Health and Ecological Status of a Crashing Koala Population on Raymond Island.

Stephen Phillips, Jon Hanger and Jeff McKee

Raymond Island comprises approximately 800 hectares of low relief mixed open woodland and grassland in the lakes area of East Gippsland Victoria ( See Figures 1a,b & c for location). In the 1950s 32 koalas were translocated to Raymond Island from Phillip Island. In 1995 the population was estimated at 400 and in late 2003 it was appraised as 600. On Raymond Island over the past 12 months there has been severe defoliation of Manna gums on crown land and in the nine week period spanning late November 2003 through to early January 2004 149 koalas were found dead or dying on the island (Arnold, S., pers. com). These observations suggested critical resource depletion and imminent koala population collapse. At that time concerned members of the community invited the Steve Irwin Wildlife Foundation to help manage the immediate problem. Under subcontract to the Foundation the Mobile Disease Ecology Unit (MDEU) was mobilised to define and mitigate the crisis. In January 2004 we surveyed 51 sites across the island in order to assess habitat type, habitat utilisation and to estimate koala population size. In addition we clinically examined 73 animals and carried out 22 Post Mortems to develop a preliminary appraisal of morbidity, mortality and reproductive success. The feeding ecology of koalas on Raymond Is. is focused equally on Manna Gum, Red Gum, Gippsland Grey Box and Swamp Gum. Severe levels of defoliation and/or dieback however, appear primarily restricted to the Manna Gum communities. Severe levels of defoliation and/or dieback are most commonly associated with areas that have a dense under-storey of Bracken Fern; defoliation and/or dieback is less severe in areas that have a sparse to mid-dense Bracken Fern under-storey, independently of koala densities. The current extent of defoliation and/or dieback of Manna Gums on Raymond Island is thus exacerbated by, but not solely attributable to, browsing by koalas. Moisture sequestration by unmanaged Bracken in combination with a four year history of reduced rainfall are likely to be contributing factors. Manna Gum communities on Raymond Island are in urgent need of assistance to ensure that adequate water balances are restored (by management of Bracken Fern) and that effective recruitment levels are re-established. The koala population on Raymond Island rose from 32 to 600 between 1953 and 2003 and the likelihood of a population- resource imbalance was first noted in 1988. In late 2003 the koala population on the island crashed by approximately 50% (our estimate for the current population size is 313 +/- 75). The preliminary clinical and post mortem evidence suggests that the prime cause of the mortality was lack of nutrient. The most recent estimated mortality rate suggests that the population crash is waning however the possibility of significant disease processes unrelated to resource restriction cannot be excluded at this time. The chlamydial, retroviral, mycobacterial, pica and genetic fitness status of the population remains ill defined and the current estimates of mortality fecundity and morbidity rates are very coarse. These attributes, along with the broader ecological questions (above), need to be resolved more fully before settling on a specific management course.We recommend a full environmental audit of the island in particular concentrating on the status of other arboreal animals, small passerines and reptiles which seemed notably absent. The results of this audit should then be integrated with a revamped fire management, vegetation and land use plan in addition to any proposed management procedures for koalas. We recommend establishing experimental plots designed to determine most appropriate method of dealing with the Bracken monoculture, restoring understorey floristic and structural diversity and restoring ecological integrity (ie water balance and recruitment levels) to remaining Manna Gum stands. We suggest that koala translocation is an option better kept in reserve as it is a resource intensive option with particular risks that cannot be defined in the absence of more specific habitat, health and genetic data. Instead we recommended a short term management option to actively shepherd the remnant population downward to a target value of 200. Initially this would involve continuing guardianship and longitudinal assessment of the population until the mortality rate stabilised at an acceptable value and the use of contraceptive implants to reduce fecundity of resident female koalas for a period of 2 – 5 years. The program would include continued trapping and triage, radio tagging and serial assessment of health status and while trapping, selection of a subgroup of animals of the appropriate age, sex and health status to be translocated should this become necessary. After initially stabilizing the population biannual health/population census should be carried out for at least ten years. In addition the introduction of genetically robust stock (ie from Strezleckis) maybe warranted to assist longer-term population management. This option should be prefaced by a detailed assessment of the current genetic status of the population.