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Management program for the Brushtail possum Trichosurus vulpecula (Kerr) in Tasmania - Review of background information

Prepared by Tasmanian Parks and Wildlife Service, October 1996


General introduction

Aims and Scope

  1. This document provides background information to the Management Program for the brushtail possum, Trichosurus vulpecula, in Tasmania.

Objectives

  1. In accordance with the National Guidelines for Kangaroo Management agreed to by the Australian and New Zealand Environment and Conservation Council (ANZECC), the objectives of brushtail possum management in Tasmania are as follows:
    1. to ensure viable populations of the brushtail possum remain throughout the species existing geographical range;
    2. to manage the species as an ecologically sustainable resource; and,
    3. to reduce economic loss or damage to nature, and the pastoral, forestry and agricultural industries.
  2. These three objectives are labelled respectively as conservation, sustained yield harvesting, and control (Caughley, 1977).

Background to Wildlife Management

  1. Wildlife management deals with all three of the above problems and in every case the problem is solved by a manipulation of the dynamics of a population. Sometimes a modification of available shelter, food, or water supply will trigger the desired change in the population's rate of increase while at other times a more direct manipulation may be called for.
  2. Wildlife management must accommodate the dynamic nature of the biological systems that are being managed, and be sufficiently flexible to adapt to unforeseen circumstances. The interaction of people, species, and environmental factors makes every management project unique.
  3. In considering the utilisation of any wildlife population there are four principles to consider. These are:
    1. any utilisation of a population reduces its abundance and the greater the utilisation, the smaller the population becomes;
    2. below a certain level of utilisation, populations are resilient and increase survival or growth to compensate for removals;
    3. rates of utilisation may be raised to levels at which they can cause the extinction of the species; and,
    4. between no exploitation and excessive exploitation, there are many levels of sustained harvest, amongst these are a maximum sustained yield harvest and an optimum yield harvest.
  4. Management programs that have sustained yield harvesting as their primary focus usually seek to identify the maximum sustainable yield or the optimum yield for the species involved and focus management controls on achieving the chosen yield. These elements, whether taken individually or together, are not incompatible with conservation. Indeed the primary objective of management is the maintenance of populations throughout their known range.

Brushtail Possum Management

  1. The common brushtail possum Trichosurus vulpecula (Kerr), is the most familiar and abundant of the Australian possums. The species is highly adaptable to a wide range of natural and human environments, and has the widest distribution of any Australian marsupial (How, 1983; Kerle, 1984). Three sub-species of common brushtail possum (subsequently referred to as the brushtail possum or possum) are currently recognised, one of which Trichosurus vulpecula fuliginosus, is confined to Tasmania where it occurs throughout most of the State in agricultural areas, woodland and forests (Watts, 1993).
  2. In the past, large numbers of brushtail possums have been killed for their skins in most Australian States (Strahan, 1983). Tasmanian possums are larger and have a denser fur than their mainland counterparts (How, 1983). As a result the species has been harvested for its skin since the earliest days of European settlement and has at times supported a substantial export-based industry. At the same time changes in land-use following settlement of Tasmania in the early 19th century have favoured the species with the result that brushtail possums have greatly increased in numbers. The use made by this species of orchards, crops, and forestry plantations as food resources means that in many areas the brushtail possum is regarded as a pest (Statham, 1983; Coman, 1994).
  3. A publicly accountable brushtail possum management plan, based on on-going research and monitoring, is generally accepted as meeting the wishes of interested parties in Tasmania. The coordinated management regime aims to balance land-use requirements and to ensure that brushtail possums remain throughout their present geographical distribution range.

Historical Background

  1. Government regulation of the brushtail possum skin industry began around 1918 when possum hunters were required to pay a surcharge on their wallaby licence. This allowed them to take brushtail possums during an open season of between one and three months duration in winter. Royalties were payable on all skins traded. In 1964 licences were issued specifically for the taking of brushtail possums. This system continued until 1973 with possums being taken during a proclaimed open season by licensed hunters using snares. However, in 1974 there was a public outcry against the use of snares because of the suffering they caused the captured animal.
  2. Since 1974, to prevent the use of snares under the licence system, while allowing the continuation of possum hunting, the taking of brushtail possums has been allowed for a specified period each year under a system of special commercial permits. Shooting with the aid of a spotlight is the specified method for these permits although permission may be granted to particular permit holders to take possums using cage traps. Harvesting is only permitted on lands used for grazing, farming or forestry.
  3. Between 1976 and 1981 over 250,000 brushtail possums were taken each year for their skins by hunters. Since then there has been a marked decline in the price of skins, the number of commercial permits issued, and the number of animals taken. Fewer than 7,000 brushtail possums were taken by commercial hunters in 1995.
  4. The decline of the commercial harvest has been followed by a significant increase in brushtail possum abundance and crop damage. This has led to an increase in the culling of possums under crop protection permits. Land holders who consider that possums are causing, or in some cases, are likely to cause damage to crops or pasture, may apply to the Parks and Wildlife Service (PWS) for a permit to cull them either by shooting or by the use of 1080 poison baits.
  5. In 1985 the Wildlife Regulations were amended to permit trading in brushtail possum meat in an effort to encourage use to be made of the carcases of possums taken for their skins. Possum meat is high in protein and has a low fat content (Hogg et al. 1992).

Alternative Methods of Control

  1. Alternatives to culling have been, and continue to be investigated. The use of fences as a barrier to the movement of possums has been examined, but Statham (1991) and Cowan and Rhodes (1992,1993) found conventional designs to be ineffective and concluded that fences must be specifically designed for this species. The high cost of electric fences designed specifically for possums, and the large areas that require protection excludes them as a viable alternative to culling. This problem is further compounded in forestry areas which are typically in hilly country with steep gullies and ridges making the construction and maintenance of electric fences problematical and of prohibitive cost.
  2. Chemical repellents have been trialled by the Tasmanian Forestry Commission (1982) and Statham (1991) but most repellents were found to be ineffectual, too costly, difficult to apply, short lived or phytotoxic (Coman, 1994). A number of olfactory repellents that simulate predator odours are being trialed and show some promise (Morgan and Woolhouse, 1993). Similarly a repellent that combines olfactory and texture repellents has been developed in Victoria although as yet this repellant has only been tested on wallabies (Coman, 1994). However, because of the wide range of crops that require protection against possums in Tasmania even successful repellents are unlikely to resolve all of the browsing problems (Coman, 1994).
  3. Fertility control techniques, including both chemical and viral-vectored immunocontraception, are being researched to control a range of pest species (Bomford, 1990; Barlow, 1994). However, the successful application of these methods remains some years away and, moreover, is unlikely to be generally acceptable for use on a native species such as the brushtail possum.

Present Status

  1. In Tasmania, brushtail possums continue to benefit from the clearing and modification of native vegetation for agriculture and forestry. Ongoing damage to crops and plantations means that the pest status of this species endures. The price of skins remains low and attracts very few commercial hunters. The developing carcase trade remains small. Brushtail possums continue to increase in abundance and the need for crop protection against them is considerable.
  2. As of August 1996 two operators have been licensed by Commonwealth authorities to export possum meat. Figures provided by the Tasmanian Department of Development and Resources suggest that this Asian market may eventually gross up to A$5M and that the industry could account for a major part of the annual possum quota by the year 2000 (Cook, personal communication 1994).

HABITAT AND DISTRIBUTION

  1. Information on the habitat and distribution of Tasmanian mammals has been systematically recorded on the TASPAWS database of the PWS. This information has been published in Rounsevell et al. (1991).

Habitat

  1. The preferred habitat of brushtail possums is eucalypt forest or woodland. This habitat provides the species with holes in trees and/or fallen limbs and trunks for nest sites and day-time refuges. It also supplies them with the eucalypt and acacia leaves, grasses and forbs upon which their diet is mainly based (Statham, 1984). Possums are most abundant where a mosaic of pasture and eucalypt forest or woodland occurs (Johnson, 1977). The species is largely absent from the extensive buttongrass plains in Southwest Tasmania and occurs at low density in temperate rainforest (How, 1983; Rounsevell et al. 1991).

Distribution

  1. As an arboreal marsupial the brushtail possum occurs in most areas where there are trees. Vegetation in Tasmania can be divided into forest and non-forest communities based on the presence or absence, respectively, of trees of at least 5m height. Subtracting the rainforested areas as unsuitable brushtail possum habitat leaves a total of 4,221,000 hectares of high rainfall and low rainfall eucalypt forest and woodland. This amounts to 61.8% of the total land area of Tasmania.
  2. The species is wholly protected in conservation reserves and at present does not require active management in these areas. However, the majority of private land, and that administered by the Forestry Commission, is inhabited by possums, often at relatively high density.

Density

  1. Observed densities of brushtail possums in Tasmania range from 8.0/hectare in pasture areas adjacent to forest, to 1.9/hectare in dry sclerophyll forest (Johnson, 1977), and 0.5/hectare in wet sclerophyll forest (Hocking, 1981).

NATURE CONSERVATION RESERVES

  1. Large tracts of Tasmania have been reserved for conservation purposes under the National Parks and Wildlife Act, 1970 . This area currently totals 1,790,246 hectares or 26.2% of Tasmania. Several classes of reserve are included in this total within the categories of State Reserve and Conservation area. The former are set aside solely for conservation purposes, including the complete protection of brushtail possums. By comparison conservation areas provide for the protection of wildlife while allowing compatible forms of land use to take place.
  2. With a total area of 1,367,186 hectares, Tasmania's National Parks constitute 20.1% of Tasmania. The majority of these areas contain habitat of medium to high suitability for brushtail possums ( see Appendix 2) and represent an extensive reserve of the species and their habitat. Brushtail possums are also well represented in conservation areas which total 423,060 hectares.

CONSERVATION STATUS

  1. The common brushtail possum, Trichosurus vulpecula fuliginosus, is a protected species under the Wildlife Regulations, 1971 of the National Parks and Wildlife Act, 1970 and as such it may be taken only under a permit or licence issued by the PWS.
  2. The species is widespread and locally abundant in Tasmania. In addition it is well represented in the Tasmanian reserve system. For these reasons the brushtail possum is regarded as secure by the Vertebrate Advisory Committee (1994) of the Tasmanian PWS.

BIOLOGY AND ECOLOGY OF THE BRUSHTAIL POSSUM

  1. Information on the biology and ecology of a species is essential to its management. The pest status of this species both in New Zealand and Australia has led to extensive research on it's biology and ecology. Only research directly relevant to brushtail possum management is reviewed here.

Diet

  1. The brushtail possum is a predominantly herbivorous marsupial (How, 1983; Fitzgerald, 1984; Statham, 1984; Cowan and Moeed, 1987). How (1983) found the diet of the brushtail possum in forest in NSW to consist of a variety of leaves, particularly of Eucalyptus sp, supplemented by fruits, buds, bark, and occasionally clover and other pasture plants. Studies in Tasmanian forests have revealed that the diet may differ according to habitat type and season (Fitzgerald, 1984; Statham, 1984). In wet forests leaves of myrtle beech, Nothofagus cunninghamii, ferns and grasses are eaten, with the myrtle beech leaves forming the greater percentage of the diet. This remains relatively constant throughout the year. In dry sclerophyll forests where grasses, herbs, and eucalypt and acacia leaves are eaten the major food consumed differs seasonally. In spring and summer grasses and herbs are the predominant foods, while in autumn the proportion of eucalypt and acacia leaves consumed increases, with these foods generally comprising the greater portion of the diet in late autumn and winter (Fitzgerald, 1984; Statham, 1984, 1992).

Crop Damage caused by Brushtail Possums

  1. Agricultural and forestry crops provide many herbivorous wildlife species with a ready food supply and damage to crops is often substantial. Various authors have reported on the use of pasture by brushtail possums (Quinn, 1968; Harvie, 1973; Jolly, 1976; Spurr and Jolly, 1981; Green and Coleman, 1981, 1986). Harvie (1973) found that pasture species formed about 30% of the diet during spring on sheep stations in New Zealand.
  2. Grazing damage by possums has not been quantified in Australia. Some recent evidence suggests that grazing by native animals including possums can reduce dry matter yield of improved pasture by up to 94% while that of native pasture can be reduced by 48% (Statham & Rayner, 1995).
  3. Similarly the incidence of crop damage by possums has been widely reported and discussed but the magnitude of the problem remains unclear.
  4. Since it's introduction from Australia in the mid-19th century the brushtail possum has markedly changed the composition of some New Zealand forests (Kean, 1951; Fitzgerald, 1984). In Australia the species has evolved with the native flora and Fitzgerald (1984) suggests that Tasmanian plants are better adapted to possum browsing than many of those in New Zealand.
  5. Damage caused by possums to native forest in Tasmania (generally through defoliation) was thought to be restricted to areas of recently regenerated forest and to seedlings (Gilbert, 1961; Cremer, 1969; Hocking, 1981). Statham (1984) found that should defoliation occur in autumn or winter it generally results in the death of the tree (or seedling), while at other times the tree will recover. However, recent evidence suggests that brushtail possums are contributing to 'dieback' in mature eucalypts in the Tasmanian midlands where tree cover on pastoral land is often minimal (Statham, 1992).
  6. Diet analyses show that possums feed on the pollen cones, leaves and bark of pine species including Pinus radiata and that this damage increases with proximity to native forest, the age of the pine tree, and during the winter months when other preferred foods are less available (Barnett et al. 1977; Forestry Commission Tasmania, 1982; Statham, 1983, 1984, 1992). Damage results from animals feeding on the 'growth tips' and bark of trees, the latter being the most serious as it can result in ring-barking and hence the death of the tree (Statham, 1983, 1984, 1992).

Reproductive Biology

  1. Births have been recorded in all months of the year, however most populations of the brushtail possum have a major autumn and minor spring breeding season (Pilton and Sharman, 1962; Meredith et al. 1969; How, 1983). In Tasmania, most births occur between the beginning of April and the end of June with 65% of all births taking place in May (Hocking, 1981). Hocking (1981) also found that the mean birth date varied between areas, those areas most recently burnt exhibiting an earlier mean birth date than those where fire had not occurred for over 8-16 years. Over 90% of females breed annually and in some populations 50% may breed in both seasons (How, 1983). This has been found to be the case in newly colonised habitat or where the number of animals has been artificially reduced by hunting or eradication programs (Tyndale-Biscoe, 1955; Kean, 1967; Hocking, 1981; Cowan, 1992, 1993).
  2. Females may begin to reproduce when about one year old (How, 1983). This is true for newly colonised habitat (or where possum numbers have been artificially reduced) and bushland regenerating after fire (Hocking, 1981; Cowan, 1993). In forests of later successional stage, and those where possum numbers are limited by food resources, Tyndale-Biscoe (1955), Gilmore (1969), and Hocking (1981), found that females begin reproducing at two to three years of age.
  3. A single young is born 17-18 days after mating, attaches to one of two teats in the well developed pouch, and develops rapidly during the four to five months spent in the pouch. A further one to two months are spent suckling and riding on the mothers back before weaning is completed (Pilton and Sharman, 1962; Jolly, 1981; How, 1983; Watts, 1993). The sex ratio of pouch young was biased towards males in wet forest habitats studied by Hocking (1981) in Tasmania although this situation is reversed in the adult population.

Population Dynamics

  1. Survival of dependant young is high in brushtail possums (Dunnet, 1964; How, 1972a, 1972b; Hocking, 1981) but a sharp increase in mortality occurs when juveniles of both sexes disperse from the area of their birth (How, 1972b, 1983; Hocking, 1981). Dispersing males range further than females and have a higher mortality rate which results in a female bias in the adult population (Hocking, 1981; How, 1983; Green, 1984; James, 1984). Juvenile mortality is reportedly higher in high density populations possibly due to an inability of individuals to establish themselves in already fully utilised habitat (How, 1972a; Hocking, 1981). Hocking (1981) also found that recruitment to possum populations favoured males in early successional habitat and females in older habitat. This indicates that brushtail possums are regulated by density-dependant factors (Cowan, 1993).
  2. In the absence of large nocturnal predators, predation on adult possums is considered unlikely, the major cause of death being starvation in the form of winter food shortage (Hocking, 1981). Human predation through harvesting and crop protection culling undoubtedly influences the dynamics of possum populations but under correct management this practice only removes that portion of the population that is readily replaced (Caughley, 1977).
  3. Although brushtail possums in New Zealand are known to be infected with the microbial agents of leptospirosis and tuberculosis which can also infect grazing livestock and humans, their Australian counterparts are not considered to be an important host or reservoir for these infectious diseases (Presidente, 1984; O'Callaghan and Moore, 1986). The pathogenic significance of many brushtail possum endoparasites is unknown although some heavy nematode infections can be pathogenic (Presidente, 1984). Ectoparasites including ticks, mites, and fleas have been noted from Tasmanian possums, but without any apparent association between parasite load and condition, they are not regarded as significant agents of mortality (Hocking, 1981; O'Callaghan and Moore, 1986).

THE COMMERCIAL HARVEST

  1. All persons taking brushtail possums commercially are required to have a permit for this purpose issued by the PWS. Commercial harvesters are required, as a condition of this permit, to supply information on the number of animals taken and details of skins or carcases sold to licenced dealers. All skins or carcases traded by dealers must be stamped and a royalty paid. The provision of information to the PWS on the quantity of skins or meat traded is a requirement of the dealers licence. This information is used to determine the number of possums traded commercially and to estimate the number of animals taken each year by commercial hunters. Since 1983, annual quotas have been applied for the number of individual brushtail possums allowed to enter the commercial trade. In 1995, this quota was expanded to also include any possums taken but not used commercially.

Commercial Permit Trends

  1. Figure 1 shows the close association between skin prices and the number of commercial permits issued. The decline in numbers of permits issued evident since 1979 is due to a depressed world fur market and a corresponding fall in the average price of skins to shooters. Commercial harvest statistics since 1923 are given in Appendix 1.

Commercial Harvest Trends

  1. The brushtail possum quota has remained constant at 250,000 animals a year since the introduction of a quota in 1983. During this time, the estimated possum harvest has approached the commercial quota on only one occasion. Since 1989, estimates of the number of animals taken each year by commercial hunters have not exceeded 65,000. In 1995, an estimated 6,012 possums were harvested under commercial permit. Similarly, the number of royalty payments made on skins and/or carcases traded has not exceeded 200,000 per year since 1983, and has been less than 20,000 a year since 1990. Figure 2 shows trends in the quota, harvest and royalties paid on brushtail possums since 1983.

Regional Trends in the Commercial Harvest

  1. The harvesting of possums varies regionally across Tasmania. Table 1 shows that over 60% of all permits are issued in the north, midlands and southeast regions which together account for less than 30% of the total area Tasmania. A similar pattern is evident in the regional distribution of the harvest with 68% of possums being harvested in the north, midlands and southeast regions. There are a number of reasons for this including ease of access to these regions and the quality and quantity of possums present.
Table 1: Percentage of commercial possum permits issued per area 1985-94.
Region % of all Permits % of Total Harvest
1. North West 3.6 2.2
2. North 22.0 15.3
3. North East 6.6 5.6
4. West 0.6 0.3
5. Central 16.2 16.5
6. Midlands 23.3 30.7
7. East 1.0 1.8
8. South East 19.0 23.4
9. SouthWest 7.7 4.1
Tasmania 100.0 100.0