Individual study: Translocation of dibblers Parantechinus apicalis to Escape Island, Jurien Bay, Western Australia
Moro D. (2003) Translocation of captive-bred dibblers Parantechinus apicalis (Marsupialia: Dasyuridae) to Escape Island, Western Australia. Biological Conservation, 111, 305-315
The dibbler Parantechinus apicalis, a small carnivorous marsupial endemic to south western Australia, was once widespread across its range. It is now restricted to two small offshore islands and three scattered mainland populations. The main reasons for their decline are thought to be due to predation by introduced predators, habitat fragmentation and habitat degradation due to too frequent burning of vegetation. Listed as endangered, island introduction of dibblers was posed as part of a conservation management plan for the species.
Boullanger and Whitlock Islands in Jurien Bay support the only two extant 'island dibbler' populations. However, they are easily accessible to people and subject to frequent human visitation with associated risks, such as fire and pets. There is also a possible but unknown threats posed from increasing numbers of non-native house mice Mus musculus on these islands.
Introduction to a third but less accessible island was therefore proposed in order to establish a new dibbler population and safeguard against such human-associated risks.
Release site: Escape Island, a remote island in Jurien Bay, Western Australia, was chosen as a site for dibbler introduction. It was selected due to its relative inaccessibility and the lack of non-native predators and rodents. Vegetation characteristics were also similar to those of the two native dibbler islands.
A total of 88 dibblers were released from 1998-2001, ('founder' dibblers, individuals from a captive breeding program at Perth Zoo). Founder dibblers were four pairs collected from Boullanger and Whitlock Islands. Introduced captive-bred individuals were born in the previous or year of release. Dates and numbers of zoo-born and founder dibbler releases are summarised in table 1 (attached).
Release method: Dibblers were transported in wooden nest boxes to Escape Island. For two days prior to release they were given twice their usual daily food supply. Individuals were also fitted with PIT (passive induction transponder) tags placed under the skin at the base of the neck.
Four release sites were located in the north of the island. A hard release method was used where no pre-release acclimatization occurred and no post-release supplementary food was provided. Releases occurred at dusk and nest boxes remained open for 5 days, after which they were removed if not still in use.
Monitoring: Single stage transmitters were fitted to eight individuals released in 1998 and 17 in 1999, under anesthesia. Day time locations were recorded one day after release and throughout the 2-week battery life of the transmitters. The collars were removed after subsequent trapping of these individuals.
Long-term monitoring was carried out in February-April, between the breeding seasons and in September-November, before and after the dispersal season. To census the population, around 100 folding aluminium traps (32 × 90 × 100 cm) were placed 20 m apart along three transects spanning the length of the island.
Weight, sex, body condition and reproductive status were recorded. Body condition was assessed on overall appearance, on a scale of poor to very good. Females were classified as reproductive if they were carrying young, lactating or had distended teats. Any new recruits to the population were fitted with identity tags.
Costs: The total cost of the captive breeding and introduction program was approximately $AUS 0.6 million, equivalent to about $AUS 6,600/individual released.
Dispersal: During the three weeks following release, most individuals stayed within 400 m of their nest box. Two dibblers remained within 50 m of their release sites for one week but then subsequently dispersed to other areas of the island where they established territories. Two juvenile deaths occurred during each of the 1998 and 1999 October releases. In all cases starvation was thought to be the cause.
Post-release survival: In 1998, of the 26 dibblers released, 29% survived the first two months. After 24 months only 4 individuals (16%) were still alive.
In 1999, survivorship was much higher with 41 dibblers (93%) surviving after 10 months of release.
In 2000, only 16% survived after two months, but of these all survived up to at least 18 months, and one survived 24 months.
To place in some kind of context, in captivity females have lived up to 36 months and males up to 32 months.
Breeding: Successful breeding and dispersal of young took place within the first year following release. Breeding occurred during March-April with young in the pouch by May. Recruitment into the population (i.e. young independant of the mother) was recorded 4-5 months later during September and October. A 70% increase in reproductive females occurred subsequent to the first year, rising from 20% in 1999 to 90% in 2001. Females were classified as being of good to very good body condition.
Recruitment: At least 121 individuals were born in the wild from 1999-2001. Population size was highest during 2000 with 131 individuals known to be alive, of which 91 were wild-born. In 1999 the number of progeny (first generation) was 24, increasing to 77 in 2000. In 2001, the number of third generation progeny was however only 20. Fifty seven (93%) of dibblers known to be alive in 2001 were born on Escape Island. The population decrease from 131 in 2000, to 67 in 2001, may have been in part due to the lack of introduction of new dibblers into the population.
Of the first generation progeny, 98% survived to 10 months old, indicating a higher survivorship of individuals born in the wild than captive-bred animals. Population estimates were higher for Escape Island compared with those for Boullanger and Whitlock Islands.
Impact on other species: A colony of wedge-tailed shearwaters Puffinus pacificus (a burrow-nesting seabird) is present on Escape Island. Although not monitored, they were considered unlikely to be affected by the dibbler introduction as breeding colonies of this sheartwater co-exist alongside dibbler populations on Boullanger and Whitlock Islands.
Conclusions: This translocation, using a small number of wild founder members supplemented by regular introduction of captive-bred individuals into the population, was considered to have been a successful strategy following the establishment of 3rd generation offspring and securing an additional dibbler population, at least in the short-term.
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