Natural immigration of brushtail possums,Trichosurus vulpecula,onto Aroha Island,Kerikeri Inlet,Bay of Islands,New Zealand

Abstract

Aroha is a 5 ha island in Kerikeri Inlet, Bay of Islands, which is joined to the North Island mainland by a 200 m long causeway. Introduced Australian brushtail possums (Trichosurus vulpecula) spread into the general area only in the mid 1970s, and were first trapped on Aroha Island in June 1979. Since then possums have regularly crossed the causeway, and have been systematically trapped to keep the island free of them. Two hundred and sixty two possums have been trapped on Aroha Island to December 1990, and the annual catch has increased steadily since 1981. About equal numbers of males and females have been trapped. Most possums were caught in March-April and fewest in July-August. Captures on the island were clustered, particularly during the breeding seasons. Average body weight was 2.2 kg; 97% were greys and 3% blacks; the young of females trapped on the island had been born in all months except December to February. In colour, body weight, and breeding pattern the possums trapped on Aroha Island were similar to other possum populations from the northern North Island. Most of the possums invading Aroha Island were 1-2 years old: in contrast to findings from previous studies of possum dispersal, Arona Island was invaded year-round by young male and female possums equally. Some older animals may also have been attracted to the island by fruits available in an orchard there. Maintenance of the possum-free state of Aroha Island will require continuous management.     

Noninvasive recovery and detection of possum Trichosurus vulpecula DNA from bitten bait interference devices (WaxTags)

The brushtail possum is a major agricultural and ecological pest in New Zealand. A novel noninvasive DNA sampling tool for detecting its presence (WaxTags, or WT) was tested. DNA was recovered from saliva left on WT, and two lengths (407 bp and 648 bp) of the cytochrome c oxidase I (COI) barcoding region were amplified by polymerase chain reaction (PCR). PCR products were considered (+) when a DNA band was clearly visible by electrophoresis. Different factors that might affect PCR (+) were investigated with captive possums: (i) both extraction protocols of the QIAGEN DNeasy Blood and Tissue Kit, (ii) effect of an overnight or longer delay of up to 3 weeks before DNA extraction on both COI amplicons, and (iii) effect of the individual, order and magnitude of the bite. Extraction protocols were not significantly different. The effect of the overnight delay was not significant, and amplification of the short amplicon was significantly higher (100%) than for the long fragment (48%). After a two or 3‐week delay, the short amplicon had 94% and 56% PCR (+), success rates, respectively. Individual, order and magnitude of a bite had no significant effect. The delay trial was repeated with WT from the wild, for which PCR (+) rate of the short amplicon was 63%, regardless of freshness. Four microsatellites were amplified from captive WT samples. We conclude that DNA from saliva traces can be recovered from WT, a potential new tool for noninvasive monitoring of possums and other wildlife.     

Are smaller subspecies of common brushtail possums more omnivorous than larger ones?

Body size is often associated with a dietary divergence within taxonomically related groups so that large animals are often folivorous, while smaller species shift progressively towards omnivory or carnivory. This trend may be influenced by allometric constrains which result in relatively high energetic requirements, but low gut capacities in small animals, compared to their large counterparts. The common brushtail possum (Trichosurus vulpecula, Phalangeridae) has six subspecies ranging widely in weight (1–4 kg). They are not strictly folivorous, but supplement their diet with more nutritious, non‐foliar foods. We predicted that T. vulpecula subspecies diverged in diet in association with body size, with smaller subspecies consuming higher proportions of non‐foliar foods. We assessed this with a review and a meta‐analysis of previous Australian studies. We also investigated the previously unquantified diet of T. v. hypoleucus at three sites in the northern jarrah forest of Western Australia. Results from the meta‐analysis and the review supported our prediction. However, the large variability in the data highlighted their limitations and those of the techniques commonly used to quantify the diet of T. vulpecula. Nonetheless, small subspecies of T. vulpecula appear to consume higher proportions of non‐foliar foods. These results should encourage further research into the body size/diet relationship within T. vulpecula and other possum species. Results from the dietary study of T. v. hypoleucus emphasized their omnivorous diet, which was dominated by foliage and flowers and smaller proportions of invertebrates, seeds and fruits. The common brushtail possum is seldom an exclusive arboreal folivore, but rather ranges from folivory to omnivory.     

Sex-biased dispersal and a density-independent mating system in the Australian brushtail possum, as revealed by minisatellite DNA profiling

Natal dispersal can have important effects on mammal population structure and dynamics following a local population crash. Such dispersal is of practical importance when applied to the control of pest species because dispersal may significantly, and undesirably, reduce the population recovery time following a control operation. The relative dispersal rate of the sexes is also critical because that too will affect the rate of population increase. Here, we describe a field experiment in which we reduce the density of two populations of the Australian brushtail possum, and use genetic similarity, as estimated by minisatellite DNA profiles, to investigate dispersal in the original (undisturbed) and recovering populations. Our results show that the genetic similarity within the undisturbed populations was lower between males than between females. Conversely, the genetic similarities between males and females in the two recovering populations were not significantly different, while relatedness among males was significantly higher in the recovering populations when compared with those in the pre-removal populations. These data indicate two important characteristics of dispersal in possums: (i) that dispersal in established populations is sex biased towards males; and (ii) that within the first 3 years following population control, 'the vacuum effect', whereby individuals from areas adjacent to a control area expand their home range and invade the depopulated area, is the most important factor in the re-colonization process for possums. We found no evidence that the mating system, which is polygynous, varied when the density was markedly reduced. These results indicate that drastic reductions in population density by conventional control will not affect the rate of spread of biological control agents that rely on sexual transmission for dissemination.     

Effectiveness of chain‐springs on leghold traps for reducing injuries to captured brushtail possums (Trichosurus vulpecula)

Abstract

Leghold traps continue to be an essential tool for managing brushtail possums (Trichosurus vulpecula) in New Zealand. Concerns about the injuries such traps cause have given rise to efforts to make these traps more humane. One trap modification that had potential for reducing injuries was the placement of an appropriately sized spring between two links of a trap chain to provide a cushioning effect when the chain is extended. Such springs were added to Lanes‐Ace and Victor No. 1½ traps and the injuries caused to captured possums were compared with those to possums captured in the same traps without springs or in Victor No. 1 traps. The addition of chain‐springs reduced the severity of injuries significantly, but injury scores were still significantly higher than for possums captured in Victor No. 1 traps. Consequently, although chain‐springs improved the welfare performance of the Lanes‐Ace and Victor No. 1½ traps, they did not do so to such an extent that those traps could be considered as humane as the currently acceptable Victor No. 1 traps.     

新西兰的负鼠——一场尚未完结的与外来入侵有袋类的较量（英）

为发展毛皮工业,新西兰19世纪中叶从澳大利亚引入刷尾负鼠（Trichosurus vulpecula),后成为新西兰一大害兽,刷尾负鼠不仅因破坏当地植被,捕食鸟蛋等而严重影响了新西兰的生物多样性,而且因其传播结核病菌对当地畜牧业造成了很大损失,新西兰每年花费巨额资金来控制这一害兽,虽然刷尾负鼠在一些孤立的岛屿和重复自然保护区已完全被消灭或得到有交和的控制,但在新西兰大部分地区其分布却还在扩展,依然威胁着新西兰的生物多样性和农牧业生产,传统的控制方法包括兽夹捕杀,射杀和药物灭杀,其效果不佳的主要原因是：1）受资金的限制;2）小面积控制后因为种群密度减小,负鼠的生育率,存活率随之增高,加上新个体从周围迁入,种群数量迅速恢复,生物防治特别是免疫绝育法被认为是经济,人道及有效的方法,新的研究结果证明刷尾负鼠具有一雄多雌的繁殖方式,而且在雌兽被绝育后,种群中雄兽与雌兽之比率反而增高,因此雌兽绝育可能不会降低雌雄负鼠之间的接触频率,这些研究结果表明免疫绝育控制刷尾负鼠是很有前景的。     

Estimation of population size and survival of sheep blowfly,Lucilia cuprina,in the field from serial recoveries of marked flies affected by weather dispersal and age-dependent trappability

A model is presented for analysis of mark-recapture data of mobile insects which, unlike the Lincoln Index, does not require marked individuals to remain within the sampling area or to mix uniformly with the wild population. The model assumes a single or multiple releases of marked insects from the centre of the sampling area and that captured individuals are not returned to the population. Dispersal rates of marked insects are estimable from serial recaptures and, for catches that are either unaffected by or have been corrected for weather effects, the model also provides estimates of mortality and age-dependent trappability. Application of the model is illustrated using mark-recapture data for adults of the Australian sheep blowfly Lucilia cuprina.     

Identification of novel major histocompatibility complex class I sequences in a marsupial,the brushtail possum (<Emphasis Type="Italic">Trichosurus vulpecula</Emphasis>)

The major histocompatibility complex (MHC) is an essential part of the vertebrate immune response. MHC genes may be classified as classical, non-classical or non-functional pseudogenes. We have investigated the diversity of class I MHC genes in the brushtail possum, a marsupial native to Australia and an introduced pest in New Zealand. The MHC of marsupials is poorly characterised compared to eutherian mammal species. Comparisons between marsupials and eutherians may enhance understanding of the evolution and functions of this important genetic region. We found a high level of diversity in possum class I MHC genes. Twenty novel sequences were identified using polymerase chain reaction (PCR) primers designed from existing marsupial class I MHC genes. Eleven of these sequences shared a high level of homology with the only previously identified possum MHC class I gene TrvuUB and appear to be alleles at a single locus. Another seven sequences are also similar to TrvuUB but have frame-shift mutations or stop codons early in their sequence, suggesting they are non-functional alleles of a pseudogene locus. The remaining sequences are highly divergent from other possum sequences and clusters with American marsupials in phylogenetic analysis, indicating they may have changed little since the separation of Australian and American marsupials.     

Costs and benefits of pre‐feeding for possum control

Abstract

Four control operations, each with paired pre‐feed and no‐pre‐feed treatments, and comprising two baited with carrot and two with cereal, provided strong support for the hypothesis that pre‐feeding produces higher possum kills than no pre‐feeding. In these replicates, even where high kills were achieved, the modelled two‐ to three‐fold extension of the period of population depression given by pre‐feeding was sufficient to warrant the expense of the additional bait sown. The benefits of pre‐feeding were clear‐cut, and were greatest where poorest kills were achieved, irrespective of bait type. Pre‐feeding is thus likely to reduce the probability of failure and should be considered a form of operational insurance.     

High variability in the MHC class II DA beta chain of the brushtail possum (<Emphasis Type="Italic">Trichosurus vulpecula</Emphasis>)

The diversity of class II major histocompatibility complex (MHC) loci was investigated in the brushtail possum, an important marsupial pest species in New Zealand. Immunocontraception, a form of fertility control that generates an autoimmune response, is being developed as a population control method for the possum. Because the immune response is partly under genetic control, an understanding of immunogenetics in possum will be crucial to the development of immunocontraceptive vaccines. MHC molecules are critical in the vertebrate immune response. Class II MHC molecules bind and present exogenously derived peptides to T lymphocytes and may be important in the presentation of immunocontraceptives. We used polymerase chain reaction primers designed to amplify the peptide binding region of possum class II MHC genes to isolate sequences from 49 animals. We have previously described 19 novel alleles from the DAB locus in the possum (Holland et al., Immunogenetics 60:449–460, 2008). Here, we report on another 11 novel alleles isolated from possum DAB, making this the most diverse marsupial locus described so far. This high level of diversity indicates that DAB is an important MHC locus in the possum and will need to be taken into account in the design of immunocontraceptive vaccines.     

Novel alleles in classical major histocompatibility complex class II loci of the brushtail possum (<Emphasis Type="Italic">Trichosurus vulpecula</Emphasis>)

We have investigated the diversity of class II major histocompatibility complex (MHC) loci in the brushtail possum (Trichosurus vulpecula), an important marsupial pest species in New Zealand. Immunocontraceptive vaccines, a method of fertility control that employs the immune system to attack reproductive cells or proteins, are currently being researched as a means of population control for the possum. Variation has been observed in the immune response of individual possums to immunocontraceptives. If this variability is under genetic control, it could compromise vaccine efficacy through preferential selection of animals that fail to mount a significant immune response and remain fertile. The MHC is an important immune region for antigen presentation and as such may influence the response to immunocontraceptives. We used known marsupial MHC sequences to design polymerase chain reaction primers to screen for possum MHC loci. Alpha and beta chains from two class II families, DA and DB, were found in possums throughout New Zealand. Forty new class II MHC alleles were identified in the possum, and the levels of variability in the MHC of this marsupial appear to be comparable to those of eutherian species. Preliminary population surveys showed evidence of clustering/variability in the distribution of MHC alleles in geographically separate locations. The extensive variation demonstrated in possums reinforces the need for further research to assess the risk that such MHC variation poses for long-term immunocontraceptive vaccine efficacy.