent results for the subsequent 6-month period were ~450 m and ~200 m for poisoned and vaccinated buffers, respectively. Movements by possums were not biased in the direction of the population ‘vacuum’ created by the poisoning operation. When subadult possums were included in the analysis, buffer widths of ~500–600 m for both poisoning and vaccination would be required to contain 95% of new den site locations. Conclusions and implications. Detailed data on possum movements provide a means for agencies managing Tb to specify the width of a buffer, subject to an acceptable level of risk that it could be breached by a potentially infected possum. As well as depending on the width of a treated buffer, the final cost-effectiveness of vaccination compared with poisoning will depend on the relative cost of applying the two control techniques, and the frequency of application required either to prevent Tb from establishing (in the case of vaccination) or to suppress possum density (in the case of lethal control).

Evaluation of two relative-abundance indices to monitor brushtail possums in New Zealand

Population monitoring of brushtail possums (Trichosurus vulpecula) is an essential part of their management in New Zealand, with a trap-catch removal method being used most commonly. An alternative monitoring method (bait-interference), using bite marks on wax blocks, has been promoted as a more cost-effective alternative to using traps. However, neither of these methods has been validated. We assessed the utility of these two methods regarding their accuracy (unbiasedness and precision) in detecting changes in possum abundance by comparing the estimates of relative change in possum density following control obtained from both methods with the kill rate among radio-collared possums in the same study areas. In each of seven control operations, 48–50 possums were collared with mortality-sensing radio-transmitters, and trap-catch and wax-block lines were assessed before and after control. The correlation between trap-catch and radio-transmitter kill estimates (R2 = 0.91) suggests that trap catch, as currently used to monitor relative possum abundance, appears to be sufficiently accurate to manage these pests, with any bias being small. The kill estimates based on the wax-block monitoring were correlated less strongly with the radiotransmitter estimates than the trap-catch estimates (R2 = 0.66), although still significant. Until the extent of the potential bias in the wax-block estimates is known, we recommend that traps continue to be used as the main method to monitor possum abundance.

Breeding capacity of female brushtail possums Trichosurus vulpecula in captivity

Abstract This study records baseline data on the reproductive capacity of female brushtail possums (Trichosurus vulpecula) at Rangiora (near Christ‐church) and compares the breeding season of captive and free‐living possums. Repeat breeding was stimulated in captive females by removal of pouch young soon after birth. Captive females that bred, produced pouch young from March to October (more or less continuously), and some females produced four pouch young in one season. Free‐living possums around Rangiora had two distinct periods of birth, in autumn (April/May/June) and spring (October/November). Reproduction ceased earlier in the year in captive possums than in possums in the wild, perhaps because of the stress of captivity. The probability of a given possum producing pouch young, and the number of pouch young produced, was not influenced by year, pelage colour, or the number of males in the breeding group. Weight was important: possums heavier than 3 kg were more likely to produce at least one pouch yo...

Comparison of camera traps and kill traps for detecting mammalian predators: a field trial

Currently there are few robust techniques being used in New Zealand to assess the results of pest control targeting predatory mammals such as stoats (Mustela erminea), feral cats (Felis catus) and hedgehogs (Erinaceus europaeus), with most operations using capture rates from kill traps as a measure of success. We conducted field trials of camera traps to detect these species at two sites—Macraes Flat and Tasman Valley—where intensive predator trapping is conducted by the New Zealand Department of Conservation. We compared camera traps with kill traps in terms of capture rate per 100 trap nights. Camera traps detected all three target species, as well as various non-target animals. Capture rates of cats and hedgehogs were higher with cameras than with kill traps. Comparisons for stoats were inconclusive due to a low number of detections. We suggest that camera traps are suitable for monitoring relative abundance of cats and hedgehogs, and recommend further testing in areas of higher stoat abundance.

Assessing Movements of Brushtail Possums (Trichosurus vulpecula) in Relation to Depopulated Buffer Zones for the Management of Wildlife Tuberculosis in New Zealand.

In New Zealand, managing the threat of bovine tuberculosis (TB) to livestock includes population reduction of potentially infectious wildlife, primarily the brushtail possum (Trichosurus vulpecula). Population control is often targeted on forested buffer zones adjacent to farmland, in order to limit movements of possums across the buffer and reduce the risk of disease transmission to livestock. To assess the effectiveness of buffers in protecting livestock we analysed GPS telemetry data from possums located in untreated forest adjacent to buffers, and used these data to characterise patterns of movement that could lead to possums reaching farmland during the season when most dispersal occurs. Analyses of movement data showed that the direction of dispersal by sub-adult and adult possums and the extent of long exploratory movements were not biased toward forest buffers, even though these provided vacant habitat as suitable for possums as untreated forest. Instead, dispersal and exploratory movements were uncommon even for sub-adult possums and such events typically lasted <10 days. Dispersing possums settled predominantly in river valleys. A simulation model was developed for the 3-6-month dispersal season; it demonstrated a probability of <0.001 that an infected possum, originating from a low-density population with low disease prevalence in untreated forest, would move across 3 km of recently controlled forest buffer to reach farmland. Our results indicate short-term reduction in the risk of TB transmission from possums to livestock in New Zealand by the use of depopulated buffer zones, while acknowledging that the threat of disease spread from untreated forest is likely to increase over time as possum population density and, potentially, TB prevalence among those possums, increase in the buffer zone.